Class A Amplifier
Common emitter amplifiers are the most commonly used type of amplifier as they have a large
voltage gain. They are designed to produce a large output voltage swing from a relatively small input signal voltage of
only a few millivolt's and are used mainly as "small signal amplifiers" as we saw in the previous tutorials. However,
sometimes an amplifier is required to drive large resistive loads such as a loudspeaker or to drive a motor in a robot
and for these types of applications where high switching currents are needed Power Amplifiers are required.
The main function of the power amplifier, which are also known as a "large signal amplifier" is to
deliver power, which is the product of voltage and current to the load. Basically a power amplifier is also a voltage
amplifier the difference being that the load resistance connected to the output is relatively low, for example a
loudspeaker of 4 or 8Ωs resulting in high currents flowing through the collector of the transistor. Because of
these high load currents the output transistor(s) used for power amplifier output stages such as the 2N3055 need to
have higher voltage and power ratings than the general ones used for small signal amplifiers such as the BC107.
Since we are interested in delivering maximum AC power to the load, while consuming the minimum DC
power possible from the supply we are mostly concerned with the "conversion efficiency" of the amplifier. However, one
of the main disadvantage of power amplifiers and especially the Class A amplifier is that their overall conversion
efficiency is very low as large currents mean that a considerable amount of power is lost in the form of heat.
Percentage efficiency in amplifiers is defined as the r.m.s. output power dissipated in the load divided by the
total DC power taken from the supply source as shown below.
Power Amplifier Efficiency
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 |
- Where:
- η% - is the efficiency of the amplifier.
- Pout - is the amplifiers output power delivered to the load.
- Pdc - is the DC power taken from the supply.
For a power amplifier it is very important that the amplifiers power supply is well designed to
provide the maximum available continuous power to the output signal.
Class A Amplifier
The most commonly used type of power amplifier configuration is the Class A Amplifier.
The Class A amplifier is the most common and simplest form of power amplifier that uses the switching transistor in
the standard common emitter circuit configuration as seen previously. The transistor is always biased "ON" so that
it conducts during one complete cycle of the input signal waveform producing minimum distortion and maximum amplitude
to the output.
This means then that the Class A Amplifier configuration is the ideal operating mode,
because there can be no crossover or switch-off distortion to the output waveform even during the negative half of
the cycle. Class A power amplifier output stages may use a single power transistor or pairs of transistors connected
together to share the high load current. Consider the Class A amplifier circuit below.
Single Stage Amplifier Circuit
This is the simplest type of Class A power amplifier circuit. It uses a single-ended transistor for its output stage with the resistive load connected directly to the Collector terminal. When the transistor switches "ON" it sinks the output current through the Collector resulting in an inevitable voltage drop across the Emitter resistance thereby limiting the negative output capability. The efficiency of this type of circuit is very low (less than 30%) and delivers small power outputs for a large drain on the DC power supply. A Class A amplifier stage passes the same load current even when no input signal is applied so large heatsinks are needed for the output transistors.
However, another simple way to increase the current handling capacity of the circuit while at the same
time obtain a greater power gain is to replace the single output transistor with a Darlington Transistor. These
types of devices are basically two transistors within a single package, one small "pilot" transistor and another larger
"switching" transistor. The big advantage of these devices are that the input impedance is suitably large while the
output impedance is relatively low, thereby reducing the power loss and therefore the heat within the switching device.
Darlington Transistor Configurations
The overall current gain Beta (β) or hfe value of a Darlington device is the product of the two individual gains of the transistors multiplied together and very high β values along with high Collector currents are possible compared to a single transistor circuit.
To improve the full power efficiency of the Class A amplifier it is possible to design
the circuit with a transformer connected directly in the Collector circuit to form a circuit called a Transformer Coupled
Amplifier. The transformer improves the efficiency of the amplifier by matching the impedance of the load with that of
the amplifiers output using the turns ratio ( n ) of the transformer and an example of
this is given below.
Transformer-coupled Amplifier Circuit
As the Collector current, Ic is reduced to below the quiescent Q-point
set up by the base bias voltage, due to variations in the base current, the magnetic flux in the transformer core
collapses causing an induced emf in the transformer primary windings. This causes an instantaneous collector voltage
to rise to a value of twice the supply voltage 2Vcc giving a maximum collector current of
twice Ic when the Collector voltage is at its minimum. Then the efficiency of this type of
Class A amplifier configuration can be calculated as follows.
The r.m.s. Collector voltage is given as:
The r.m.s. Collector current is given as:
The r.m.s. Power delivered to the load (Pac) is therefore given as:
The average power drawn from the supply (Pdc) is given by:
and therefore the efficiency of a Transformer-coupled Class A amplifier is given as:
While the transformer improves the efficiency of the amplifier by matching the impedance of the load
with that of the amplifiers output impedance, using the turns ratio of an output signal transformer, efficiencies reaching
40% are possible with most commercially available Class-A type power amplifiers being of this type of configuration, but
the use of inductive components is best avoided. Also one big disadvantage of this type of circuit is the additional cost
and size of the audio transformer required.
The type of "Class" or classification that an amplifier is given really depends upon the
conduction angle, the portion of the 360o of the input waveform cycle, in which the transistor is
conducting. In the Class A amplifier the conduction angle is a full 360o or 100% of the input signal
while in other amplifier classes the transistor conducts during a lesser conduction angle.
It is possible to obtain greater power output and efficiency than that of the
Class A amplifier by using two complementary transistors in the output stage with one transistor
being an NPN or N-channel type while the other transistor is a PNP or P-channel (the complement) type connected in
what is called a "push-pull" configuration. This type of configuration is generally called a Class B Amplifier
and is another type of audio amplifier circuit that we will look at in the next tutorial.
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