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Analog electronic circuit simulation 1.1 Basic transistor amplifier circuit The three basic amplifier circuits of common emitter, common collector and common base are the basis of analog electronic technology. Through EWB simulation analysis, we can further understand the different characteristics of the three circuits in terms of static operating point, voltage amplification factor, frequency characteristics, input and output resistance, etc. 1.1.1 Common emitter basic amplifier circuit Build the common emitter basic amplifier circuit according to Figure 7.1-1 , select the Show/Hide button in the Circuit/Schematic Option menu , and set and display the component numbers and values. . 1. Static working point analysis Select the DC operating point analysis option ( Analysis/DC Operating Point ) in the analysis menu (of course, you can also use the digital multimeter in the instrument library to measure directly). The analysis results show that transistor Q1 is working in the amplification state. 2. Dynamic analysis Use the function generator in the instrument library to provide the circuit with a sinusoidal input signal Vi ( amplitude 5mV, frequency 10kH) , and use an oscilloscope to observe the input and output waveforms. From the waveform diagram, it can be observed that the input and output voltage signals of the circuit have an anti-phase relationship. Another simple method to directly measure the voltage amplification factor is to directly measure it with a digital multimeter in the instrument library. 3. Parameter sweep analysis In the common emitter basic amplifier circuit shown in Figure 7.1-1 , the resistance of the bias resistor R1 directly determines the size of the static current IC. Keeping the input signal unchanged and changing the resistance of R1, the distortion of the output voltage waveform can be observed. Select the parameter sweep option ( Analysis/Parameter Sweep Analysis ) in the analysis menu, set the sweep element to R1 in the parameter sweep setting dialog box, the parameter is resistance, the sweep start value is 100K, the final value is 900K, the sweep mode is linear, the step increment is 400K, the output node is 5, and the sweep is used for transient analysis. 4. Frequency response analysis Select the AC Frequency Analysis item ( Analysis/AC Frequency Analysis ) in the Analysis menu and set the following in the AC Frequency Analysis Parameter Setting dialog box: the start frequency of the scan is 1 Hz , the end frequency is 1 GHz , the scan format is decimal, the vertical scale is linear, and node 5 is the output node. From the analysis of the figure, it can be concluded that: when the input signal voltage VI of the common emitter basic amplifier circuit is a variable frequency voltage with an amplitude of 5 mV , the circuit outputs an intermediate frequency voltage amplitude of approximately 0.5V , the intermediate frequency voltage amplification factor is approximately -100 times, the lower limit frequency (X1) is 14.22 Hz , the upper limit frequency ( X2 ) is 25.12 MHz , and the passband of the amplifier is approximately 25.12 MHz . From theoretical analysis, it can be obtained that the input resistance of the above common emitter basic amplifier circuit is limited by the input resistance rbe of the transistor , and the output resistance is limited by the collector resistance R3. 1.1.2 Common collector basic amplifier circuit (emitter follower) Figure 7.1-7 is a common collector basic amplifier circuit. The function generator in the instrument library is used to provide the circuit with a sinusoidal input signal VI (amplitude 1V, frequency 10 kHz ). The same analysis method as the common emitter basic amplifier circuit is used to obtain the static operating point analysis results of the circuit. The output and input voltage waveforms of the circuit are measured with an oscilloscope, and the AC frequency analysis item is selected to analyze the frequency response curve and related parameters of the circuit. From the frequency response curve of the common-collector basic amplifier circuit shown in the figure, it can be obtained that the upper limit frequency (X2) of the circuit is 4.50 GHz , the lower limit frequency (X1) is 2.73 Hz , and the passband is approximately 4.50 GHz . 1.1.3 Common base basic amplifier circuit Figure 7.1-11 is a basic common base amplifier circuit. The function generator in the instrument library is used to provide the circuit with a sinusoidal input signal Vi ( amplitude 5mV, frequency 10kHz ) . The same analysis method as the basic common emitter amplifier circuit is used to obtain the static operating point analysis results of the circuit. The output and input voltage waveforms of the circuit are measured with an oscilloscope, and the AC frequency analysis item is selected to analyze the frequency response curve and related parameters of the circuit. From the frequency response curve of the common base basic amplifier circuit shown in the figure, it can be obtained that the upper limit frequency (X2) of the circuit is 27.94 MHz , the lower limit frequency (X1) is 261.01 Hz , and the passband is approximately 27.94 MHz .
