Figure 2-7.Shunt peaking coil.
The "phantom" capacitor, CT, represents the total capacitance of the circuit. Notice that it tends to
couple the output signal to ground.
L1 is the shunt peaking coil. While it is in series with the load resistor (R3), it is in parallel (shunt)
with the output-signal path.
Since inductive reactance increases as frequency increases, the reactance of L1 develops more output
signal as the frequency increases. At the same time, the capacitive reactance of CT is decreasing as
frequency increases. This tends to couple more of the output signal to ground. The increased inductive
reactance counters the effect of the decreased capacitive reactance and this increases the high-frequency
response of the amplifier.
You have seen how a series peaking coil isolates the output capacitance of an amplifier from the
input capacitance of the next stage. You have also seen how a shunt peaking coil will counteract the
effects of the total capacitance of an amplifier. If these two techniques are used together, the combination
is more effective than the use of either one alone. The use of both series and shunt peaking coils is known
as COMBINATION PEAKING. An amplifier circuit with combination peaking is shown in figure 2-8. In
figure 2-8 the peaking coils are L1 and L2. L1 is a shunt peaking coil, and L2 is a series peaking coil.