Introduction To Power Amplifiers
An amplifier system consists of signal pick-up transducer, followed by a small signal amplifier(s), a large signal amplifier and an output transducer. A transducer is used to convert one form of energy into another type. For example a microphone is used to convert acoustical energy into electrical energy. Conversely, a loudspeaker is used to convert electrical energy into acoustical energy. A motor is a transducer that is used to convert electrical energy into mechanical energy.
The input transducer produces small electrical (typically voltage) signal, that needs sufficient amplification to operate some output device such as a loudspeaker, a servomotor, a solenoid or relay. The factors of prime interest in small signal voltage amplifiers are usually linearity and gain. Since the signal voltage and current from the input transducer is usually very small, the amount of power handling capacity and power efficiency are of slight concern. The functions of voltage amplifiers are to present a high resistance to the input transducer to minimize loading effects and to provide a large enough voltage signal to the large-signal amplifier stages to operate such output devices (loudspeaker, servomotor etc.). A large-signal amplifier must operate efficiently and be capable of handling large amounts of power-typically, a few watts to hundred of watts. Large signal amplifier that drives the output transducer demands even more consideration than the small-signal voltage amplifiers that we have focused so far. The factors of greatest concern to the large signal power amplifiers are the power efficiency of the circuit, the maximum amount of power that the circuit is capable of handling, and impedance matching to the output device.
Power amplifier is meant to raise the power level of the input signal. In order to get large power at the output, it is necessary that the input-signal voltage is large. That is why, in an electronic system, a voltage amplifier always precedes the power amplifier, also, that is why power amplifiers are called large-signal amplifiers.
In fact, power amplifier does not amplify power. What a power amplifier actually does is that it draws power from dc supply connected to the output circuit and converts it into useful ac signal power. The type of ac power available at the output terminals of the power amplifier is controlled by the input signal. Thus a power amplifier may be defined as a device that converts dc power and whose action is controlled by the input signal.
The transistors employed in power amplifiers are called power transistors. They differ from other transistors in the following respects.
(i) The base is made thicker to handle large currents i.e.in power amplifiers; transistors with comparatively smaller gain are used.
(ii) The area of collector region of a power transistor is made considerably larger in order to dissipate the heat developed in the transistor during operation. Moreover, heat sinks are used for improving the heat dissipation.
(iii) The emitter and base layers are heavily doped. The contact area between the base layers and base leads is in ring like form so that the area is increased. By doing so ohmic resistance between emitter and base is reduced and due to low resistance, small power is required at input.
Classification Of Power Amplifiers
The power amplifiers are primarily divided into two categories
- Audio-power amplifiers – also called the small signal power amplifiers, raise the power levels of signals that have audio-frequency range (20 Hz- 20 kHz).
- Radio-power amplifiers – also called large signal power amplifiers raise the power level of signals that have radio frequency range. They amplify a specific frequency or narrow band of frequencies while rejecting all other frequencies.
Classification According To Mode of Operation
Transistor power amplifiers handle large signals. Many of them are driven so hard by the input large signal that collector current is either cut- off or is in saturation region during a large portion of the input cycle. So such amplifiers are generally classified according to their mode of operation. This classification is based on the amount of transistor bias and amplitude of the input signal. It takes into account the portion of the cycle for which the transistor conducts. They are classified as below:
- Class A Power Amplifiers – In this case, transistor is so biased that the output current flows for the entire cycle of the input signal. Thus the operating point is so selected that the transistor operates only over the linear region of its load line. So such an amplifier can amplify input signal of small amplitude. As the transistor operates over the linear portion of load line, the output waveform is exactly similar to input waveform. So class A amplifiers are characterised by a high fidelity of the output. Such amplifiers are used where freedom from distortion is prime aim. Operation is restricted only over a small central region of the load line so such amplifiers can be used for amplifying signals of small amplitude. Also ac power output per transistor is small. The maximum possible overall efficiency with resistive load is 25%. The maximum possible collector efficiency with resistive load is 50%. In case an output transformer is used, both of these efficiencies are 50%.
- Class B Power Amplifiers – In this case, the transistor bias and signal amplitude are such that output current flows only during positive half cycle of the input signal. At zero signals, the collector current is zero and no biasing system is required in class B amplifiers. The operating point is selected at collector cut-off voltage; Because of total absence of negative half cycle from the output the signal distortion is high. Zero signal input represents the best condition for class B amplifiers because of zero collectors current. The transistor dissipates more power with increase in signal strength. In comparison to class A amplifiers average current is less, power dissipation is less. So overall efficiency is increased. The theoretical efficiency in class B operation is about 78.5% while it is only 50% in class A operation.
- Class AB Power Amplifiers – An amplifier may be biased at a dc level above the zero base current level of class B power amplifiers and above one-half the supply voltage level of class A; this bias condition is class AB. Class AB operation still needs a push-pull connection to achieve a full output cycle, but the dc bias level is usually closer to zero base current level for better power efficiency. For class AB operation the output signal swing occurs between 180 degree and 360 degree and is neither class A nor class B operation.
- Class C Power Amplifiers – A class C power amplifier is biased for operation for less than 180 of the input signal cycle and will operate only with a tuned or resonant circuit which provides a full cycle of operation for the tuned or resonant frequency. Such power amplifiers are, therefore, employed in special areas of tuned circuits, such as radio or communications.
- Class D Power Amplifiers – Class D power amplifiers are designed to operate with digital or pulse type signals. Using digital techniques makes it possible to have a signal that varies over the entire cycle (using sample-and-hold-circuitry) to recreate the output from many pieces of input signal. The main advantage of class D power amplifiers is that it is on (using power) only for short intervals and the overall efficiency can practically be very high (above 90 degree).