BJT AMPLIFIERS
Introduction
An
amplifier is used to increase the signal level. It is used to get a larger
signal output from a small signal input. Assume a sinusoidal signal at the
input of the amplifier. At the output, signal must remain sinusoidal in
waveform with frequency same as that of input. To make the transistor work as
an amplifier, it is to be biased to operate in active region. It means
base-emitter junction is forward biased and base-collector junction is reverse
biased.
Let us
consider the common emitter amplifier circuit using voltage divider bias.
In the
absence of input signal, only D.C. voltage is present in the circuit. It is
known as zero signal or no signal condition or quiescent condition. D.C.
collector-emitter voltage VCE, D.C. collector current IC
and base current I B is the quiescent operating point for the
amplifier. Due to this base current varies sinusoidaly as shown in the below
figure. Fig. IBQ is quiescent DC base current
If the
transistor is biased to operate in active region, output is linearly
proportional to the input. The collector current is β times larger than the
input base current in CE configuration. The collector current will also vary
sinusoidally about its quiescent value ICQ. The output voltage will
also vary sinusoidally as shown in the below figure.
Variations
in the collector current and voltage between collector and emitter due to
change in base current are shown graphically with the help of load line in the
above figure.
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