Download Multistage Transistor Amplifiers


The output from a single amplifier is
generally insufficient to drive an output
device. In other words, the gain of a
single amplifier is inadequate for
practical purposes. Consequently,
additional amplification over two or
three stages is necessary.

To achieve this, the output of each
amplifier is coupled in some way to the
input of the next stage. The resulting
system is referred to as the multistage
amplifier. In fact, a multistage amplifier
consists of single amplifiers in which
output of first stage is connected to the
input of the second stage through a
suitable coupling device and so on.
 (i) R-C
coupled amplifiers
 (ii)Transformer
amplifiers
 (iii)
coupled
Direct coupled amplifiers.
 The
R-C coupled amplifiers
employ R-C coupling and are
generally used for voltage
amplification because of very
little distortion.
 The
transformer coupled
amplifiers employ transformer
coupling and are generally
used for power amplification
because transformer permits
excellent impedance
matching.
 in
direct coupled amplifiers, one
stage is directly coupled to the
next stage without any
intervening coupling device. Such
type of coupling is used for
amplifying extremely low
frequency signals because the
electrical sizes of capacitors and
transformers become very large
at extremely low frequencies.
Gain
It is the ratio of output electrical
quantity to the input of the
amplifier. Accordingly; it can be
current gain, voltage or power.
The gain of a multistage
amplifier is equal to the product
of gains of individual stages.
 For
instance, if G1,G2 and G3
are the individual voltage
gains of a three stage
amplifier, then total voltage
gain is given by;
G= G1 x G2 xG3
The gain of an amplifier can be
expressed as a number or in
decibel. The latter method is the
practical way of expressing the
gain of an amplifier. It is
because the unit ‘decibel’ is a
logarithmic unit and tallies with
human ear response.
 It
is the curve between voltage
gain and signal frequency of an
amplifier. The gain of the
amplifier changes with signal
frequency because the values of
capacitive
and
inductive
reactance of the circuit elements
change with the frequency.
 An
amplifier is said to
have
good
frequency
response if the gain
remains uniform over a
considerable range of
signal frequency.
 The
frequency response of
R-C coupling is excellent
because
gain
remains
constant over the audio
frequency (i.e. 20 Hz to 20
KHz) range. On the other
hand,
the
frequency
response of a transformer
coupling is poor i.e
.
its
gain
varies
considerably
with
frequency. It is for this
reason that transformer
coupling is not used in the
initial stages of a multistage
amplifier.
 its
gain varies considerably
with frequency. It is for this
reason that transformer
coupling is not used in the
initial stages of a multistage
amplifier.
 An
amplifier is said to
have
good
frequency
response if the gain
remains uniform over a
considerable range of
signal frequency.
.
The range of frequency
over which the voltage
gain is equal to or
greater than 70.7% of
maximum gain is called
“bandwidth”.

There are two frequencies, called
‘lower cut-off’ and ‘upper cut-off’
frequency at which gain is exactly
70.7% of maximum gain. If these
values are represented by f1 and f2
respectively, then ‘bandwidth’ = f1 to
f2. For distortion less amplification, it
is important that signal frequency
range must be within the bandwidth
of the amplifier.

Important relations. A few important relations are
given below:

Power gain = 10 log10
db

Voltage gain = 20 log10
db

Turn ratio of output transformer,

