Comparison between field effect tube and BJT amplifier circuit

Performance comparison between field effect tube amplifier circuit and BJT amplifier circuit

The common source circuit, common drain circuit, and common gate circuit of the field effect tube amplifier circuit correspond to the common emitter circuit, common collector circuit, and common base circuit of the triode amplifier circuit respectively.

Both the common source circuit and the common emitter circuit have voltage amplification effect, that is, , and the output voltage is opposite to the input voltage. Therefore, these two amplifier circuits can be collectively referred to as inverting voltage amplifiers, as shown in the schematic diagram shown in Figure 1(a).

Neither the common drain circuit nor the common collector circuit has a voltage amplification effect, that is . Under certain conditions, it can be considered that, that is , and the output voltage is in phase with the input voltage. Therefore, these two amplifier circuits can be called voltage followers, which are represented by the schematic diagram shown in Figure 1(b).

The output current of the common-gate circuit and the common-base circuit are nearly equal to the input current ( ). For this reason, they can be called current followers, represented by the schematic diagram shown in Figure 1(c). Moreover, since the input current of these two amplifier circuits is relatively large, their input resistance is relatively small.

The most prominent advantage of the field effect tube amplifier circuit is that the input resistance of the common source, common drain and common gate circuit is higher than the input resistance of the corresponding common emitter, common collector and common base circuit. In addition, the field effect tube has the advantages of low noise, good temperature stability, strong radiation resistance, etc. that are superior to triodes, and it is easy to integrate.

It must be pointed out that since the low-frequency transconductance of the field effect tube is generally small, the amplification ability of the field effect tube is worse than that of the triode. For example, the voltage gain of the common source circuit is often smaller than the voltage gain of the common emitter circuit. In addition, since the equivalent capacitance Cgs between the gate and source of the MOS tube is only a few picofarads to tens of picofarads, and the gate-source resistance rgs is very large, if there is an induced charge, it is not easy to release, thus forming a high voltage, so that the insulation layer between the gate and source is broken down, causing permanent damage to the tube. Pay attention to protection when using it.

In practical applications, the above-mentioned various configurations of circuits can be appropriately combined according to specific requirements to form a high-performance amplifier circuit.

Summary of this chapter
Due to the differences in structure and working principle, field effect tubes have some characteristics different from triodes, as shown in the following table. Combining the two, taking advantage of their strengths and making up for their weaknesses, can improve and enhance certain performance indicators of the amplifier circuit.

• According to the different structures, field effect transistors are divided into two types: junction type and insulated gate type. MOS transistors belong to the insulated gate type. Each type has two channels, N channel and P channel. The main difference between the two is the polarity of the voltage and the direction of the current. MOS transistors are divided into two types: enhancement type and depletion type.

• The key to correctly understanding the working principle of field effect tubes is to master the different effects of voltage vGS and vDS on the conductive channel and current iD, and to master the differences and conditions of the two states of pre-pinch-off and pinch-off. The transfer characteristic curve and the output characteristic curve describe the relationship between vGS, vDS and iD. Similar to triodes, field effect tubes have three working areas: cut-off area (i.e. pinch-off area), constant current area (i.e. amplification area) and variable resistance area. In the constant current area, iD can be regarded as a current source controlled by voltage vGS. gm, VP (or VT), IDSS, IDM, PDM, V(BR)DS and inter-electrode capacitance are the main parameters of field effect tubes.

• In the field effect tube amplifier circuit, the DC bias circuit often uses a self-bias circuit (only suitable for depletion-type field effect tubes) and a voltage divider self-bias circuit. In order to ensure that the field effect tube works in the amplification area, the polarity of the voltage vGS and vDS and the size of vDS should be as shown in the following table.

• The common source and common drain amplifier circuits of field effect tubes correspond to the common emitter and common collector amplifier circuits of triodes, but have higher input resistance, lower noise coefficient and smaller voltage amplification factor than triode amplifier circuits.