As you have seen, positive feedback is accomplished by adding part of the output signal in phase with the input signal. In a common-base transistor amplifier, it is fairly simple to provide positive feedback. Since the input and output signals are in phase, you need only couple part of the output signal back to the input. This is shown in figure 1-18.
The feedback network in this amplifier is made up of R2 and C2. The value of C2 should be large so that the capacitive reactance (XC) will be low and the capacitor will couple the signal easily. (This is also the case with the input and output coupling capacitors C1 and C3.) The resistive value of R2 should be large to limit the amount of feedback signal and to ensure that the majority of the output signal goes on to the next stage through C3.
Figure 1-18. - Positive feedback in a transistor amplifier.
A more common configuration for transistor amplifiers is the common-emitter configuration. Positive feedback is a little more difficult with this configuration because the input and output signals are 180° out of phase. Positive feedback can be accomplished by feeding a portion of the output signal of the second stage back to the input of the first stage. This arrangement is shown in figure 1-19.
Figure 1-19. - Positive feedback in two stages of transistor amplification.
The figure shows that each stage of amplification has a 180° phase shift. This means that the output signal of Q2 will be in phase with the input signal to Q1. A portion of the output signal of Q2 is coupled back to the input of Q1 through the feedback network of C3 and R3. R3 should have a large resistance to limit the amount of signal through the feedback network. C3 should have a large capacitance so the capacitive reactance is low and the capacitor will couple the signal easily.
Sometimes positive feedback is used to eliminate the effects of negative feedback that are caused by circuit components. One way in which a circuit component can cause negative feedback is shown in figure 1-20.
In view (A) a common-emitter transistor amplifier is shown. An emitter resistor (R2) has been placed in this circuit to provide proper biasing and temperature stability. An undesired effect of this resistor is the development of a signal at the emitter in phase with the input signal on the base. This signal is caused by the changing current through the emitter resistor (R2) as the current through the transistor changes.
You might think that this signal on the emitter is a form of positive feedback since it is in phase with the input signal. But the emitter signal is really negative feedback. Current through the transistor is controlled by the base-to-emitter bias. If both the base and emitter become more positive by the same amount at the same time, current will not increase. It is the difference between the base and emitter voltages that controls the current flow through the transistor.
To eliminate this negative feedback caused by the emitter resistor, some way must be found to remove the signal from the emitter. If the signal could be coupled to ground (decoupled) the emitter of the transistor would be unaffected. That is exactly what is done. A DECOUPLING CAPACITOR (C3 in view B) is placed between the emitter of Q1 and ground (across the emitter resistor). This capacitor should have a high capacitance so that it will pass the signal to ground easily. The decoupling capacitor (C3) should have the same qualities as the coupling capacitors (C1 and C2) of the circuit. Decoupling capacitors are also called bypass capacitors.
Figure 1-20A. - Decoupling (bypass) capacitor in a transistor amplifier.
Figure 1-20B. - Decoupling (bypass) capacitor in a transistor amplifier.
Regardless of the method used to provide positive feedback in a circuit, the purpose is to increase the output signal amplitude.
Negative feedback is accomplished by adding part of the output signal out of phase with the input signal. You have seen that an emitter resistor in a common-emitter transistor amplifier will develop a negative feedback signal. Other methods of providing negative feedback are similar to those methods used to provide positive feedback. The phase relationship of the feedback signal and the input signal is the only difference.
Figure 1-21 shows negative feedback in a common-emitter transistor amplifier. The feedback network of C2 and R2 couples part of the output signal of Q1 back to the input. Since the output signal is 180° out of phase with the input signal, this causes negative feedback.
Figure 1-21. - Negative feedback in a transistor amplifier.
Negative feedback is used to improve fidelity of an amplifier by limiting the input signal. Negative feedback can also be used to increase the Frequency response of an amplifier. The gain of an amplifier decreases when the limit of its Frequency response is reached. When negative feedback is used, the feedback signal decreases as the output signal decreases. At the limits of Frequency response of the amplifier the smaller feedback signal means that the effective gain (gain with feedback) is increased. This will improve the Frequency response of the amplifier.
Q.23 What is feedback?