What is a common collector, common base, common emitter amplifier?

We usually use a lot of triode amplifier circuits in our studies and life, but there are also many people who can't identify amplifier type.

Application status:

Among three types of amplifiers, most widely used is common-emitter amplifier, which is used in various frequency amplification systems and is preferred circuit for signal amplification.

Because of large input impedance and small output impedance, common-collector amplifier is mainly used for isolation in amplification system, such as input stage, output stage, and buffer in stage of a multi-stage amplification system, which minimizes mutual influence between front-stage circuit and amplifier's post-stage circuit.

Common-base amplifiers have excellent high-frequency characteristics, so they are mainly used in circuits with relatively high operating frequencies.

Amplifier type estimation method:

By method of connecting triode, you can find out its type of amplification and characteristics.

Principle: An amplifier has an input circuit and an output circuit, and each circuit must have two pins, while a triode has only three pins, and one pin must be common for input and output. , for example Common emitter is a common emitter amplifier.

Method. The amplifier ground wire is common reference point in circuit. Therefore, this triode pin must be AC ​​ground. As long as you can see which triode pin is AC ground, you can know what type of amplifier it is.

1

Common Emitter Amplifier

(Common Emitter Amplifier)

As shown in figure above, emitter of Q1 in circuit is directly grounded, while emitter of Q2 is grounded through capacitor C3. Since C3 has a large capacitance, capacitive reactance on AC signal is small, and this is path, so for AC signal That is, emitter is equivalent to ground, so this is a common-emitter amplifier.

It can be seen from diagram that base and collector of triode are not grounded, input signal enters triode between base and ground, and input signal is output between collector of triode. triode and ground to output from collector.

2

Common Collector Amplifier

(Common collector amplifier)

The collector is connected to a +V DC power supply, and +V terminal is equivalently grounded for AC (C1 connects +V terminal to AC ground), so collector of triode is grounded, which is a common collector amplifier. The AC signal comes from base (input between base and ground) and signal is output from emitter (output between emitter and ground). A shunt capacitor must not be connected between emitter and ground, otherwise amplifier will be shorted out and there will be no output signal.

3

Common Core Amplifier

The base is AC grounded through decoupling capacitor C2, so base is common, so this is a common base amplifier. The AC signal comes from emitter (input between emitter and ground) and output from collector (output between collector and ground).

(basic booster)