1.2 Basic field effect tube amplifier circuit 1.2.1 Common Source Amplifier Circuit The common source amplifier circuit is shown in Figure 7.2-1
, Q1 uses a three-terminal enhancement type N- channel insulated gate field effect transistor. After building the circuit in the EWB main interface according to Figure 7.2-1, double-click Q1 , and the three-terminal enhancement type N-MOSFET parameter setting dialog box will appear. Select the Model item, set the library component to default , ideal mode , and then click the Edit button on the right side of the dialog box. Set the transconductance coefficient (KP) to 0.001A / V in Sheet 1 . The analysis of the common-source amplifier circuit can refer to the analysis process of the common-emitter amplifier circuit in Section 7.1 . The theoretical calculated value of AV can be obtained based on the circuit parameters in Figure 7.2-1 and the voltage gain expression of the common-source amplifier , and then compared with the simulated measured value. 1.2.2 Common Drain Amplifier Circuit The common drain amplifier circuit is shown in Figure 7.2-2
, after building the circuit in the EWB main interface according to the diagram , select Q1 as an ideal three-terminal enhanced N- channel insulated gate field effect transistor, and set the transconductance value to 0.001A/V . The circuit simulation analysis process can refer to the analysis process of the common collector amplifier circuit in Section 7.1 . The theoretical calculated value of A can be obtained based on the circuit parameters in Figure 7.2-2 and the voltage gain expression of the common source amplifier , and then compared with the simulated measured value. 1.2.3 Common-gate amplifier circuit The common gate amplifier circuit is shown in Figure 7.2-3
, after building the circuit in the EWB main interface according to the diagram , select Q1 as an ideal three-terminal enhanced N- channel insulated gate field effect transistor, and set the transconductance value to 0.001A/V . The circuit simulation analysis process can refer to the analysis process of the common base amplifier circuit in Section 7.1 . The theoretical calculated value of A can be obtained based on the circuit parameters in Figure 7.2-2 and the voltage gain expression of the common source amplifier , and then compared with the simulated measured value. 1.3 Field effect tube and transistor combination amplifier circuit Field effect tubes have significant characteristics such as high input impedance and low noise, but their amplification ability is weak (small), while semiconductor triodes have strong amplification ability (high) and load capacity. If field effect tubes and semiconductor triodes are used in combination, some performance indicators of the amplifier circuit can be greatly improved and the application range of field effect tubes can be expanded.
Figure 7.3-1 is a bipolar combination amplifier circuit composed of a field effect tube common source amplifier circuit and a transistor common emitter amplifier circuit. In the figure, the three-terminal enhanced insulated gate field effect tube Q1 uses an ideal model, and the transconductance gm is set to 0.001A/V . The transistor Q2 uses N2222A , and its current amplification factor is 255.9 . First, the circuit is statically analyzed, and then dynamic analysis, frequency characteristic analysis, and parameter scanning analysis of key components are performed.
1. Static analysis. Select the DC operating point analysis item in the analysis menu to obtain the circuit static analysis results. 2. Dynamic analysis. ( 1 ) Theoretical analysis. ( 2 ) Simulation test analysis. Use the function generator in the instrument library to provide a sinusoidal input signal ( the amplitude of Vi is 5mV and the frequency is 10kHz) to the circuit . Use an oscilloscope to measure the output and input voltage of the circuit. Then calculate the amplification factor of the circuit. 3. Frequency characteristics analysis. 4. Component parameter scanning analysis. 1.4 Differential Amplifier Circuit The differential amplifier circuit is the most widely used unit circuit in analog integrated circuits. It is the input stage of almost all integrated operational amplifiers, data amplifiers, analog multipliers, voltage comparators and other circuits, and almost completely determines the differential input characteristics, common mode input characteristics, input offset characteristics and noise characteristics of these circuits. The following only simulates and analyzes the emitter-coupled differential amplifier and constant current source differential amplifier composed of transistors. The same method can be used to analyze the differential amplifier circuit composed of field effect tubes. In the differential amplifier circuit shown in Figure 7.4-1 , the emitters of transistors Q1 and Q2 are selectively connected to the constant current source composed of emitter resistors R3 and Q3 through switch S1 ( by tapping the "K" key , select connection point 9 or 11), completing the conversion between the emitter coupled differential amplifier and the constant current source differential amplifier circuit .
1.4.1 Emitter-coupled differential amplifier simulation analysis Build the circuit according to Figure 7.4-1 , select transistors Q1 , Q2 and Q3 are all 2N2222A , the current amplification factor is 200. Connect switch S1 and R3 to form an emitter-coupled differential amplifier circuit. 1. Static analysis. Select the DC operating point analysis item in the analysis menu to obtain the circuit static analysis results. 2. Dynamic analysis. (1) Theoretical analysis (2) Differential input simulation test analysis. A. Use an oscilloscope to measure the differential voltage gain and observe the phase relationship of the waveform. According to the single-ended input mode (see Figure 7.4-1 ), use the function signal generator in the instrument library to provide a sinusoidal input signal to the circuit ( the amplitude of Vi is 10mV and the frequency is 1kHz) . Use an oscilloscope to measure the output voltage waveforms at the two output ends of the circuit. B. Differential input frequency response analysis. Select the AC frequency analysis item ( Analysis/AC Frequency Analysis ) in the analysis menu , and set the following in the AC frequency analysis parameter setting dialog box: the scan start frequency is 1Hz , the middle finger frequency is 10GHz , and the scan format is decimal.
