Since this is the basic principle of tracking damage to an amplifier using transistors, before discussing the stereo audio system, below is an example of a single-stage amplifier circuit with all types of possible damage and measured voltages at predetermined points. Of course, from here we can take the meaning to move on to a more complicated circuit. Seen in Figure 6.48 below is a single-stage amplifier with measured DC voltage under normal conditions.
Figure 6.48: Single-Stage Amplifier with Normal DC Voltage
The single-stage amplifier above uses a silicon transistor type with an hFE between 50 and 500. Through calculations, the voltages at points 1, 2, and 3 will be obtained as follows:
- Point 1: obtained by using an easy formula, namely the voltage divider principle as follows: V1 = {VCC / (R1+R2) } R2, so that V1 = 2.4 Volts is obtained.
- Point 2: Obtained by the formula V2 = VCC -- IC.R3, while to find IC by finding IE, namely IE = V3 / R4, because IB is very small compared to IE then IC = IE. So that we get IC = 3.05 mA and V2 = 5.3 Volts (remember to find V3 first).
- Point 3: because it uses a silicon type transistor (VBE = 0.6 V or 0.7 V) then V3 can be obtained very easily, namely V3 = V1 - VBE = 2.4 V -- 0.7 V = 1.7 V.
In reality, the measured circuit using a multimeter is: V1 = 2.3 V, V2 = 5.5 V and V3 = 1.7 V, this all happens because a resistor with a tolerance of 10% is used, so there is no problem. While the output signal is amplified in reverse phase with its input, and this is indeed a characteristic of a single-stage amplifier. Below are the damages that occur and the results of DC voltage measurements and their reasons, as follows:
R1 is open given in Figure 6.49, then the measured voltage is: V1 = 0 V, V2 = 12 V, V3 = 0 V and the output is no signal. Because the current and DC base voltage = 0 V (cannot be supplied from R1), the transistor is in the off condition (cut off), so that V3 is also = 0V.
Figure 6.49: R1 Open Condition
R2 is open given in figure 6.50, then the measured voltage becomes V1 = 3.2 V, V2 = 2.6 V, V3 = 2.5 V and the output of the defect is cut off the negative part. Because it means the transistor current increases so that the voltage on R1 = V1 also increases. The transistor is on and almost saturated so that the voltage V2 is almost the same as the voltage on V3.
Figure 6.50: R2 Open Condition
R3 is open given in figure 6.51, then the measured voltage becomes V1 = 0.75 V, V2 = 0.1 V, V3 = 0.1 V, and the output has no signal. Because without R3 then the collector current = 0, so the emitter current is obtained from the base. As a result, the base emitter connection is a forward diode, so that R4 is parallel with R2, and because R4 is small then the voltage V3 is also small. While the voltage at V2 can be said to be almost the same as V3.
Figure 6.51: R3 Open Condition
R4 is open given in figure 6.52, then the measured voltage becomes V1 = 2.3V, V2 = 12V, V3 = 2V, and the output is no signal. Because the emitter is open to ground, there is no current flowing in the transistor. The voltage at the collector = VCC, while at V1 the condition is normal, and at V3 because it is measured against ground, there is a voltage read on the meter because there is current through the meter.
Figure 6.52: R4 Open Condition
C1 or C2 is open given in figure 6.53, then the measured voltage becomes V1 = 2.3 V, V2 = 5.5 V, V3 = 1.7 V, and the output is no signal. The DC voltage here does not change as normal because only the coupling capacitor is open so that the input signal is not forwarded to the transistor.
Figure 6.53: Condition C1 or C2 Open
C3 is open given in figure 6.54, then the measured voltage becomes V1 = 2.3V, V2 = 5.5V, V3 = 1.7V, and the output with small gain. Because C3 is open then the circuit has negative feedback through R4, so its gain becomes small (R3:R4 ???? 4) while its DC voltage remains normal.
Figure 6.54: Open C3 Condition
C3 short circuit is given in figure 6.55, then the measured voltage becomes V1 = 0.7V, V2 = 0.1V, V3 = 0V, and there is no signal at the output. This means that the emitter is short circuited to ground so that V3 = 0 V. The transistor is in saturation so that V2 is very small.
