A little learning method for analog electronic circuits

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A little learning method for analog electronic circuits [Copy link]

A little learning method for analog electronic circuits

1. Summary 1. Identification of feedback circuit (1) If there is a reverse transmission path for the signal in addition to the amplification path in the circuit, If the circuit is positive, it is a feedback amplifier circuit (closed loop); otherwise, it is an open loop. (2) Through the instantaneous polarity method, if it is determined that the change trend of the feedback signal is to weaken the change trend of the input signal, It is negative feedback; otherwise it is positive feedback. (3) If the feedback contains an AC component, it is AC feedback; if it contains a DC component, it is DC feedback. (4) Short-circuit the output AC to ground. If the feedback signal still exists, it is current feedback. On the contrary, if the feedback signal disappears, it is voltage feedback. (5) Observe the connection between the feedback signal, input signal and net input signal at the input end. If they are connected in series, That is series feedback; on the contrary, if the feedback signal is connected in parallel with the input loop, the feedback signal, The input signal and the net input signal are connected in parallel, which is parallel feedback; 2. Ideal model of single-loop feedback amplifier: 　 Block Diagram of a Feedback Amplifier The basic equation for feedback: Based on the above block diagram, the basic equation of the feedback amplifier circuit can be derived. The open-loop gain of the amplifier circuit is: Feedback coefficient of the feedback network: The closed-loop gain of the amplifier circuit is: The above quantities are all expressed in complex numbers because the phase shift of the actual circuit must be considered. Where is called the loop gain. 2. Four common connection methods for AC negative feedback (1). Voltage series negative feedback: The feedback quantity is proportional to the output voltage , and the connection with the input quantity adopts the voltage summing method. (a) Discrete component amplifier circuit (b) Integrated operational amplifier amplifier circuit Voltage series negative feedback (2). Voltage parallel negative feedback: The feedback quantity is proportional to the output voltage , and the connection with the input quantity adopts the current summing method. 　 　 　 　 Voltage shunt negative feedback (3). Current series negative feedback: The feedback quantity is proportional to the output current , and the connection with the input quantity adopts the voltage summation method. Current series negative feedback (4). Current parallel negative feedback: The feedback quantity is proportional to the output current , and the connection with the input quantity adopts the current summing method. 　 　 　 　 　 Parallel current negative feedback 3. The impact of negative feedback on the main performance of the amplifier: (1) Improve the stability of gain: voltage feedback can stabilize the voltage gain, and current feedback can stabilize the current. Gain, if the load is a constant resistance, can stabilize both the output voltage and the output current. (2). Improve nonlinear distortion and reduce internal noise interference. (3) Change the input and output impedance of the amplifier: Series resistance increases input impedance; parallel feedback reduces input impedance; Voltage feedback reduces the input impedance; current feedback increases the output impedance. (4) Broaden the bandwidth of the amplifier 4. To qualitatively and quantitatively analyze the negative feedback amplifier circuit, the following steps should be taken: (1) Determine the type of feedback amplifier. The type of multi-stage amplifier is determined by the main feedback loop. (2) Separate the feedback network B from the basic amplifier A and draw a new basic amplifier circuit. In order to separate the feedback network B from the basic amplifier A, the load effect of network B on network A must be considered. You can follow the four principles of dismantling the net. (3) Calculate the open-loop gains A U , A I and the input and output impedances r I , r o from the new basic amplifier after the network is removed . (4). Find the feedback coefficient B, and calculate parameters such as A Ui , A If , r If and r of from the feedback equation . 　 Back to Top Require 1. Be proficient in the types and polarity judgment of feedback amplifier circuits composed of integrated operational amplifiers. 2. Be proficient in the impact of negative feedback on the performance of amplifier circuits and the calculation of closed-loop gain under deep negative feedback. 3. Correctly understand virtual short and virtual disconnection; correctly understand the meaning of the closed-loop gain formula; Feedback will be introduced upon request. 　 