(3) , the vertical scale is linear, and node 2 is the output point. C. Differential input transfer function analysis. Select a DC voltage source from the EWB signal source library (and set it to 0.001V ) , replace the function generator in the instrument library, and use it as the input signal source of the differential amplifier circuit to meet the requirements for the input source when performing transfer function analysis. The circuit connection method for the emitter coupling circuit for differential input transfer function analysis is shown in Figure 7.4-5 . The analysis method is the same as above. D. Common mode input simulation analysis. According to the common mode input method (see Figure 7.4-8 ), use the function generator in the instrument library to provide a sinusoidal input signal to the circuit. Use an oscilloscope to measure the output voltage waveform at the two output ends of the circuit. 1.4.2 Constant current source differential amplifier simulation analysis The differential amplifier circuit introduces a constant current source to replace the emitter bias resistor, which has no effect on the differential gain. It is mainly to further reduce the common-mode gain and improve the common-mode rejection ratio. Therefore, only the common-mode gain of the constant current source differential amplifier is simulated and analyzed here. For the circuit shown in Figure 7.4-1 built in the EWB main interface , by tapping the " K " key, the emitters of Q1 and Q2 are connected to node 11 through switch S to make it a constant current source differential amplifier circuit. Adjust the R6 resistor to make the static current of the constant current source differential amplifier and the emitter-coupled differential amplifier circuit the same, so as to facilitate comparison between the two. Adjust the function generator so that the amplitude of the input sine wave VI is 100 , the frequency is 1 , and the input signal is connected in common mode. The oscilloscope is connected to the input voltage and the output voltage. The final constant current source differential amplifier circuit common-mode gain test circuit is shown in Figure 7.4-10
. The analysis method is the same as above. It can be seen that after the constant current source is introduced, the common-mode gain of the differential amplifier circuit is greatly reduced, the common-mode rejection ratio is greatly improved, and the ability to suppress zero drift is enhanced. 1.5 Integrated Operational Amplifier There are many types of operational amplifiers, and the circuits are also different, but they have something in common in terms of circuit structure. Generally, they can be divided into three parts, namely the differential input stage, the voltage amplification intermediate stage, and the output stage. The input stage is generally a differential amplifier circuit composed of transistors or field effect tubes. The symmetry of the differential amplifier circuit can improve the common mode rejection ratio and other performances of the entire circuit. Its two input terminals constitute the inverting input terminal and the non-inverting input terminal of the entire circuit. The main function of the voltage amplifier stage is to increase the voltage amplification factor. It can be composed of one or more amplifier circuits. The output stage is generally composed of an emitter follower or a complementary emitter follower, and its main function is to increase the output power. Figure 7.5-1 is a simple integrated operational amplifier built in the EWB main interface. Q1 and Q2 form a differential amplifier, with dual-end input and single-end output. Q3 and Q4 form a composite tube common emitter amplifier circuit to increase the voltage amplification factor of the entire circuit. The output pole is composed of a two-pole emitter follower composed of Q5 and Q6 , which can not only improve the load capacity, but also cooperate with R5 to reduce the DC potential step by step, so that when the input signal voltage Vi is zero, the output voltage Vo=0 . The input terminal Vi- is the inverting input terminal of the operational amplifier, and Vi+ is the non-inverting input terminal. Simulation analysis of integrated operational amplifier: 1. Static Analysis Set the input signal voltage to zero (both input terminals are grounded), select the DC operating point analysis item ( Analysis/DC Operating Point ) in the analysis menu, and after analyzing the results, observe whether the DC potential at the output terminal Vo (node 19 ) is zero. If it is not zero, adjust the resistance value of R5 to make the output terminal potential zero. 3. Dynamic Analysis (1) Transfer function analysis Connect the non-inverting and inverting input terminals of the simple integrated operational amplifier to the DC voltage source in the signal source library respectively, and set its voltage value to 1mV . The connection method is shown in Figure 7.5-3 . A Transfer function analysis under the same-phase input mode Select the Transfer Function Analysis item in the Analysis menu ( Analysis/Transfer Function Analysis ), and in the