The working stability of the amplifier circuit-EEWORLD

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The working stability of the amplifier circuit

3.5.1 Effect of temperature on operating point

Problem: In a fixed bias current circuit, when the tube is replaced, especially when the ambient temperature changes, the tube parameters will change, causing the circuit's operating point to move up or down. When it moves to an inappropriate position, the amplifier circuit will not work properly.

Figure 1 Fixed bias current common emitter circuit

3. Effect of temperature change on b

1. To address the influence of ICBO, we can try to make the base current IB automatically decrease as the temperature rises.

2. To address the influence of VBE, we can try to make the applied voltage of the emitter junction automatically decrease as the temperature increases.

2. The principle of emitter bias circuit stabilizing the operating point:

The emitter bias circuit is shown in Figure 1, and its DC path is shown in Figure 2.

The circuit parameters are selected so that the circuit meets the condition I1>>IB.

It can be obtained that VB » Rb2VCC/(Rb1+ Rb2).

When the temperature rises, IC (IE) increases, IERe increases, VBE decreases (because VBE=VB–IERe, and VB is constant), IB automatically decreases, and the increase of IC is restrained, and IC is basically constant. When designing this kind of circuit, you can generally choose:

I1=（5~10）IB (silicon tube)

I1=（10~20）IB (germanium tube)

VB=（3~5）V (silicon tube)

VB=（1~3）V (germanium tube)

1. Static analysis of emitter bias circuit

The static operating point of the circuit should be analyzed based on the DC path. Open the coupling capacitors Cb1 and Cb2 in the emitter bias circuit of Figure 1 and draw its DC path as shown in Figure 2.

Under the conditions of I1>> IB and VB>> VBE, we can consider I1»I2, VB»VE, so we have

In summary, the process of finding the static point of the emitter bias circuit is:

2. Dynamic Analysis of Emitter Bias Circuit

Based on the above static analysis, dynamic analysis is performed

1) Draw the small signal model equivalent circuit of the circuit

）

3) Find the input resistance and output resistance

Input resistance

As shown in Figure 2, when the circuit input is subjected to an external test voltage, the corresponding test current is

It can be seen that after adding Re, the input resistance increases. This is because the current flowing through Re is (1+b) times the base current. After Re is converted to the base circuit, it is equivalent to a resistance of (1+b)Re.