Per Amplifiers

The function of voltage amplifiers discussed in chapter 7 is to increase a signal voltage from a low level to a higher level which is required for operating some low-power circuit. Such amplifiers are generally operated under class A conditions so that the amplification is without distortion. The final stage of multi-stage amplifier is usually required to drive a loudspeaker or some load which requires sufficient power for its operation. Thus such a stage must supply large and undistorted power to the load. The final stage which does this function is called a large signal or a power amplifier.
A power amplifier is defined as a frequency converter which changes d.c. power into a.c. signal power. The input power is supplied by the d.c. source and the average power in the circuii is distributed according to the relation
d.c. power input=a.c power output-f losses.
A bipolar junction transistor that is suitable for a power amplifier is called a power transistor. It differs from other transistor in the following respects :
(i) The area of the collector region of a power transistor is large and the collector is attached firmly to a metallic heat sink so that heat, which is generated mainly in the depletion region between the collector and the base is taken away quickly.
(ii) In a power transistor the emitter and base layers are heavily doped and the contact area between the base layers and the base leads is in ring like form so that the area is increased. Consequently the ohmic resistance between the emitter and the base is very low. Because of this low resistance, a power transistor requires low input power, the saturation voltage in the output circuit becomes smaller and the current gain becomes more uniform.
Power amplifiers may be operated as class .4, class B, class AB, or class C. In this chapter we NN ill describe two classes of power amplifiers which are common in transistor circuits. They are class A, and class B push-pull power amplifiers.
In class A power amplifier the load is not directly connected in the collector circuit because of the following reasons. The quiescent current through the load resistance gives rise to a large wastage of power. If the load is the voice coil of a loudspeaker, a large d.c. quiescent current through the voice coil would cause damage to it. Hence in class A power amplifier the load is always coupled to the collector circuit by means of a transformer. Therefore, we will discuss class A power amplifier with transformer coupling only.
92. Transformer-Coupled class A Power Amplifier.
Dynamic speakers used in audio-amplifiers and in radio receivers are low impedance devices ; the voice coil of a loudspeaker is of the order of 10 ohms. The collector circuit of a power transistor has high impedance. Therefore, to transfer audio power from the collector circuit of the transistor to the speaker a voltage-step down and current-step up transformer is used.
The primary and secondary coils of the transformer are wound on an iron core and it is called an output transformer. The high impedance primary of the output transformer is connected in the high-impedance collector circuit and the low-impedance secondary is connected across the voice coil of the speaker. If RL is the impedance of the speaker, then according to the theory of an ideal transformer, the a.c. load impedance in the collector circuit will be
R' = 1, n2
Rr
number of turns of the.secondary N2
where number of turns of the primary N,
For a step-down transformer a is less than I.
The ratio is called the turns ratio of the
transformer. (The derivation of Eq. (I) is given at the end of this
section) Fig. 9.1 shows the circuit diagram of transformer coupled
class A power amplifier. The resistors R, and R2 provide potential
divider arrangement for forward biasing of the emitter-base