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, as illustrated in the block diagram below. 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 it that it draw 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 into ac power and whose action is controlled by the input signal.
The transistors employed in power amplifiers are called power transistors. They differ from the other transistors in the following respects:
- The base is made thicker to handle large currents.
- 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.
- The emitter and base layers are heavily doped.
Terms used in Power Amplifiers
- Collector efficiency: The effectiveness of power amplifier is measured by its capability of converting dc power into ac power at the output. This is known as collector efficiency and may be defined as the ratio of ac output power to dc power drawn from the supply battery of a power amplifier.
- Power Dissipation Capacity: Power dissipation capacity of a power transistor is defined as its ability to dissipate the heat developed in it.
- Distortion: It is defined as the change of output wave shape from the input wave shape of the amplifier. Since power amplifier handles large signals and for large input signals, the transistor operation becomes nonlinear, the problem of distortion arises. Lesser the distortion, better is the amplifier.
Classification of Power Amplifiers
The power amplifiers are primarily divided into two categories viz. audio power amplifiers and radio power amplifiers.
Classification According to Mode of Operation:
1. Class A Power Amplifiers: In this 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 as amplifier can amplify input signal of small aptitude. 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 characterized by a high fidelity of the output. Operation is restricted only over a small central region of the load line so such amplifiers can be used for amplifying signals of small aptitude. 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%.
2. 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 signal, the collector current is zero and no biasing system is required in class B amplifiers. The operating point is selected at collector cutoff 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 collector current. The transistor dissipates more power with increase in signal strength. The theoretical efficiency in class b operation is about 78.5%.
3. 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 and 360 degree and neither class A nor class B operation.
4. Class C Power Amplifiers: It is biased for operation for less than 180 degree 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 circuit, such as radio or communications.
5. Class D Power Amplifiers: Class D amplifiers is biased are designed to operate with digital or pulse type signals. Unlike digital techniques makes it possible to have signal that varies over the entire cycle to recreate the output from many pieces of input signal. The main advantage of class D power amplifiers is that it is on only for short intervals and overall efficiency can practically be very high (above 90%).
Classification Based on Deriving Output:
Single-ended power amplifiers uses single transistor and derives output power w.r.t. one end permanently grounded.
Double-ended or push-pull amplifier uses two transistors in a single stage. It consists of two loops in which the transistor collector currents flow in opposite directions but add in the load,
Complementary symmetry push-pull power amplifier uses two transistors having complementary symmetry. The term complementary arises from the fact that one transistor is the N-P-N type and the other is P-N-P type. They have symmetry as they are made with the same material and technology and are of same maximum ratings.
Classification based on Use: The amplifiers, according to use, may be classifies as voltage, power. current or general purpose amplifiers. In general, the load of an amplifier is impedance. The two most important special cases are the idealized resistive load and the tuned circuit operating near its resonant frequency. Class AB and class B operation are used with untuned power amplifiers whereas class C operation is employed with tuned radio frequency amplifiers. Many important wave shaping functions may be accomplished by class B or class C overdiven amplifiers.
Classification According to Mode of Operation:
1. Class A Power Amplifiers: In this 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 as amplifier can amplify input signal of small aptitude. 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 characterized by a high fidelity of the output. Operation is restricted only over a small central region of the load line so such amplifiers can be used for amplifying signals of small aptitude. 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%.
2. 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 signal, the collector current is zero and no biasing system is required in class B amplifiers. The operating point is selected at collector cutoff 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 collector current. The transistor dissipates more power with increase in signal strength. The theoretical efficiency in class b operation is about 78.5%.
3. 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 and 360 degree and neither class A nor class B operation.
4. Class C Power Amplifiers: It is biased for operation for less than 180 degree 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 circuit, such as radio or communications.
5. Class D Power Amplifiers: Class D amplifiers is biased are designed to operate with digital or pulse type signals. Unlike digital techniques makes it possible to have signal that varies over the entire cycle to recreate the output from many pieces of input signal. The main advantage of class D power amplifiers is that it is on only for short intervals and overall efficiency can practically be very high (above 90%).
Classification Based on Deriving Output:
Single-ended power amplifiers uses single transistor and derives output power w.r.t. one end permanently grounded.
Double-ended or push-pull amplifier uses two transistors in a single stage. It consists of two loops in which the transistor collector currents flow in opposite directions but add in the load,
Complementary symmetry push-pull power amplifier uses two transistors having complementary symmetry. The term complementary arises from the fact that one transistor is the N-P-N type and the other is P-N-P type. They have symmetry as they are made with the same material and technology and are of same maximum ratings.
Classification based on Use: The amplifiers, according to use, may be classifies as voltage, power. current or general purpose amplifiers. In general, the load of an amplifier is impedance. The two most important special cases are the idealized resistive load and the tuned circuit operating near its resonant frequency. Class AB and class B operation are used with untuned power amplifiers whereas class C operation is employed with tuned radio frequency amplifiers. Many important wave shaping functions may be accomplished by class B or class C overdiven amplifiers.
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