MOS tube (field effect transistor) amplifier circuit and principle

Crystal oscillator

MOSFET with original FET imported

Photocoupler

The field-effect transistor (FET) amplifier is a voltage-controlled device that boasts high input impedance and low noise, making it a popular choice for electronic circuits, especially in preamplifiers. Junction field-effect transistors (JFETs) and insulated-gate field-effect transistors (IGFETs/MOSFETs) can both form corresponding FET amplifiers. Below is an example of the equivalent circuits and performance index calculation expressions for the three basic configurations of amplifiers, as detailed in Table 5.2-7. The image demonstrates that FETs exhibit a positive control effect similar to transistors, enabling them to create three fundamental amplifier configurations: common source, common drain, and common gate.

1. Bias Circuit: Different types of FETs operate in the amplification region under varying conditions, requiring distinct polarities for the gate voltage. For instance, JFETs require the gate-source and drain-source voltages to have opposite polarities, whereas MOSFETs need these voltages to share the same polarity. Depletion-mode MOSFETs can be positively biased, zero-biased, or negatively biased depending on the application.

Based on these characteristics, there are two primary types of bias circuits for single-supply configurations: (1) Self-bias circuit, which is self-biasing for JFET and depletion-mode MOSFET amplifiers. Figure 5.2-6 illustrates an N-channel JFET self-biasing circuit. The gate-source voltage is UGS = RID. (2) Hybrid bias circuit, suitable for various FET amplifiers. Figure 5.2-7 shows the hybrid bias circuit for an N-channel enhancement-mode MOSFET amplifier.

Figure 5.2-7 Hybrid Bias Circuit

2. Equivalent Circuit and Performance Index Calculation Formula for Field-Effect Transistor Three Basic Configurations (as per Table 5.2-7)

Gate-source voltage: