Triode Single Stage Amplifier

There are three different configurations of bipolar transistors. Correspondingly, there are three types of triode amplifiers, namely common emitter amplifier, common collector amplifier and common base amplifier, as shown in Figure 1.

Figure 1a is a common emitter amplifier. The signal ui to be amplified is input from the base of the transistor, and the amplified signal uo is output from the collector. The base and the emitter form an input circuit, and the collector and the emitter form an output circuit. It can be seen that the emitter is the common end of the input and output circuits, so it is called a common emitter amplifier, or common emitter amplifier for short. The working characteristics of this circuit are that it can amplify both the voltage and current of the signal, and the output signal is inversely proportional to the input signal; the input resistance and the output resistance are moderate. Generally, it is several thousand ohms, and the voltage amplification factor is generally tens or hundreds of times. It can be used to amplify voltage signals and is often used as an intermediate stage of a multi-stage amplifier.

Figure 1b is called a common collector amplifier, ui is input from the base. uo is output from the emitter, and the collector is the common end of the input and output loops, so it is called a common collector amplifier, abbreviated as a common collector amplifier, also known as an emitter follower. The working characteristics of this circuit are that it can amplify the current of the signal but cannot amplify the voltage of the signal, the voltage amplification factor is about 1, and the output signal is in phase with the input signal; the input resistance is relatively large, generally tens of kilo-ohms, and the output resistance is very small, generally tens of ohms. It is often used as the input stage (strong ability to obtain signals from the signal source), output stage (strong load capacity) and buffer stage (to achieve impedance conversion) of a multi-stage amplifier.

Figure 1c is a common base amplifier. ui is input from the emitter, uo is output from the collector, and the base is the common end of the input and output loops, so it is called a common base circuit, or a common base amplifier for short. The working characteristics of this structure circuit are that it can amplify the voltage of the signal but not the current of the signal, and the output signal is in phase with the input signal. The input resistance is very small, generally tens to tens of ohms, and the output resistance is moderate, generally thousands of ohms. It is often used in high-frequency signal voltage amplifier circuits and oscillators.