Figure 6.55: Short Circuit Condition C3
The open base collector connection is given in figure 6.56, then the measured voltage becomes V1 = 0.75V, V2 = 12V, V3 = 0.1V, and there is no output. Since the collector is open, there is no current flowing in the collector, so V2 = 12 V. While the base emitter connection is like a diode with forward voltage, so it is the same as the damage of R3 is open.
Figure 6.56: Open Base Collector Connection
The short-circuit collector-base connection is given in Figure 6.57, so the measured voltage becomes V1 = 3 V, V2 = 3 V, V3 = 2.3 V, and there is no output. The base and collector voltages are the same because of the short circuit. This short circuit causes R3 to be in series with R4, so that the current flowing in R4 is I = (VCC-VBE) / (R3 + R4) = 4 mA, and V3 = I x R4 = 2.3 V.
Figure 6.57: Short Circuit Base Collector Connection
The open base emitter relationship is given in figure 6.58, then the measured voltage becomes V1 = 2.3 V, V2 = 12 V, V3 = 0V, and there is no output. There is no current flowing in the transistor, so the voltage at the collector = VCC, and the voltage at the emitter = 0 V. While at V1 the condition is normal.
Figure 6.58: Open Base Emitter Connection
The short circuit base emitter relationship is given in figure 6.59, then the measured voltage becomes V1 = 0.13 V, V2 = 12 V, V3 = 0.13 V, and there is no output. The base and emitter have the same and small voltage because R2 and R4 are connected in parallel so that the voltage on R4 becomes small. With the short circuit of the base emitter, the transistor is not active, so the collector voltage = VCC.
Figure 6.59: Short Circuit Base Emitter Connection
The short-circuit collector emitter connection is given in Figure 6.60, so the measured voltage becomes V1 = 2.3 V, V2 = 2.5V, V3 = 2.5V, and there is no output.
Figure 6.60: Short Circuit Collector Emitter Connection
The emitter voltage is the same as the voltage on the collector, it indicates a short circuit on the emitter and collector. This voltage is obtained from the voltage divider between R3 and R4. While the voltage V1 is normal because when the emitter voltage increases, the base emitter diode connection is fed backwards (reverse), so the voltage V1 is the voltage divider between R1 and R2.
Through the single stage amplifier circuit above, we can learn a lot about:
- Various types of damage to an amplifier, if the damage is to one of the components in the circuit.
- Characteristics of the damage that occurs, where if damage occurs to one of the components, the voltages at the required points can be identified, and each damage has a different voltage value.
- Transistor damage can vary, but what is certain is that every transistor damage, the output signal is definitely not there because the active component is actually damaged. It is only necessary to study the voltage that occurs, so that if damage occurs to the transistor, it can be immediately detected again whether it damages other components.
- Damage to the coupling capacitor during a short circuit in a single-level amplifier will not make any difference. But if the circuit is more than one level, then the damage will be quite fatal, because the DC voltage from the previous or subsequent circuits will mix with each other so that the transistor can shift its working point or even the transistors can be damaged by the shift in the working point.
- This single-stage amplifier usually works in class A and is widely used as a driver before the final amplifier (power amplifier).
Power amplifier is a final amplifier that is always used in any audio system, not only in audio amplifiers because all electronic systems definitely need a final amplifier to produce a desired output. For that, a power amplifier circuit is provided for audio frequencies as shown in Figure 6.61 below.
Figure 6.61: Audio Frequency Power Amplifier
How the circuit works can be explained in parts as follows:
- This circuit is built from a 741 op-amp in noninverting mode, which will run the final amplifier in the form of a complementary amplifier which will then run an 8 ? loudspeaker.
- This amplifier is designed to have a frequency response of 15 Hz to 15 kHz with an output power of 3.5 W.
- The input signal is fed through C1 to pin 3 of IC 741, and will produce an output at pin 6 with the same polarity. This output signal will then be fed to the base of the output transistors Tr3 and Tr2 through an emitter follower Tr1.
- Part of the output signal is fed back to the inverting input of IC 741 through the voltage divider R3 and R2. These two resistors will determine the overall circuit gain, in addition, this type of feedback will improve the performance of the AC gain circuit and maintain the stability of its output and make the voltage at TP4 equal to zero or close to zero.