Transfer Function Analysis Settings dialog box that appears, set the input source to V4 and the output terminals to nodes 15 , 10 , and 19. Each time the Simulate button is reset , a transfer function simulation analysis is performed. B Transfer function analysis under inverting input mode Select the transfer function analysis phase ( Analysis/Transfer Function Analysis ) in the analysis menu , and in the transfer function analysis settings dialog box that appears, set the input source to V3 and the output end to node 19 . ( 2 ) Working voltage waveform test. A. Inverting input waveform test. According to the differential mode single-ended input mode, the sinusoidal input signal ( VI amplitude is 2mV , frequency is 1kHz ) provided by the function generator of the instrument library is connected between the inverting and non-inverting terminals, and the non-inverting input terminal is grounded. The connection method is shown in Figure 7.5-6 . The voltage waveforms of the inverting input terminal ( V- ) and output terminal ( Vo ) of the circuit are measured by an oscilloscope . B Common-phase input waveform test. According to the differential mode single-ended input mode, the sinusoidal input signal ( VI amplitude is 2mV , frequency is 1kHz ) provided by the function generator in the instrument library is connected between the in-phase and inverting terminals, and the inverting input terminal is grounded. The voltage waveforms of the in-phase input terminal ( V+ ) and output terminal ( Vo ) of the circuit are measured with an oscilloscope . The results of the waveform test of a simple integrated operational amplifier are completely consistent with the results of the transfer function analysis. By observing the input and output waveforms with an oscilloscope, the phase relationship between the op amp's in-phase input, inverting input and output terminals is intuitively reflected. 1.6 Power Amplifier Circuit In electronic circuits, the main requirement for voltage amplifiers is to make the load obtain an undistorted voltage signal. The main indicators for assessment are voltage amplification factor, input and output resistance, etc. There is basically no high requirement for output power. The power amplifier is different. The main requirement for it is to have a certain undistorted (or less distorted) output power. It usually works under large signals. Therefore, it is important to solve the contradiction between high output power, high efficiency and nonlinear distortion. The following is a simulation analysis of the dual-power supply and single-power supply complementary symmetrical power amplifier circuits. 1.6.1 Dual-supply complementary symmetry ( OCL ) power amplifier circuit Figure 7.6-1 is a complementary symmetrical power amplifier circuit (also called OCL circuit) using dual power supplies. Adjust the function generator to make the input sine wave voltage Vi peak at 10V and the frequency at 1kHz . In the figure, D1 , D2 and RW provide appropriate static bias for T1 and T2 to overcome the crossover distortion caused by the transistor threshold voltage. Use an oscilloscope to observe the input and output waveforms at the same time, press the R key, adjust the size of RW , and change the bias voltage of T1 and T2 until the crossover distortion is eliminated. Press the A key to change the on and off of switch S1 , and the crossover distortion phenomenon can be observed. 1.6.2 Single-supply complementary symmetry ( OTL ) power amplifier circuit Figure 7.6-4 is a single-power complementary symmetrical power amplifier circuit with a bootstrap circuit (also called an OTL circuit). After connecting the circuit as shown in the figure, press the R key and adjust RW2 so that the DC potential at point K is 1/2VCC . Adjust the function generator so that the input sinusoidal voltage ( Vi ) has a peak value of 10mV and a frequency of 1kHz . Use an oscilloscope to simultaneously observe the input ( VA ) and output ( VB ) voltage waveforms, press the W key, and adjust RW1 to overcome crossover distortion. The resistor R and capacitor C in the figure form a bootstrap circuit to increase the peak value of the positive half-cycle of the output voltage. The effect of the bootstrap circuit can be observed by the change of the positive half-cycle of the output voltage when the capacitor C is disconnected and connected. The input ( VA ) and output ( VB ) working voltage waveforms of the single-power complementary symmetrical power amplifier circuit are measured with an oscilloscope . Compared with the previous waveform, it can be seen that the single-power complementary symmetrical power amplifier circuit has a slightly worse symmetry in the positive and negative half-cycles of the output voltage than the dual-power amplifier circuit. 