Due to the high input impedance of the common-emitter amplifier circuit, it can be directly connected to the signal source to be amplified, and then use the ‘Wolmanisation’ of the common-base amplifier circuit to eliminate the ‘Miller effect’ and improve the bandwidth, and then connect to the load through the common-collector amplifier circuit because the common-collector amplifier circuit has low output impedance and acts as a buffer to drive the heavy load. Because of the low output impedance of the common-collector amplifier circuit, it acts as a buffer and can drive heavy loads.

1. Common Emission Amplifier Circuit

The schematic diagram of the circuit is as follows:

①, the amplification is: A = -Rc/Re. Design the value of Rc and Re according to the demand. 

②, the input impedance: Zin = beta * Re. (R1 and R2 for the triode to provide bias voltage, here first ignored, of course, the actual should be considered). Due to the circuit amplification characteristics of the three-stage tube, Re converted to the input needs to be amplified beta times, so the input impedance is high. 

③, the output impedance: Zout = Rc. In order to reduce the current of the three-stage tube, reduce power consumption, so Rc generally take a large value. 

④, frequency characteristics: due to the existence of the Miller effect, the parasitic capacitance between the base and collector of the triode in the amplification area will be enlarged A times the response to the input, so the frequency characteristics are poor, and can not amplify high-frequency signals.

2、Common-collector amplifier circuit

Circuit schematic diagram is as follows:

①, the amplification factor is: A=1, the output voltage is the input voltage minus the tube voltage drop of the triode 0.7V.

②, the input impedance: Zin = beta * Re. Due to the circuit amplification characteristic of the triode, Re needs to be amplified by beta times when converted to the input, so the input impedance is high. 

③, Output impedance: Zout = Re / beta, also due to the amplification characteristics of the triode circuit, Re needs to be converted to the output to reduce beta times, so the output impedance is low. (Although strange, but the derivation is true, which is also an important advantage of the common set of amplifier circuits) 

④, frequency characteristics: common set of amplifier circuits do not exist Miller effect, so the frequency characteristics are very good.

3、Common-base amplifier circuit

The schematic diagram of the circuit is as follows:

①, Amplification: A=Rc/Re. The amplification principle is the same as that of the common-emitting amplifier circuit. However, it is forward amplification, and the common-emitting amplifier circuit is reverse amplification. 

②, Input impedance: Zin = Re (also ignoring the base bias voltage), due to the lack of beta times the amplification of the ‘isolation’ effect, the input impedance is not large, which is a disadvantage. Output impedance: Zout = Rc. 

③, Output impedance is the same as that of the common-emitting amplifier circuit, which is also a disadvantage.

④, frequency characteristics: the base of the common base amplifier circuit has a large capacitance for the AC base, so there is no Miller effect, good frequency characteristics.

4, other amplifier circuits

Other amplifier circuits are in the first three amplifier circuits on the basis of the deformation, as long as the above three amplifier circuits to understand the deep, and then how to change the other can be quickly mastered. Differential amplifier circuit in order to reduce the impact of temperature drift of the common emitter amplifier circuit. Amplifier, audio in the rear amplifier circuit for the distortion of the common set of amplifier circuit, a good amplifier will be more complex than my introduction of class A, class B, class A and B amplifiers much more, but all changes are not the same.

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