718 Chapter 20
Fig. 20-20 represents a circuit configuration that is
superficially similar to Fig. 20-19 but is drastically
different in its operation. The currents in Q 1 and Q 2 are
again equal at quiescence and there is no current in the
load. When the bases of Q 1 and Q 2 are driven with a
positive-going signal Q 1 conducts more heavily while
the current in Q 2 decreases, thus producing a net current
in the load directed from left to right such that the right
end of the load assumes a negative voltage relative to
ground. On the other hand, if the bases of Q 1 and Q 2 are
driven with a negative-going signal, Q 1 conducts less
heavily while Q 2 conducts more, thus producing a net
current in the load directed from right to left such that
the right end of the load assumes a positive voltage rela-
tive to ground. The load now in contrast to Fig. 20-19
has been shifted from the transistor emitters to the tran-
sistor collectors.
Instead of dealing with a common collector stage as
in Fig. 20-19 the circuit of Fig. 20-20 is that of a
common emitter amplifier. That is, the load is really in
the collector circuits of the transistors. Such a stage
produces an output signal swing that is inverted in
polarity and possibly of much larger amplitude than the
input signal swing. The drive requirements of such a
stage are greatly relaxed as compared with those of the
circuit of Fig. 20-19. In fact, if power field-effect tran-
sistors (FET) are employed instead of bipolar transistors
in the circuit of Fig. 20-20, it is possible to produce a
high-power high-voltage output stage which can be
easily driven by a single low-power operational ampli-
fier. This is made possible because a field-effect tran-
sistor is a voltage-controlled device having a high input
impedance whereas a bipolar transistor is a current-
controlled device with inherently a low input impedance
in the common emitter configuration.
Fig. 20-21 is a totem pole configuration that has
been supplemented by an additional pair of transistors
to produce the AB plus B mode of operation. The totem
pole configuration became popular before the advent of
high-power complementary symmetry pairs and is still
employed where it is desirable to use only a single type
of power transistor. This configuration must be driven
by a circuit that furnishes two drive signals of opposite
polarity. Resistors R 1 and R 2 are typically a few hundred
ohms while the resistors R 3 and R 4 are of the order of
1 :. At quiescence neither Q 3 or Q 4 is conducting and
the emitter currents of Q 1 and Q 2 are equal in the range
of 50–100 mA and there is no current in the load. If the
base of Q 1 is driven with a positive-going signal while
that of Q 2 is driven with a negative-going signal, the
emitter current of Q 1 will increase while that of Q 2 will
decrease and there will exist a net current in the load
directed from left to right such that the left of the load
will assume a positive voltage relative to ground. On the
other hand, if the base of Q 1 is driven with a negative-
going signal while that of Q 2 is driven with a positive-
going signal, the emitter current of Q 1 will decrease
while that of Q 2 will increase, thus producing a net
current in the load directed from right to left such that
the left end of the load will assume a negative voltage
relative to ground. In the first instance, the current
supplied to the load by Q 1 was forced to pass through
R3, and in the second instance, the current supplied by
Q 2 is forced to pass through R 4. Under small or
moderate signal swings the voltage drops across R 3 or
R 4 are not sufficiently large to forward bias either Q 3 or
Q 4. When larger voltage swings are occurring, Q 3 and
Q 4 will be brought into conduction on alternate halves
of the cycle and thus will aid in supplying load current.
The circuit does possess a basic asymmetry in that even
though Q 1 (and Q 3 when it conducts) are operated as
common collector amplifiers, transistors Q 2 (and Q 4
when it conducts) are operated as common emitter
amplifiers.Figure 20-20. Common-emitter complementary symmetry
output.
LoadQ 1Q 2Bias and driveFeedbackBias and driveFigure 20-21. Totem pole with AB plus B.Bias and driveBias and driveFeedback LoadQ 1Q 2Q 3Q 4R 1R 2R 3R 4++