1.7 Negative Feedback Amplifier Figure 7.7-1 is a two-stage common-emitter amplifier circuit composed of discrete components. The circuit introduces AC voltage series negative feedback, and the feedback network consists of REF , RF and CF. The on-off of switch SO controls the connection and disconnection of the feedback network. The on-off of switch S1 controls the connection and on-off of the load resistor ( RL ). The following simulation analysis of the circuit verifies the basic theory of negative feedback and further deepens the understanding of these basic theories. Feedback coefficient of the circuit: FV=0.07 1. Measure the open-loop voltage gain Press the C key to disconnect the switch SO , the peak value of the input sinusoidal voltage ( VI ) is 20MV , and the frequency is 1KHZ . Use an oscilloscope to measure the peak value VO of the input and output voltage (expand the oscilloscope panel and drag the reading pointer to read). 2.Measure the closed voltage magnification Press the C key to close the switch S0 , adjust the input voltage amplitude to 200MV , repeat the above process, and measure the input and output voltage waveforms after introducing feedback. 3. Measure the output resistance of the feedback amplifier when it is open loop When the amplifier is open-loop, press the B key to control the opening and closing of switch S1 . Turn on the digital multimeter and set it to the sinusoidal voltage RMS test position. Measure the output voltage when the load is open and when the load is connected, and calculate RO 4. Measure the output resistance of the feedback amplifier when the loop is closed When the amplifier is working in closed loop, press the B key to control the opening and closing of switch S1 . Turn on the digital multimeter and set it to the sinusoidal voltage effective value test position. Measure the output voltage when the load is open and when the load is connected, and calculate RO 5. Measure the frequency response of the feedback amplifier when it is open loop Make the amplifier work in open loop state, select AC frequency analysis item in EWB analysis menu, set the start and end frequencies of the scan in AC frequency analysis setting dialog box to 1HZ and 1GHZ respectively , select decimal for scan mode, set the number of display points to default, select linear for vertical scale, and select node 8 as output node. After pressing simulation key, the open loop frequency response curve of feedback amplifier is obtained. 6. Measuring the frequency response of a closed-loop feedback amplifier Make the amplifier work in closed loop state, select AC frequency analysis item in EWB analysis menu, and set the parameters in dialog box the same as those in open loop. Press simulation key to get the closed loop frequency response curve of the amplifier. 7. Observe the improvement of amplifier nonlinear distortion by introducing negative feedback and no feedback In the two cases of negative feedback and no feedback, the amplitude of the input sinusoidal signal voltage is increased respectively, and the output voltage peak reaches about 4.5V . Comparing the output waveforms with and without negative feedback, it can be seen that after the introduction of negative feedback, the nonlinear distortion is significantly improved (the symmetry of the positive and negative half-cycles of the waveform is significantly improved).
1.8 RC Sine Wave Oscillator Circuit RC sine wave oscillation mainly discusses the following circuits: diode-stabilized RC bridge oscillator, RC phase-shift oscillator, field-effect transistor-stabilized bridge oscillator, and RC double- T feedback oscillator. As long as the circuit is connected according to the component parameters shown in the figure, the oscilloscope in the instrument library is connected to the output terminal VO of the oscillator , and the power switch is turned on, the output sinusoidal voltage waveform of the oscillator can be observed. Through these circuits, we can do a more in-depth study of the oscillation conditions, starting process, amplitude stabilization measures, and frequency selection characteristics of the frequency selection network of the RC oscillator. In addition, the oscillation period and oscillation frequency of the circuit can be measured by the oscilloscope, and then compared with the theoretical value, so as to deepen the understanding of the basic theory. 1.8.1 Diode-stabilized RC bridge oscillator Figure 7.8-1 is a diode-stabilized RC bridge oscillator circuit, in which R1 , R2 , C1 , and C2 form an R , C series, parallel frequency selection network. We first analyze the frequency selection characteristics of the frequency selection network and reconstruct the frequency selection network circuit in the EWB main interface as shown in Figure 7.8-2 . Define the input and output nodes of the circuit, and use the function generator in the instrument library to add an AC sinusoidal voltage at the input end ( Vi amplitude is 5V , frequency is 10KHz ). Select the AC frequency analysis item in the analysis menu to analyze the frequency selection network and obtain the amplitude-frequency response and phase-frequency response curves. In the oscillation circuit, diodes D1 and D2 form an amplitude stabilization link. By adjusting R4 , the effect of amplitude condition changes on oscillation can be observed. By controlling the on and off of switch S1 (or turning the power on and off), the oscillator start-up and amplitude stabilization process can be observed by an oscilloscope. 