CLASS B AMPLIFIER

A half-wave output is not problematic for some applications. In fact, some applications may
necessitate this very kind of amplification. Because it is possible to operate an amplifier in
modes other than full-wave reproduction and specific applications require different ranges of
reproduction, it is useful to describe the degree
to which an amplifier reproduces the input
waveform by designating it according to class. Amplifier class operation is categorized with
alphabetical letters: A, B, C, and AB.

For Class A operation, the entire input waveform is faithfully reproduced. Class A operation can only be obtained when the transistor spends
its entire time in the active mode, never reaching either cutoff or saturation. To achieve this,
sufficient DC bias voltage is usually set at the level necessary to drive the transistor exactly
halfway between cutoff and saturation. This way, the AC input signal will be perfectly
“centered” between the amplifier’s high and low signal limit levels.
Class A: The amplifier output is a faithful reproduction of the input.
Class B operation is what we had the first time an AC signal was applied to the common emitter
amplifier with no DC bias voltage. The transistor spent half its time in active mode
and the other half in cutoff with the input voltage too low (or even of the wrong polarity!) to
forward-bias its base-emitter junction.

Class B: Bias is such that half (180o) of the waveform is reproduced.
By itself, an amplifier operating in class B mode is not very useful. In most circumstances,
the severe distortion introduced into the wave shape by eliminating half of it would be
unacceptable. However, class B operation is a useful mode of biasing if two amplifiers are
operated as a push-pull pair, each amplifier handling only half of the waveform at a time:
Class B push pull amplifier: Each transistor reproduces half of the waveform. Combining the
halves produces a faithful reproduction of the whole wave.

Transistor Q1 “pushes” (drives the output voltage in a positive direction with respect to
ground), while transistor Q2 “pulls” the output voltage (in a negative direction, toward 0 volts
with respect to ground). Individually, each of these transistors is operating in class B mode,
active only for one-half of the input waveform cycle. Together, however, both function as a
team to produce an output waveform identical in shape to the input waveform.
A decided advantage of the class B (push-pull) amplifier design over the class A design is
greater output power capability. With a class A design, the transistor dissipates considerable
energy in the form of heat because it never stops conducting current. At all points in the wave
cycle it is in the active (conducting) mode, conducting substantial current and dropping
substantial voltage. There is substantial power dissipated by the transistor throughout the
cycle. In a class B design, each transistor spends half the time in cutoff mode, where it
dissipates zero power (zero current = zero power dissipation). This gives each transistor a
time to “rest” and cool while the other transistor carries the burden of the load. Class A
amplifiers are simpler in design, but tend to be limited to low-power signal applications for
the simple reason of transistor heat dissipation.
Another class of amplifier operation known as class AB, is somewhere between class A and
class B: the transistor spends more than 50% but less than 100% of the time conducting
current.
If the input signal bias for an amplifier is slightly negative (opposite of the bias polarity for
class A operation), the output waveform will be further “clipped” than it was with class B
biasing, resulting in an operation where the transistor spends most of the time in cutoff mode:
Class C: Conduction is for less than a half cycle (< 180o).
At first, this scheme may seem utterly pointless. After all, how useful could an amplifier be if
it clips the waveform as badly as this? If the output is used directly with no conditioning of
any kind, it would indeed be of questionable utility. However, with the application of a tank
circuit (parallel resonant inductor - capacitor combination) to the output, the occasional output
surge produced by the amplifier can set in motion a higher-frequency oscillation maintained
by the tank circuit. This may be likened to a machine where a heavy flywheel is given an
occasional “kick” to keep it spinning: