Chapter 11

Topics: Input impedance, Transistor, Electronic amplifier Pages: 26 (9287 words) Published: November 3, 2014
Principles of Electronics

280

11

Multistage Transistor
Amplifiers
11.1

Multistage Transistor
Amplifier

11.2

Role of Capacitors in
Transistor Amplifiers

11.3

Important Terms

11.4

Properties of dB Gain

11.5

RC Coupled Transistor
Amplifier

11.6

Transformer-Coupled Amplifier

11.7

Direct-Coupled Amplifier

11.8

Comparison of Different Types
of Coupling

11.9

Difference Between Transistor
And Tube Amplifiers

INTR
ODUCTION
INTRODUCTION
he output from a single stage amplifier is usually insufficient to drive an output device. Inther words, the gain of a single amplifier is inadequate for practical purposes. Conse quently, additional amplification over two or three stages is necessary. To achieve this, the output of each amplifier stage is coupled in some way to the input of the next stage. The resulting system is referred to as multistage amplifier. It may be emphasised here that a practical amplifier is always a multistage amplifier. For example, in a transistor radio receiver, the number of amplification stages may be six or more. In this chapter, we shall focus our attention on the various multistage transistor amplifiers and their practical applications.

T

11.1 Multistage Transistor Amplifier
A transistor circuit containing more than one stage of amplification is known as multistage transistor amplifier.

Multistage Transistor Amplifiers

281

In a multistage amplifier, a number of single amplifiers are connected in *cascade arrangement i.e. output of first stage is connected to the input of the second stage through a suitable coupling device and so on. The purpose of coupling device (e.g. a capacitor, transformer etc.) is (i) to transfer a.c. output of one stage to the input of the next stage and (ii) to isolate the d.c. conditions of one stage from the next stage. Fig. 11.1 shows the block diagram of a 3-stage amplifier. Each stage consists of one transistor and associated circuitry and is coupled to the next stage through a coupling device. The name of the amplifier is usually given after the type of coupling used. e.g. Name of coupling

Name of multistage amplifier
RC coupling
R-C coupled amplifier
Transformer coupling
Transformer coupled amplifier
Direct coupling
Direct coupled amplifier

Fig. 11.1
(i) In RC coupling, a capacitor is used as the coupling device. The capacitor connects the output of one stage to the input of the next stage in order to pass the a.c. signal on while blocking the d.c. bias voltages.

(ii) In transformer coupling, transformer is used as the coupling device. The transformer coupling provides the same two functions (viz. to pass the signal on and blocking d.c.) but permits in addition impedance matching.

(iii) In direct coupling or d.c. coupling, the individual amplifier stage bias conditions are so designed that the two stages may be directly connected without the necessity for d.c. isolation.

11.2 Role of Capacitors in Transistor Amplifiers
Regardless of the manner in which a capacitor is connected in a transistor amplifier, its behaviour towards d.c. and a.c. is as follows. A capacitor blocks d.c. i.e. a capacitor behaves as an “open**” to d.c. Therefore, for d.c. analysis, we can remove the capacitors from the transistor amplifier circuit. A capacitor offers reactance (= 1/2πfC) to a.c. depending upon the values of f and C. In practical transistor circuits, the size of capacitors is so selected that they offer negligible (ideally zero) reactance to the range of frequencies handled by the circuits. Therefore, for a.c. analysis, we can replace the capacitors by a short i.e. by a wire. The capacitors serve the following two roles in transistor amplifiers :

1. As coupling capacitors
2. As bypass capacitors
1. As coupling capacitors. In most applications, you will not see a single transistor amplifier. Rather we use a multistage amplifier i.e. a number of transistor amplifiers are connected in...
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