Field effect tube amplifier circuit

4.3.1 DC bias circuit and static analysis

1. DC bias circuit

When the amplifier circuit is composed of field effect tubes, it is also necessary to establish a suitable static operating point Q. In addition, field effect tubes are voltage-controlled devices, so a suitable gate-source bias voltage is required. There are two commonly used DC bias circuits, namely self-bias circuits and voltage-dividing self-bias circuits.

1. Self-bias circuit

(a)                                    (b)

Figure 1

The circuit shown in Figure 1(a) is a self-bias circuit, in which the gate of the field effect tube is grounded through a resistor Rg, and the source is grounded through a resistor R. This biasing method relies on the voltage generated by the drain current ID on the source resistor R to provide a bias voltage VGS between the gate and the source, so it is called a self-bias circuit. In static state, the source potential VS=IDR. Since the gate current is zero, there is no voltage drop on Rg, and the gate potential VG=0, so the gate-source bias voltage VGS= VG–VS= –IDR. Depletion-type MOS tubes can also use this form of bias circuit.

The circuit shown in Figure 1(b) is a special case of a self-bias circuit, where VGS = 0. Obviously, this bias circuit is only applicable to depletion-type MOS tubes, because within a certain range of gate-source voltage greater than zero, equal to zero, and less than zero, depletion-type MOS tubes can work normally.

The enhancement type MOS tube will only generate drain current ID when the gate-source voltage reaches its turn-on voltage VT, so this type of tube cannot be used in the self-bias circuit shown in Figure 1.

2. Voltage divider bias circuit

Figure 2

2. Static Analysis

Static analysis of field effect transistor amplifier circuits can also be done by graphical analysis or formula estimation. The steps of the graphical analysis are similar to those of the bipolar transistor amplifier circuit. Here we only discuss the use of the formula estimation method to find the static operating point.

When working in the saturation region, the drain current of the junction field effect transistor and the depletion mode MOS transistor , and the drain current of the enhancement mode MOS transistor .

When finding the static operating point, for the circuit shown in Figure 1(a), the equation group can be solved

Get ID and VGS.

Pipe pressure drop

For the circuit shown in Figure 2, the equation system can be solved

Get ID and VGS.

Tube voltage drop VDS = VDD – ID (Rd + R)

4.4.2 Small signal model analysis method for field effect transistors

1. Field Effect Transistor and Small Signal Model

The field effect transistor is also a nonlinear device. When the input signal voltage is very small and the field effect transistor works in the amplification area, it can be equivalent to a small signal model like the triode, as shown in the animation below.

When the field effect tube works under high frequency small signal conditions, the influence of its inter-electrode capacitance cannot be ignored. At this time, the field effect tube should be equivalent to the high frequency small signal model shown in the right figure.

2. Common Source Amplifier Circuit and Small Signal Model Analysis Method

Corresponding to the bipolar transistor amplifier circuit, the field effect tube amplifier circuit also has three basic configurations, namely the common source, common drain and common gate amplifier circuits. The steps of analyzing its amplifier circuit using the field effect tube small signal model are the same as those of the triode small signal model analysis method.

Common source Pole amplifier circuit As shown in Figure 1(a), the small-frequency signal equivalent circuit is obtained by continuously operating the first button in Figure 1.

1. Intermediate frequency voltage gain

The output resistance rd of the field effect tube is usually in the order of hundreds of kilo-ohms, which is much larger than the resistance Rd and RL. Therefore, rd can be treated as an open circuit.

The negative sign in the formula indicates that the output voltage of the common source amplifier circuit is opposite to the input voltage in phase, that is, the common source amplifier circuit belongs to the inverting voltage amplifier circuit.

2. Input resistance

Since the gate of the field effect tube hardly takes any signal current, the AC resistance between the gate and the source can be regarded as infinite. Therefore, the input resistance of the common source amplifier circuit shown in Figure 1 is

3. Output resistance

Applying the method of calculating the output resistance of the amplifier circuit introduced earlier, the output resistance of the circuit shown in Figure 1 can be obtained .

From the above analysis, we can see that, similar to the common emitter amplifier circuit, the common source amplifier circuit has a certain voltage amplification capability, and the output voltage is inverse to the input voltage, so it is called an inverting voltage amplifier. The input resistance of the common source amplifier circuit is very high, and the output resistance is mainly determined by the drain resistance Rd. It is suitable for the input stage or intermediate stage of a multi-stage amplifier circuit.

3. Common Drain Amplifier Circuit

The common drain amplifier circuit is shown in Figure 1(a), and the small-frequency signal equivalent circuit is shown in Figure 1(b). Since the output voltage is taken from the source, it is also called a source follower.

1. Intermediate frequency voltage gain

It can be seen from Figure 1(b) that

so

From this formula, we can know that the intermediate frequency voltage gain of the common drain amplifier circuit , the output voltage is in the same phase as the input voltage. When , , therefore, the common drain amplifier circuit is also called a source voltage follower.

2. Input resistance Ri

3. Output resistance Ro

By continuously operating the triangle button in Figure 1(b), we can obtain an equivalent circuit for calculating the output resistance of the common-drain amplifier circuit.

In this circuit, due to the gate current , there is no signal voltage on the resistors of the gate circuit, so ,

That is, the output resistance Ro of the common drain circuit is equal to the source resistance R and the inverse of the transconductance in parallel, so the output resistance Ro is small. However, because gm is generally small, the output resistance of the common drain circuit is higher than that of the common collector circuit.

From the above analysis, we can know that, similar to the triode common collector amplifier circuit, the field effect tube common drain amplifier circuit has no voltage amplification function, its voltage gain is less than 1, the output voltage is in the same phase as the input voltage, the input resistance is high, and the output resistance is low. It can be used for impedance transformation.