8.3 BJT AMPLIFIERS 399
Noting the supply voltage to be 20 V and the drop across the transistor 10 V, about 10 V can be
allowed for ac swing acrossRD. Allowing some drop of, say,3VacrossRSfor dc stability,
RD=
7
0. 5
= 14
RS=
4
0. 5
= 8
From Equation (8.2.18),
VG=VGSQ+IDQRS= 5. 826 + 4 = 9 .826 V
Selecting arbitrarilyR 2 =10,000to maintain a large gate impedance,
R 1 =
R 2 (VDD−VG)
VG
=
104 ( 20 − 9. 826 )
9. 826
= 10 .354 k
One should also check to ensure that the voltage, current, and power ratings of the device are not
exceeded.
Biasing methods using resistors have been presented for the sake of simplicity and ease of
understanding. However, nowadays biasing techniques for modern amplifiers utilize transistors.
8.3 BJT Amplifiers
The purpose of electronic amplifiers is essentially to increase the amplitude and power of a signal
so that either useful work is done or information processing is realized. The output signal power
being greater than the input signal power, the additional power is supplied by the bias supply.
Thus, the amplifier action is one of energy conversion in which the bias power is converted to
signal power within the device.
A single-stage amplifier is one in which there is only one amplifying element. By combining
several single-stage amplifier circuits, a multistage amplifier is produced. Audio amplifiers are
designed to amplify signals in the frequency range of 30 to 15,000 Hz perceptible to the human
ear. A video amplifier is designed to amplify the signal frequencies needed for television imaging
(see Chapter 15). An amplifier, in general, is then made up of a cascade of several stages. A stage
usually consists of an elementary amplifier, which normally has only one transistor. The cascade
is formed by making the output of the first stage as the input of the second stage, the output of
the second stage as the input of the third stage, and so on. This section is devoted to the study of
three basic forms of amplifier stages which use a BJT.
Common-Emitter (CE) Configuration
The emitter part of a circuit being common to both the input and the output portions, Figure
8.3.1(a) illustrates a common-emitter (CE) BJT amplifier. The resistorsR 1 ,R 2 ,RC, andREare
primarily set by biasing. The input ac source is represented by its Thévenin equivalent. The
amplified output ac voltagevLappears across the load resistorRL, which could represent the
input resistance of the next stage in a cascade. CapacitorsCB,CC,andCEare so chosen that they
represent short circuits at the lowest frequency of interest.CEwould be made large enough so
that 1/ωCEis small relative toREin parallel with the impedance looking into the emitter at the
smallestωof interest. Similarly, the reactances ofCCandCBwould be chosen small relative to
the resistances in their parts of the circuit. CapacitorsCBandCCappear as short circuits to the ac
signals, but block the dc voltages and currents out of one part of the circuit from coupling with
