Field Effect Transistor Amplifier

Field effect transistor amplifiers are voltage-controlled devices with the advantages of high input impedance and low noise. They are widely used in electronic circuits, especially in preamplifiers with the above requirements. According to the two major types of field effect transistors - junction field effect transistors and insulated gate field effect transistors, corresponding field effect transistor amplifiers can be constructed. The following takes the junction transistor as an example to give the equivalent circuits and performance index calculation expressions of three basic configuration amplifiers, see Table 5.2-7. The figure shows that the field effect transistor has a forward controlled effect similar to that of a transistor, and it can also form three basic amplifiers: common source, common drain, and common gate.

1. Bias circuit

Because different types of field effect transistors work in the amplification region, they require different gate voltage polarities. For example, junction field effect transistors require the gate source and drain source voltage polarities to be opposite, while addition MOS transistors require the gate source and drain source voltage polarities to be the same. As for the gate bias polarity of depletion MOS transistors, it can be positive bias, zero bias or negative bias. Based on these characteristics, there are mainly two types of bias circuits using a single power supply:

(1) Self-bias circuit Self-bias is used in junction and depletion-type MOS tube amplifier circuits. Figure 5.2-6 shows the self-bias circuit of an N-channel junction field effect transistor.

Figure 5.2-6 Self-bias circuit

The gate-source voltage is UGS=RID

(2) Hybrid bias circuit Hybrid bias circuit is used in various field effect tube amplifiers. The hybrid bias circuit of N-channel enhancement MOS tube amplifier circuit is shown in Figure 5.2-7.

Figure 5.2-7 Hybrid bias circuit

The gate-source voltage is

2. Equivalent circuits and performance index calculation formulas of three basic configuration amplifiers of field effect transistors (see Table 5.2-7)