Multistage amplifier circuit fault analysis

1. Fault Analysis of Resistor-Capacitor Coupled Multistage Amplifier Circuit

As shown in the figure, taking the resistance-capacitance coupling amplifier circuit as an example, the circuit fault analysis is performed.

The fault analysis of multi-stage amplifier circuits is basically the same as that of single-stage amplifier circuits. Here are some additional explanations.

1. When the DC circuit in the VT1 amplifier stage fails, the DC circuit operation of the VT2 amplifier stage will not be affected due to the DC blocking effect of C3. However, since the first-stage amplifier circuit can no longer work normally, it does not have normal signal amplification. In the second-stage amplifier circuit, although the second-stage amplifier circuit can work normally, it has no signal output.

2. When the DC circuit of the second-stage amplifier circuit fails, the existence of C3 will not affect the operation of the DC circuit of the first-stage amplifier circuit. The first-stage amplifier circuit can output a normal signal, but due to the failure of the second-stage amplifier circuit The circuit cannot work properly, so the second-stage amplifier circuit cannot output a normal signal.

Tip: In a multi-stage amplifier circuit, as long as there is a problem with one stage of the amplifier circuit, the entire multi-stage amplifier circuit will not be able to output normal signals.

3. When C2 is open circuit, it will have no effect on the second-stage amplifier circuit, but will increase the output signal voltage of the first-stage amplifier circuit. Because the collector load resistance of VT1 increases with R4, the collector load resistance is large within a certain range. The voltage amplification factor is large. When C2 leaks or breaks down, the DC operating voltage of the first-stage amplifier circuit becomes smaller or has no DC voltage. At the same time, due to the increase in current flowing through R4, +V will also decrease and affect the normal operation of the second-stage amplifier circuit. When working, the entire amplifier circuit has no output signal or the signal is small.

4. When R4 is open circuit, the first-stage amplifier circuit has no DC operating voltage, which does not affect the operation of the second-stage amplifier circuit, but the entire amplifier circuit has no output signal.

2. Fault analysis of direct coupled multi-stage amplifier circuit

As shown in the figure, it is a directly coupled two-stage amplifier circuit.

The collector of the first-stage amplifier tube VT1 is directly connected to the base of the second-stage amplifier tube VT2, so it is a directly coupled amplifier. The signals of VT1 and VT2 are input from the base and output from the collector, so it is a co-emitter-co-emitter dual-tube direct coupling amplifier circuit.

The fault analysis of this multi-stage amplifier circuit mainly explains the following points.

a. When R4 is open circuit, VT1 has no DC working voltage, and there is no DC current in the base of VT2. At this time, both transistors are in a cut-off state and there is no signal output.

b. When C2 breaks down or severely leaks electricity, both VT1 and VT2 are in a cut-off state.

c. When C2 has a breakdown or leakage fault, the DC operating voltage is zero or too low, which affects the normal operation of VT1 and VT2.

d. When R2 is open, neither VT1 nor VT2 can work.

e. When R1 is open, VT1 is in a cut-off state, and VT1 has no collector current. In this way, all the current flowing through R2 flows into the base of VT2, causing the base current of VT2 to be very large and in a saturated state, and the amplifier has no signal output. When R1 is short-circuited, VT1 is in a saturated state, and its collector DC voltage is very low, causing the base current bias voltage of VT2 to be very low, and VT2 will be in a cut-off state.

Tip: Since the two-stage amplifier circuits VT1 and VT2 use a direct coupling circuit, a failure of one of the first-stage circuits will affect the DC working status of the two-stage circuits at the same time. Therefore, when checking the failure of this direct coupling circuit, The two-stage circuit should be inspected as a whole.

3. Three-stage amplifier circuit fault analysis

As shown in the figure, it is a three-stage amplifier circuit composed of three triodes.

In the circuit, VT1 is connected to a common collector amplifier circuit, which is the input stage amplifier circuit. VT2 is connected to a common-emitter amplifier circuit, which is the second-stage amplifier circuit. Capacitor C3 is used to couple the first-stage and second-stage amplifier circuits. VT3 is connected to a common-emitter amplifier circuit, which is the third-stage amplifier circuit and uses direct coupling with the second-stage circuit.

The fault analysis of this multi-stage amplifier circuit mainly explains the following two points:

1. Since capacitive coupling is used between the first-stage amplifier circuit and the following two-stage circuits, when the DC circuit in the first-stage amplifier circuit fails, it will have no impact on the DC circuits of the following two-stage circuits, but there will be no normal The signal is added to the subsequent amplifier circuit. Similarly, if there is a problem with the DC circuit in the subsequent two-stage amplifier circuit, it will not affect the DC circuit of the input stage amplifier.

2. As long as one stage amplifier in the circuit fails, the output signal of this multi-stage amplifier circuit will be abnormal, but the amplifier circuit before the fault point works normally. If there is a fault in the VT2 amplifier stage, the VT1 emitter output signal is normal.