transistor low frequency amplifier

Transistor low-frequency amplifiers are mainly used to amplify low-frequency small signal voltages, with frequencies ranging from tens of Hz to about one hundred kilohertz. 1. Transistor bias circuit In order for the amplifier to obtain linear amplification, the transistor must not only have a suitable static The working point must be stable. Since the influence of temperature on the tube parameters β, Icbo, and Ube is ultimately reflected in the change of Ic, in order to eliminate this influence, we stabilize the static operating point through the negative feedback of the DC or voltage of the transistor bias. It is common. The two bias circuits and working point stabilization principles of the transistor amplifier are as follows: Table 1. Transistor amplifier bias circuit circuit working point stabilization principle calculation formula Current negative electrical feedback Assume the temperature T↑, the DC negative feedback process results in such a way that Ic remains unchanged U=( 1/3-1/5)Ec Re=(1/3-1/5)Ec/Ic Rb=Rb1//Rb2 ===(2-5)Re Ub=Rb1Ec/(Rb1+Rb2) Voltage negative feedback Assuming the temperature T, the result of the DC negative feedback process keeps Ic unchanged Rb=β(Ec-Ube)/Ic-βRc Ic=Ec/(Rc+Rb/β) According to experience, Rb/Rc=(2-10 ) 2. Three circuit forms of amplifiers The amplifier is a three-terminal circuit, one of which must be the common "ground" terminal of the input and output. If this common "ground" terminal is connected to the emitter, it is called a common emitter. The circuit connected to the collector is called a common collector circuit, and the one connected to the base is called a common base circuit. These three types have different performances. See the table below for the three circuit forms and their performance comparison: circuit voltage amplification times current Amplification factor input resistance output resistance common emitter circuit 10-100 large 10-1000 large 100Ω-50KΩ medium 10KΩ-500KΩ CCP set circuit 0.9-0.999 small 10-1000 large due to different loads, up to about 50MΩ large 1-100Ω small common base Circuit 100-10000 (Practical) Large 0.9-0.999 Small 10-500Ω or so Small 500KΩ-5MΩ Junior 3. Graphical method The so-called graphical method is a method of analyzing amplifier performance by drawing using the characteristic curves of transistor input and output, graphical illustration The method can intuitively and comprehensively demonstrate the working process of triode amplification, and can calculate certain performance indicators of the amplifier. Here is an example to illustrate the graphical process of the graphical method. For example: It is known that the parameters and input voltage Ui=15sinωt in the circuit below are known (millivolt) requires the graphic method to determine the static operating point parameters Ibq, Icq, and Iceq of the circuit, and calculate the amplification factors Ku and Kio of the voltage and current. Step 1 of the graphical method: Determine the static working point of the base loop, and select the midpoint Q of the straight line segment from the input characteristic curve (Ubeq=0.7 volts, Ibq=40 microamps at this point) as the static working point of the base loop. By selecting the appropriate Eb or Rb (usually by adjusting Rb) to meet the requirements of the operating point, 2. Draw a DC load line. From the above figure, we can get the load line equation as Uce=Ec-IcRc, and its trajectory is a straight line. If Let Ic=0, we get Uce=Ec=20 volts, mark N point on the horizontal axis; let Uce=0, we get Ic=Ec/Rc=20 volts/6 kΩ=3.3 mA, on the vertical axis Mark point M, connecting M and N is the DC load line. It intersects the output characteristic curve of Ib=40 microamps at Q. Find Icq=1.8 milliamps and Uceq=9 volts from the Q point. The Q point is the static operating point of the collector circuit. For the sake of simplicity, the static current , the voltage is no longer represented by the subscript Q, Ic, Ie are Icq, Ieqo 3. Make the waveform, make the waveform Ut=15sinωt (millivolt) based on the input characteristics, and make the waveforms of ib, ic and Uce based on the waveform of Ut The following points can be obtained from the graphical method: (1) Whether the selection of the static operating point Q is appropriate can be judged from the sinusoidal nature of the waveform. (2) From the diagram, we know that the input voltage Ui and the collector output voltage Uo are in opposite phases, and the base current ib, collector current Ic and the input voltage Ui are in phase. (3) The above graphical method is carried out under no-load condition. If the effect of load resistor RL is taken into account, the AC load should be RL=RC//RL. Since the AC load line and the DC load line both intersect at Q, a straight line M'N' with an inclination angle a"=(arctg)1/RL is drawn through the Q point, which is called the AC load line. 4. Equivalent circuit method and h Parameter 1. Simplified h-parameter equivalent circuit "slight change" refers to the transistor's