Page 52 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Common-emitter amplifier: This type of circuit is by far the most frequently used. It has the greatest power gain,
substantial current and voltage gains, and is specially advantageous in multistage application when a high gain is
a primary requirement. A common-emitter amplifier stage with biasing from a single D.C supply battery is in Fig 6.

Fig 6

The a.c. signal is applied between the base and the emitter and the output is taken from the collector. For the
transistor to operate, the emitter base junction must be forward-biased, the resistors R and R setting the base
2
1
voltage so that the emitter is forward-biased. The collector current flows through the load resistors R and R and
e
L
the voltage developed by R at the collector is the output.
L
The voltage gain of a transistor is largely determined by the value of this particular resistor since the voltage
developed across it due to change in the collector current is far greater than that developed across the base
resistor from the input signal.
Resistor Re is included to minimise the effect of temperature changes in the collector current. To prevent Re from
reducing the signal gain by current feedback, a capacitor C may be included in parallel with R .
3
e
Common-collector amplifier: In this configuration, the collector is the common point for the input and output
circuits, the input signal being applied between the base and collector and taken off between the emitter and
collector, Fig 7. The notable feature is the large input impedance virtually equal to that of the parallel circuit of R1
and R . The output resistance is, however, low and, hence it follows that the voltage gain is low, but a high current
2
amplification can be obtained.

Fig 7

The functions of the capacitors C and C are the same as for the common-emitter stage, as the potential networks
1
2
R and R which provide forward bias for the emitter-base junction. The main advantage of the common-collector
2
1
circuit is the readiness with which it may be directly coupled to any point in a circuit regardless of voltage.
Common-base amplifier: In this circuit the base is the common terminal between the emitter terminal and the
collector terminal. The emitter current I is the input current and the collector current I is the output current.
c
c
(Fig 8) Since Ie = I + I and since in this circuit Ie is greater than I , by the value of Ib, the current gain I /I will
c e
c
b
c
always be slightly less than one. Therefore, there can be no current gain in a common-base circuit. However,
because of the low impedance of the forward-biased emitter-base junction and the high impedance of the reverse-
biased collector-base junction a sizable voltage gain is obtained.
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CITS : Power - Electrician & Wireman - Lesson 60-69