(5) Since class AB operation is
more efficient than class A
operation,
it
seems
that
class AB would always be
preferred.
The
output
waveshape looks as good for
class AB as it does for class
A because we have assumed
perfectly matched transistors
and
symmetrical
Suppose, however, that the
transistors are not perfectly
matched or that the circuit
is not perfectly symmetrical;
the dynamic curves for Q1 and
Q2
would
no
longer
be
identical.
For
such
a
condition, the outputs from a
class AB and a class A push-
pull amplifier are shown in A
of figure 3-6 and B of figure
3-6, respectively.
It is
quite obvious that the output
for class AB operation is
distorted.
This
nonsym-
metrical
distortion
is
a
result of the presence of
even harmonics.
Thus, we
need to realize that class AB
operation
requires
matched
transistors
and
circuit
symmetry if high fidelity is
desired.
Such requirements
somewhat offset the higher
efficiency
that
class
AB
amplifiers offer.
b. Phase Inverter Driver Amplifier.
It
is
not
necessary
to
use
a
transformer to obtain input signals of
opposite
phase
for
a
push-pull
amplifier.
The circuit in figure 3-7
shows how a driver amplifier can be
designed to produce output signals
that are 180 out of phase. The values
of resistors R1 and R2 are chosen to
provide the proper base bias for
linear class A operation.
Resistors
R3 and R4 are practically equal in
Figure 3-5.
Typical output waveforms.
value so that the signal outputs are
the same in amplitude.
The normal
phase inversion from base to collector
occurs across R3, whereas across R4
the signal output is in phase with the
input.
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