- The working principle of the complementary amplifier is: in the positive half cycle Tr3 conducts and Tr2 turns off. In the negative half cycle Tr2 conducts and Tr3 turns off. So the power distribution from the complementary amplifier to the loudspeaker is done through Tr3 in the positive half cycle and through Tr2 in the negative half cycle.
- To get good output, both transistors must be properly matched and installed using good cooling. If the transistors are not properly matched, then a cross-over defect occurs. Diodes D1 and D2 are installed to help overcome the cross-over defect by setting their forward bias at a small value.
- The offset voltage at the input will be amplified and will appear at TP4 in the order of several millivolts, both positive and negative. This causes unwanted DC current to flow through the loudspeaker, which will reduce the quality of the loudspeaker produced. To eliminate it, RV1 is used as a null offset regulator.
- The maximum output power available can be determined by estimating the amplitude increase of the input signal where the output waveform is monitored by an oscilloscope. The RMS voltage across the load ignoring distortion can be used to obtain the output power. And the formula for the output power is:
where RL = 8 ?. While the voltage gain is: Av = (R2 + R3) / R2.
Transistors Tr3 and Tr2 will be damaged if they are supplied with current that exceeds their capacity. This can happen if Tr1 is short-circuited. Therefore, a power supply must be selected that is in accordance with the maximum current limit of 1A so that the maximum capacity of the transistor is not exceeded.
By explaining each part, it will certainly be clearer, so that if there is damage, it will be easier to know which component is damaged. Under normal conditions without input signals, the DC voltage measured at the TP-TP to ground is as follows:
There are several damages that can be explained, namely if the measurements are given to the ground as follows:
And here it turns out that the fuse is broken but none of the transistors are very hot. From this case it turns out that TP 2, 3, and 4 are all negative, so the positive voltage is not distributed, meaning that Tr1 does not work (open not short circuit) even though TP1 is very high (as a trigger for Tr1 for the conductor). This means that the Op-Amp continues to work normally only its output becomes large positive because its inverting input gets a large negative voltage compared to its non-inverting input. So this happens because of two possibilities, namely R7 is open or the base and emitter of Tr1 are open.
Here Tr3 is cut off and Tr2 conducts so that a negative voltage arises. While the fuse is broken because the current flowing exceeds 0.6 A.
- If the amplifier gain becomes very low. The output voltage is almost the same as the input voltage. No transistors are hot. This must happen because of the emergence of negative feedback (remember in a single-stage amplifier), this is possible if R2 is open or C2 is short-circuited, so that the gain approaches unity.
- The output is very unstable in its gain so that the signal is erratic. It should be noted that to maintain the stability of the circuit in general, negative feedback is always given. Because it is unstable, there is only one possibility that causes it all, namely the feedback circuit is not right. And the feedback of this circuit is R3, so R3 must be open.
- There is a positive half-wave distortion (positive wave is cut off) at the output, while the negative part is normal. It is known from the way the circuit works that the one that produces the positive half-wave is the Tr3 area, so if Tr1 is not hot and continues to work because Tr2 can input from Tr1 and still works normally, then the damage must be in the Tr3 area and its output, namely the base and emitter of Tr3 are open or R5 is open.
- What happens if Rv2 opens. This is very dangerous, because Rv2 is the determinant of the working point setting of Tr2 and Tr3, so if Rv2 opens then the output will be crossover distortion and both transistors Tr2 and Tr3 will quickly heat up and be damaged. So do not underestimate the damage to a resistor because it can have a huge impact on the circuit.
- If the Op-Amp is damaged, with the condition of its output section open (pin 6 is open). This does not mean it is safe, because even though TP1 = 0, which means Tr1 and Tr3 are cut off, but Tr2 is very conductive so that the fuse will definitely blow again (as in the first damage R7 is open or the base and emitter of Tr1 are open).
So it turns out that the final amplifier circuit for this complement model is very sensitive, even a little wrong setting will have fatal consequences for the circuit. Here, precision and experience are needed, so even without measuring the DC voltages on certain TPs, it can still be determined which areas are not correct and which components are damaged when there is a case of damage.
Through the two simple circuit examples above, hopefully it can add insight into our thinking about an amplifier in an audio system and make us more curious to know more about a stereo audio system. Because in the audio system circuit there will be many varieties, and of course there are many cases of damage that will be faced with all forms of damage that can be said to be very varied, but in essence, first master the basics of an amplifier, both the driver and the final/power amplifier.