1.8.2 Field Effect Transistor Amplitude Stabilized RC Bridge Oscillator Figure 7.8-5 is an RC bridge oscillator using field effect tube amplitude stabilization . In this circuit, Q1 , R3 , and R6 form the amplitude stabilization link. C3 , R5 , R7 , R4 , and D1 form the output voltage negative half-wave rectification filter circuit, providing an adjustable DC negative bias voltage for the N- channel junction field effect tube Q1 to adjust the channel resistance of the field effect tube. When the circuit is connected for simulation experiments, you can first adjust R5 to make the gate bias of Q1 zero (gate grounded), and then adjust R6 to make the circuit oscillate (the output voltage waveform is severely distorted at this time). At this time, adjust R5 to increase the negative gate bias value of Q1 , and the output voltage waveform distortion will be significantly improved until you are satisfied. The circuit's oscillation and amplitude stabilization process is described as follows: When the circuit oscillates, the output voltage is zero, the diode D1 is cut off, the gate bias voltage of Q1 is zero, the channel resistance is small, and the amplifier voltage gain is large. Because the circuit meets the oscillation conditions, the output voltage waveform amplitude will increase sharply from zero. As the output voltage amplitude increases, the diode D1 is turned on, and the negative gate voltage of Q1 increases with the increase in the output voltage amplitude. Affected by the increasing negative gate voltage, the channel resistance of Q1 is also increasing. At the same time, affected by the increase in the channel resistance of Q1 , the amplifier's voltage gain is also decreasing. If the parameters of R6 and R5 are adjusted appropriately, before the output voltage peak produces nonlinear distortion, the circuit's loop gain: AF decreases from greater than 1 to equal to 1. At this time, the output voltage is stable, and the oscillation and amplitude stabilization process of the entire oscillation circuit is completed. 1.8.3 RC Phase-Shifted Oscillator RC phase-shifted sinusoidal oscillation is shown in Figure 7.8-6 . This circuit is composed of an inverting amplifier and a three-section RC phase-shifting network. Because no amplitude stabilization measures are taken, there is obvious nonlinear distortion at the top of the output waveform. To meet the oscillation phase condition, the RC phase-shifting network is required to complete a 180- degree phase shift. Because the limit of a single RC phase-shifting network is 90 degrees. Therefore, a three-section (or more than three-section) RC phase-shifting network must be used to achieve a 180- degree phase shift. 1.8.4 RC Double- T Feedback Oscillator Figure 7.8-7 is an RC double- T feedback oscillator, in which C1 , C2 , C3 , R3 , R4 , and R5 form a double- T negative feedback network (to complete the frequency selection function). The two voltage regulator tubes Dz1 and Dz2 in the circuit have the function of amplitude stabilization to improve the output waveform. We first analyze the frequency selection characteristics of the double- T negative feedback network and reconstruct the double- T network circuit in the EWB main interface as shown in Figure 7.8-8 . Define the input and output nodes of the circuit, and use the function generator in the instrument library to add an AC sinusoidal voltage to the input terminal ( VI amplitude is 5V , frequency is 10KHZ ). Use node 8 as the output terminal. Select the AC frequency analysis item in the analysis menu to analyze the double- T network and obtain the amplitude-frequency response and phase-frequency response curves. 1.9 LC Sine Wave Oscillator LC oscillators are mainly used to generate high-frequency sinusoidal signals. The frequency selection network of the oscillator is composed of inductors and capacitors, which can generally be divided into transformer feedback type and three-point type. 1.9.1 Frequency-selective characteristics of LC parallel resonant circuit The LC parallel resonant circuit determines the oscillation frequency of the LC oscillator. The following AC frequency analysis is used to illustrate the frequency selection characteristics of the LC parallel resonant circuit. In the EWB main interface, build an LC parallel resonance test circuit as shown in Figure 7.9-1 . Select a sinusoidal AC voltage source as the excitation signal in the signal source library, select the AC frequency analysis item in the analysis menu, and set the start and end frequencies of the scan to 200HZ and 1GHZ respectively in the AC frequency analysis parameter setting dialog box . The scan format is decimal, the display points are the default settings, the vertical scale is linear, and the analysis output is node 1. Click the simulation button to get the AC frequency simulation results. 1.9.2 Transformer Feedback LC Oscillator The transformer feedback oscillation circuit is constructed in the EWB main interface as shown in Figure 7.9-3 . Transformer T1 is used as a feedback element, and its secondary winding and capacitor C1 form a parallel resonant frequency selection network. The inductance of the transformer is set to 0.001H , and the capacity of capacitor C1 is set to 0.001μF . The feedback amount is introduced into the input end of the common base amplifier from the tap of the secondary winding, which can reduce the influence of the amplifier input impedance on the quality factor ( Q ) value of the LC parallel resonant circuit . Select the DC operating point analysis item in the analysis menu to analyze the static situation of the oscillation circuit. The analysis results show that the amplifier is working normally. 1.9.3 Three-point LC oscillator Figure 7.9-5 is a three-point LC sine wave oscillator, the analysis is as follows: (1) Determine what type of three-point circuit the circuit is (2) Use the DC operating point analysis item in the analysis menu to analyze the static operating point of the circuit. (3) Use the oscilloscope in the instrument library to measure the oscillation frequency of the circuit. (4) Obtain the circuit’s oscillation frequency through theoretical analysis and compare it with the measured value. 1.10 Signal operation circuit composed of operational amplifier 1.10.1 Inverting Proportional Operation Circuit In the EWB main interface, build an inverting proportional operation circuit as shown in Figure 7.10-1 , set the input DC voltage source to 1V , select a voltmeter in the display device library and connect it to the output terminal (contact 2 ). After the circuit is connected, close the power switch, and the circuit operation result will be displayed in the voltmeter (the output voltage in this example is 10V ). Calculation relationship: VO= (— R1/R2 ) *V1= — 10V1= — 10V , the inverting proportional coefficient is — 10 . Select the Transfer Function Analysis option in the Analysis menu, set the input source to V1 and the output terminal to Node 2 in the Transfer Function Analysis Parameter Setting dialog box , and click the Simulate button to get the transfer function analysis results. 1.10.2 In-phase proportional operational circuit The in-phase proportional operational circuit is shown in Figure 7.10-3 . Calculation relationship: VO= ( 1+R3/R2 ) *V1=11V1=11V , the in-phase proportional coefficient is 11 . Select the Transfer Function Analysis option in the Analysis menu, set the input source to V1 and the output terminal to node 4 in the Transfer Function Analysis Parameter Setting dialog box , and click the Simulate button to get the transfer function analysis results. 1.10.3 Addition circuit The addition operation circuit is shown in Figure 7.10-5 . Calculation relationship: VO= (— R3/R1 ) *V1+ (— R3/R2 ) *V2= (— 5 ) V1+ (— 4 ) V2= — 7V . Select the Transfer Function Analysis option in the Analysis menu. In the Transfer Function Analysis Parameter Setting dialog box, set the input sources to V1 and V2 respectively , and the output end to Node 1. After clicking the Simulate button twice, the transfer function analysis results are obtained. 1.10.4 Subtraction Circuit The circuit is shown in Figure 7.10-7 . Calculation relationship: VO=[ ( R1+R4/R1 ) * ( R3/R2+R3 ) ] — ( R4/R1 ) *V1=5V2 — 5V1=5V . Select the Transfer Function Analysis option in the Analysis menu. In the Transfer Function Analysis Parameter Setting dialog box, set the input sources to V1 and V2 respectively , and the output end to Node 1. After clicking the Simulate button twice, the transfer function analysis results are obtained. 1.10.5 积分运算电路 电路如图7.10-9所示。 敲击B键，拨动开关S2，令积分电路输入端接—1V直流电压。敲击D键，通过开关S1的通，断，在示波器上观察积分过程。 积分关系式：VO=—V1/RC*t。 设置函数发生器输出（频率5HZ，占空比50%，幅度1V）连续方波电压，拨动开关S2，将方波输入积分器，由示波器同时观察积分器的输入（VA）和输出(VB)电压波形。由图可知，积分器可以将连续的方波信号电压转换为连续的三角波电压。 1.10.6 微分运算电路 微分电路如图7.10-12所示 。 将函数发生器设置为连续方波（频率5HZ，占空比50%，幅度1V）输出方式，将其连接到微分器的输入端。由示波器同时观察积分器的输入（VA）和输出(VB)电压波形。由图可知，积分器可以将连续的方波转换为正负相间的连续尖脉冲。 1.10.7 仪用测量放大器 图7.10-14所示电路，是一个具有高输入阻抗，低输出阻抗的仪用测量放大器。 理论分析得： 基本运算关系式为：VO=（R4/R3）*（1+2R2/R1）*（V2—V1）=110（V2—V1） 放大倍数（传递函数）：AV=VO/（V2—V1）=110 选择分析菜单中的传递函数分析选项，在传递函数分析参数设置对话框中将输入源设置为V1，输出端设置为节点9，点仿真按钮后，得到传递函数分析结果。 1.11 模拟乘法器及其应用电路 1.11.1 模拟乘法器作乘法运算 在控制器件库中选择模拟乘法器，设置其参数 。图7.11-2为利用乘法器实现乘法运算电路，验证运算关系式：VO=K*V1*V2。 1.11.2 除法运算电路 利用模拟乘法器与运放构成的除法电路如图7.11-3所示。 验证运算关系式：VO=—V1/V2。 1.11.3 负电压平方根运算电路 利用模拟乘法器与运放构成的负电压平方根运算电路如图7.11-4所示。 验证运算关系式：VO=√-V1。 1.11.4 正电压平方根运算电路 利用模拟乘法器与运放构成的正电压平方根运算电路如图7.11-5所示， 试验证关系式：VO=√V1。 1.11.5 立方根运算电路 利用模拟乘法器与运放构成的立方根运算电路如图7.11-6所示。 验证基本关系式：VO= 1.12 有源滤波电路 滤波器是一种能使有用频率信号通过而同时抑制（大为衰减）无用频率信号的电子装置。工程上常用它来作信号处理，数据传送和抑制干扰等。利用运算放大器与无源器件R，L，C构成有源滤波器，由于运算放大器具有高增益，高输入阻抗和低输出阻抗等特点，使有源滤波器具有一定的电压放大和输出缓冲作用。 利用EWB分析菜单中的交流频率分析项，可以方便地求得滤波器的频率响应曲线，根据频率响应曲线，调整和确定滤波电路的元件参数，很容易获得所需的滤波特性，省去了非常烦琐的人工计算，充分体现了计算机仿真技术的优越性。以下仅就低通，高通，带通和带阻四种典型滤波电路加以讨论。 1.12.1 一阶有源低通滤波器 图7.12-1为 一一阶有源低通滤波电路。 电路的截止频率：Fn=1/2∏RC=15.92kHz 选择分析菜单中的交流频率分析项，在交流频率分析参数设置对话框中将扫描起始与终止频率设置为1Hz和1MHz，扫描形式为十进制，纵向尺度为线性，输出端为节点3。点仿真按钮后，得一阶有源低通滤波电路的幅频响应和相频响应曲线。 By changing the parameter values of R and C in Figure 7.12-1, different cutoff frequencies can be obtained. 