Multi-stage amplifier circuit fault analysis

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I. Fault analysis of RC-coupled multistage amplifier circuit

As shown in the figure, taking the RC-coupled amplifier circuit as an example, the circuit fault analysis is explained.

The fault analysis of the multi-stage amplifier circuit is basically the same as the fault analysis of the single-stage amplifier circuit. Here are the additional points to be added here.

1. When the DC circuit in the VT1 amplifier stage fails, the DC circuit of the VT2 amplifier stage will not be affected due to the DC blocking function. However, since the first stage amplifier circuit is not working properly, it has no normal signal plus. 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 presence 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 second-stage amplifier. The circuit does not work properly, so the second stage amplifier circuit is not able to output a normal signal.

Tip: In a multi-stage amplifier circuit, as long as there is a problem with the primary amplifier circuit, the entire multi-stage amplifier circuit cannot output a normal signal.

3. When C2 is open, there is no influence on the second-stage amplifier circuit, which will increase the output signal voltage of the first-stage amplifier circuit, because the VT1 collector load resistance increases R4, and 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 of the current flowing through R4, the +V is also decreased, which affects the normality of the second-stage amplifier circuit. Working, at this time the entire amplifier circuit has no output signal or the signal is small.

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

Second, direct coupled multi-stage amplifier circuit fault analysis

As shown, it is a direct coupled two-stage amplifier circuit.

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

The failure analysis of this multi-stage amplifier circuit mainly describes the following points.

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

b. When C2 breaks down or severely leaks, both VT1 and VT2 are in the off state.

c. When the breakdown or leakage of C2 occurs, the DC working voltage is zero or too low, which affects the normal operation of VT1 and also affects the normal operation of VT2.

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

e. When R1 is open, VT1 is in the off state, and VT1 has no collector current, so that the current flowing through R2 flows into the base of VT2, so that the base current of VT2 is large and saturated, 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, so that the VT2 base current bias voltage is very low, and VT2 will be in an off state.

Tip: Since the direct coupling circuit is used between the VT1 and VT2 two-stage amplifier circuits, the failure of the primary circuit will affect the DC operating state of the two-stage circuit at the same time. Therefore, when checking the fault of the direct-coupled circuit, The two-stage circuit is to be checked as a whole.

Third, three-stage amplifier circuit failure 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 an input stage amplifier circuit. VT2 is connected to a common emitter amplifier circuit, which is a second stage amplifier circuit, and a capacitor C3 is coupled between the first stage and the second stage amplifier circuit. VT3 is connected to a common emitter amplifier circuit, which is a third stage amplifier circuit, which is directly coupled to the second stage circuit.

The failure analysis of this multi-stage amplifier circuit mainly describes the following two points:

1. Since the first-stage amplifier circuit and the latter two-stage circuit use capacitive coupling, when the DC circuit in the first-stage amplifier circuit fails, there is no influence on the DC circuit of the latter two-stage circuit, but it is not normal. The signal is applied to the subsequent amplifier circuit. Similarly, the DC circuit in the latter two-stage amplifier circuit has a problem, and has no effect on the input stage amplifier DC circuit.

2. As long as there is a fault in the first-stage amplifier in the circuit, the output signal of this multi-stage amplifier circuit is not normal, 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.