The Class B Amplifier
To improve the full power efficiency of the previous Class A amplifier by reducing the wasted power
in the form of heat, it is possible to design the power amplifier circuit with two transistors in its output stage
producing what is commonly termed as a Class B Amplifier also known as a push-pull amplifier
configuration.
Push-pull amplifiers use two "complementary" or matching transistors, one being an NPN-type and the
other being a PNP-type with both power transistors receiving the same input signal together that is equal in magnitude,
but in opposite phase to each other. This results in one transistor only amplifying one half or 180o of the
input waveform cycle while the other transistor amplifies the other half or remaining 180o of the input waveform
cycle with the resulting "two-halves" being put back together again at the output terminal.
Then the conduction angle for this type of amplifier circuit is only 180o or 50% of the input
signal. This pushing and pulling effect of the alternating half cycles by the transistors gives this type of circuit its
amusing "push-pull" name, but are more generally known as the Class B Amplifier as shown below.
Class B Push-pull Transformer Amplifier Circuit
The circuit above shows a standard Class B Amplifier circuit that uses a balanced centre-tapped input transformer, which splits the incoming waveform signal into two equal halves and which are 180o out of phase with each other. Another centre-tapped transformer on the output is used to recombined the two signals providing the increased power to the load. The transistors used for this type of transformer push-pull amplifier circuit are both NPN transistors with their emitter terminals connected together.
Here, the load current is shared between the two power transistor devices as it decreases in one device
and increases in the other throughout the signal cycle reducing the output voltage and current to zero. The result is that
both halves of the output waveform now swings from zero to twice the quiescent current thereby reducing dissipation. This
has the effect of almost doubling the efficiency of the amplifier to around 70%.
Assuming that no input signal is present, then each transistor carries the normal quiescent collector
current, the value of which is determined by the base bias which is at the cut-off point. If the transformer is accurately
centre tapped, then the two collector currents will flow in opposite directions (ideal condition) and there will be no
magnetization of the transformer core, thus minimizing the possibility of distortion.
When an input signal is present across the secondary of the driver transformer T1,
the transistor base inputs are in "anti-phase" to each other as shown, thus if TR1 base goes
positive driving the transistor into heavy conduction, its collector current will increase but at the same time the base
current of TR2 will go negative further into cut-off and the collector current of this transistor
decreases by an equal amount and vice versa. Hence negative halves are amplified by one transistor and positive halves by
the other transistor giving this push-pull effect.
Unlike the DC condition, these AC currents are ADDITIVE resulting in the two output half-cycles
being combined to reform the sine-wave in the output transformers primary winding which then appears across the load.
Class B Amplifier operation has zero DC bias as the transistors are biased at the cut-off,
so each transistor only conducts when the input signal is greater than the base-emitter voltage.
Therefore, at zero input there is zero output and no power is being consumed. This then means that the actual Q-point of
a Class B amplifier is on the Vce part of the load line as shown below.
Class B Output Characteristics Curves
The Class B Amplifier has the big advantage over their Class A amplifier cousins
in that no current flows through the transistors when they are in their quiescent state (ie, with no input signal),
therefore no power is dissipated in the output transistors or transformer when there is no signal present unlike
Class A amplifier stages that require significant base bias thereby dissipating lots of heat - even with no input
signal present. So the overall conversion efficiency ( η ) of the amplifier
is greater than that of the equivalent Class A with efficiencies reaching as high as 70% possible resulting in nearly
all modern types of push-pull amplifiers operated in this Class B mode.
Transformerless Class B Push-Pull Amplifier
One of the main disadvantages of the Class B amplifier circuit above is that it uses balanced
centre-tapped transformers in its design, making it expensive to construct. However, there is another type of
Class B amplifier called a Complementary-Symmetry Class B Amplifier that does not use transformers in its
design therefore, it is transformerless using instead complementary or matching pairs of power transistors. As
transformers are not needed this makes the amplifier circuit much smaller for the same amount of output, also
there are no stray magnetic effects or transformer distortion to effect the quality of the output signal.
An example of a "transformerless" Class B amplifier circuit is given below.
Class B Transformerless Output Stage
The Class B amplifier circuit above uses complimentary transistors for each half of the waveform
and while Class B amplifiers have a much high gain than the Class A types, one of the main
disadvantages of class B type push-pull amplifiers is that they suffer from an effect known commonly as
Crossover Distortion.
Hopefully we remember from our tutorials about Transistors that
it takes approximately 0.7 volts (measured from base to emitter) to get a bipolar transistor to start conducting.
In a pure class B amplifier, the output transistors are not "pre-biased" to an "ON" state of operation.
This means that the part of the output waveform which falls below this 0.7 volt window will
not be reproduced accurately as the transition between the two transistors (when they are switching over from
one transistor to the other), the transistors do not stop or start conducting exactly at the zero crossover point
even if they are specially matched pairs. The output transistors for each half of the waveform (positive and negative)
will each have a 0.7 volt area in which they are not conducting. The resuslt is that both transistors are turned "OFF"
at exactly the same time.
A simple way to eliminate crossover distortion in a Class B amplifier is to add two small
voltage sources to the circuit to bias both the transistors at a point slightly above their cut-off point. This
then would give us what is commonly called an Class AB Amplifier circuit. However, it is impractical to
add additional voltage sources to the amplifier circuit so pn-junctions are used to provide the additional bias
in the form of silicon diodes.
The Class AB Amplifier
We know that we need the base-emitter voltage to be greater than 0.7v for
a silicon bipolar transistor to start conducting, so if we were to replace the two voltage divider biasing resistors
connected to the base terminals of the transistors with two
silicon Diodes, the biasing voltage
applied to the transistors would now be equal to the forward voltage drop of the diode. These two diodes are generally
called Biasing Diodes or Compensating Diodes and are chosen to match the characteristics of the matching
transistors. The circuit below shows diode biasing.
Class AB Amplifier
The Class AB Amplifier circuit is a compromise between the Class A and the Class B
configurations. This very small diode biasing voltage causes both transistors to slightly conduct even when no input
signal is present. An input signal waveform will cause the transistors to operate as normal in their active region
thereby eliminating any crossover distortion present in pure Class B amplifier designs.
A small collector current will flow when there is no input signal but it is much less than that for
the Class A amplifier configuration. This means then that the transistor will be "ON" for more than half a cycle of the
waveform but much less than a full cycle giving a conduction angle of between 180 to 360o or 50 to 100% of
the input signal depending upon the amount of additional biasing used. The amount of diode biasing voltage present at
the base terminal of the transistor can be increased in multiples by adding additional diodes in series.
Class B amplifiers are greatly preferred over Class A designs for high-power
applications such as audio power amplifiers and PA systems. Like the Class A Amplifier
circuit, one way to greatly boost the current gain ( Ai ) of a
Class B push-pull amplifier is to use Darlington transistors pairs instead of single transistors in its output
circuitry.
In the next tutorial about Amplifiers we will look more closely at the
effects of Crossover Distortion in
Class B amplifier circuits and ways to reduce its effect.
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