1.12.2 Second-Order Active Low-Pass Filter The first-order filter circuit is simple, but when the input signal frequency is higher than the cut-off frequency, the amplitude-frequency response attenuation rate is low. Therefore, second-order filtering is introduced. Figure 7.12-3 shows a first- and second-order active low-pass filter. Cut-off frequency of the circuit: Fn=709Hz Select the AC frequency analysis item in the analysis menu, set the start and end frequencies of the scan to 1Hz and 1MHz in the AC frequency analysis parameter setting dialog box , the scan format to decimal, the vertical scale to linear, and the output end to node 3. After clicking the simulation button, the amplitude-frequency response and phase-frequency response curves of the second-order active low-pass filter circuit are obtained. When the input signal voltage is higher than the cut-off frequency, the amplitude-frequency response decrease rate of the second-order filter is significantly higher than that of the first-order filter (the decrease rate increases from 20dB/decade to 40dB/decade). 1.12.3 First-Order Active High-Pass Filter After swapping the positions of the components R and C in the low-pass filter, the circuit is converted into a high-pass filter. Figure 7.12-5 is a first-order high-pass filter. Cut-off frequency: Fn=1/2∏RC=9.95kHz Select the AC frequency analysis item in the analysis menu, set the start and end frequencies of the scan to 1Hz and 1MHz in the AC frequency analysis parameter setting dialog box , the scan format to decimal, the vertical scale to linear, and the output end to node 3. After clicking the simulation button, the amplitude-frequency response and phase-frequency response curves of the first-order active high-pass filter circuit are obtained. Similarly, to increase the rising rate of the amplitude-frequency characteristic near the cut-off frequency point, the first-order filter can be changed to a second-order filter. 1.12.4 Second-Order Active High-Pass Filter The first and second order active high pass filter circuits are shown in Figure 7.12-7. Cut-off frequency: Fn=1/2∏RC=3.52kHz Select the AC frequency analysis item in the analysis menu, set the start and end frequencies of the scan to 1Hz and 1MHz in the AC frequency analysis parameter setting dialog box , the scan format to decimal, the vertical scale to linear, and the output end to node 3. After clicking the simulation button, the amplitude-frequency response and phase-frequency response curves of the second-order active high-pass filter circuit are obtained. 1.12.5 Narrow Bandpass Filters Figure 7.12-9 shows the principle circuit of a narrowband bandpass filter. Readers can analyze the filter-related parameters such as the center frequency by themselves. 1.12.6 Bandstop Filter Figure 7.12-11 shows the principle circuit of the band-stop filter. Readers can analyze the filter-related parameters such as the center frequency by themselves. 1.13 DC regulated power supply 1.13.1 Bridge rectifier capacitor filter circuit Figure 7.13-1 is a single-phase bridge rectifier capacitor filter circuit. Disconnect the filter capacitor C1, close the circuit simulation switch, and use an oscilloscope to observe the waveform across the resistor RL. At this time, the voltage waveform across RL is a full-wave rectified waveform. Connect the filter capacitor C1 and use an oscilloscope to observe the voltage waveform across RL. Under the action of the filter capacitor, the output voltage is relatively smooth and the output DC average voltage is improved. 1.13.2 Series Feedback Regulated Power Supply Figure 7.13.4 is a series feedback regulated power supply. Build the circuit in the EWB main interface and do the following tests: ( 1) RW makes the output voltage VO = 10V, measures the potentials at the following points, and determines the working state of the transistor based on the potentials at each point. (2) Measure the output voltage adjustment range. Change RW, measure the maximum and minimum values of VO, and compare them with the theoretical calculated values. (3) Measure the output resistance of the voltage-regulated power supply. Disconnect the load resistor RL and adjust RW to make the output voltage VO = 12V. Connect the load RL = 20 ohms, measure the output voltage VO, and calculate the output resistance according to the formula r0 = △V0/△I0. (4) Measure the voltage regulation coefficient of the voltage regulator. Connect the load resistor RL to make the output voltage VO = 12V. Adjust the input AC voltage V1 = 18 × (1 + 10﹪) V and measure the output voltage value of the voltage regulator. Calculate the voltage regulation coefficient by the following formula. Sr = (△V0/V0) / (△V1/V1). Change the resistor R3 to 5.1K and re-measure the output resistance and voltage regulation coefficient of the voltage regulator. Compare with the above measurement results to understand the role of R3. (5) Output short-circuit protection test. Short-circuit the output end of the voltage regulator, measure the potential of the following points in the circuit, and determine the working status of the tube to understand the working principle of the short-circuit protection circuit composed of transistor Q1.
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