Class A power amplifier.
Class A power amplifier is a type of power amplifier where the output transistor is ON full time and the output current flows for the entire cycle of the input wave form. Class A power amplifier is the simplest of all power amplifier configurations. They have high fidelity and are totally immune to crossover distortion. Even though the class A power amplifier have a handful of good feature, they are not the prime choice because of their poor efficiency. Since the active elements (transistors) are forward biased full time, some current will flow through them even though there is no input signal and this is the main reason for the inefficiency. Output characteristics of a Class A power amplifier is shown in the figure below.
From the above figure its is clear that the Q-point is placed exactly at the center of the DC load line and the transistor conducts for every point in the input waveform. The theoretical maximum efficiency of a Class A power amplifier is 50%. In practical scenario, with capacitive coupling and inductive loads (loud speakers), the efficiency can come down as low as 25%. This means 75% of power drawn by the amplifier from the supply line is wasted. Majority of the power wasted is lost as heat on the active elements (transistor).As a result, even a moderately powered Class A power amplifier require a large power supply and a large heatsink.
Class A power amplifier circuit.
The circuit diagram of a two stage single ended Class A power amplifier is shown above. R1 and R2 are the biasing resistors. They form a voltage divider network which supplies the base of the transistor with a voltage 0.7V higher than the “negative maximum amplitude swing” of the input signal. This is the reason behind the transistor being ON irrespective of the input signal amplitude. Capacitor Cin is the input decoupling capacitor which removes the DC components present in the input signal. If Cin is not there, and there are DC components in the input signal, these DC components will be directly coupled to the base of the transistor and will surely alter the biasing conditions.
Rc is the collector resistor and Re is the emitter resistance. Their value is so selected that the collector current is in the desired level and the operating point is placed at the center of the load line under zero signal condition. Placing operating point as close as possible to the center of load line is very essential for the distortion free operation of the amplifier. Cc is the coupling capacitor which connects the two stages together. Its function is to block passage of DC components from first stage to the second stage.
Ce is the emitter by-pass capacitor whose function is to by-pass the AC components in the emitter current while amplifier is operating. If Ce is not there, the AC components will drop across the emitter resistor resulting in reduced gain (degenerative feedback). The most simple explanation is that, the additional voltage drop across Re will get added to the base-emitter voltage and this means additional forward voltage is required to forward bias the transistor.
Cout is the output coupling capacitor which couples the output to the load (loud speaker). Cout blocks the DC components of the second stage from entering the load (loud speaker). The Coupling capacitor Cout, Cin and Cc all degrades the low frequency response of the amplifier. This is because these capacitors form high pass filters in conjunction with the input impedance of succeeding stages resulting in the attenuation of low frequency components. Input and output waveforms of a two stage RC couple amplifier is shown in the figure below.
- Class A design is the simplest.
- High fidelity because input signal will be exactly reproduced at the output.
- Since the active device is on full time, no time is required for the turn on and this improves high frequency response.
- Since the active device conducts for the entire cycle of the input signal, there will be no cross over distortion.
- Single ended configuration can be practically realized in Class A amplifier. Single ended means only one active device (transistor) in the output stage.
Disadvantages of Class A power amplifier.
- Main disadvantage is poor efficiency.
- Steps for improving efficiency like transformer coupling etc affects the frequency response.
- Powerful Class A power amplifiers are costly and bulky due to the large power supply and heatsink.
Transformer coupled Class A power amplifier.
An amplifier where the load is coupled to the output using a transformer is called a transformer coupled amplifier. Using transformer coupling the efficiency of the amplifier can be improved to a great extend. The coupling transformer provides good impedance matching between the output and load and it is the main reason behind the improved efficiency. Impedance matching means making the output impedance of the amplifier equal to the input impedance of the load and this is an important criteria for the transfer of maximum power. Circuit diagram of typical single stage Class A amplifier is shown in the circuit diagram below.
Impedance matching can be attained by selecting the number of turns of the primary so that its net impedance is equal to the transistors output impedance and selecting the number of turns of the secondary so that its net impedance is equal to the loudspeakers input impedance.
Advantages of transformer coupled amplifier.
- Main advantage is the improvement of efficiency.
- Provides good DC isolation as there is no physical connection between amplifier output and load. Audio signals pass from one side to other by virtue of induction.
Disadvantages of transformer coupled amplifier.
- It is a bit hard to make/find an exactly matching transformer.
- Transformers are bulky and so it increases the cost and size of the amplifier.
- Transformer winding does not provide any resistance to DC current. If any DC components if present in the amplifier output, it will flow through the primary winding and saturate the core. This will result in reduced transformer action.
- Transformer coupling reduces the low frequency response of the amplifier.
- Transformer coupling induces hum in the output.
- Transformer coupling can be employed only for small loads.