1-22
Figure 1-19.—Positive feedback in two stages of transistor amplification.
The figure shows that each stage of amplification has a 180º 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.