Back to Top example Example 1: Try to determine the feedback configuration of the circuit shown in the figure. Solution : According to the instantaneous polarity method, see the red "+" and "-" signs in the figure, we can know that the resistor R1 is added to the base B1 is a DC parallel negative feedback. Since the feedback signal is proportional to the output current, it is a current feedback . Conclusion : It is DC current parallel negative feedback. Added to E1 via Rf is AC negative feedback . The feedback signal and input signal are added to the two input electrodes of the transistor, so it is series feedback. Conclusion: AC voltage is connected in series with positive feedback. Figure 1 Example 1 Figure Example 2: Try to determine the feedback configuration of the circuit shown in Figure 09.03. Solution : According to the instantaneous polarity method, seeing the red “+” and “-” signs in the figure, we can know that it is negative feedback. Because the feedback signal and input signal are added to the two input terminals of the op amp, it is series feedback. Since the feedback signal is proportional to the output voltage, it is voltage feedback. Conclusion: AC and DC series voltage negative feedback. Figure 2 Example 2 　 Example 3: Find the voltage gain of the circuit in Figure 3. Solution : When calculating the voltage gain expression, A1 and A2 can be considered as an operational amplifier, as shown in the brown figure. Since both A1 and A2 have inverting inputs, the polarity between the input and output signals can be determined. This circuit is equivalent to an in-phase proportional circuit, so : Figure 3 Circuit diagram of Example 3 Example 4: Answer the following questions. ① Find the common-mode input voltage of the op amp in Figure 09.09 when it is static; ② If series voltage feedback is to be achieved, where should R f be connected? ③To realize series voltage negative feedback, how to determine the polarity of the op amp’s input? ④Calculate the closed-loop voltage gain after introducing voltage series negative feedback. Figure 4 Example 4 Answer: ① The common-mode input voltage of the op amp in static state, that is, the collector potential of T1 and T2 in static state . I c1 = I c2 = I c3 /2 ② The differential amplifier circuit can be regarded as the input stage of op amp A. The input signal is added to the base of T1 . To realize series feedback, the feedback signal must be added to B 2 . Therefore , to realize series voltage feedback , Rf should be connected to B2 . ③Since it is series feedback, the feedback and input signals are connected to the two input terminals of the differential amplifier. To achieve negative feedback, the signals must be of the same polarity. The instantaneous polarity of the differential amplifier input terminal is shown in red in the figure. According to the requirements of series feedback, the polarity of B2 can be determined , as shown in the green mark in the figure, from which the output of the op amp can be determined . Input polarity. ④ To calculate the closed-loop voltage gain after introducing voltage series negative feedback, the differential amplifier and operational amplifier can be considered as a whole. In order to ensure the polarity of the green label of the op amp, B1 is equivalent to the non-inverting input terminal and B2 is equivalent to the inverting input terminal. For this reason, the circuit is equivalent to a non-inverting input proportional operation circuit. So the voltage gain is : 　 Example 5: For the circuit shown in Figure 5, try to determine the feedback type and calculate its voltage gain. Answer: Since the feedback signal and the input signal are added at one point, it is parallel feedback. According to the instantaneous polarity method, it is negative feedback, and it is voltage negative feedback. Because parallel feedback uses current addition and subtraction at the input. 　I i = I f +I ' i A vi = V o / I' i has the dimension of resistance A vif = V o / I i has the dimension of resistance F iv = I f / V o has the dimension of conductivity Figure 5: Example 5 A vif is called the mutual resistance gain, F iv is called the mutual conductance feedback coefficient, and the product of A vif and F iv is dimensionless. For deep negative feedback, the transimpedance gain is : The voltage gain is : 　 Example 6: Figure 6 is a feedback amplifier. Try to determine the feedback type and calculate the voltage gain. 　 Figure 6 Example 6 Figure Solution: The feedback voltage is taken from Re . According to the instantaneous polarity and the feedback voltage access method, it can be judged as a series load. Feedback. Because the output voltage is short-circuited, the feedback voltage still exists, so it is a series current negative feedback. Its mutual conductance gain Therefore , the shunt effect of R f is neglected here . The voltage gain is: Example 7: Figure 7 shows a feedback amplifier. Try to determine the feedback type and calculate the voltage gain. Figure 7 Example 7 Figure Solution: For the circuit in Figure 7, it is current parallel negative feedback The current feedback coefficient is F ii = I f / I o , taking Figure 6 as an example Current Gain: Obviously, the current gain is basically only related to the parameters of the external circuit and has nothing to do with the internal parameters of the op amp. The voltage gain is:

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tony0727

Too bad the picture is no longer available