Electronic Circuit Analysis

154 Electronic Circuit Analysis
short circuits. But because of these reactive coupling elements, as signal frequency decreases Xc
increases. Hence there will be large voltage drop across the capacitor and so the actual input to the
amplifier reduces and hence gain decreases. Similarly at high frequencies because of the shunting
capacitance C s ' gain falls. Therefore there is a particular frequency range in which gain is of desirable
value only.
Instead of capacitors, transformer can also be used for coupling.
/I /I
Fig. 4.9 Transformer coupling
Transformer does not respond to DC. Therefore only AC signals fro¢. source to the amplifier
circuit and from the amplifier to the load will be coupled. But what is the advaptage of the transformer
coupling? Suppose, the load resistance RL is very small :::::: 40, 80 or ISO as in the case of a loud
speaker. The output impedance Ro of the transistor amplifier is much larger (for common emitter and
common base configuration A v ' Ai »1). Therefore impedance matching will not be there and so
maximum power will not be transferred. Even if capacitive coupling is used, impedance matching
cannot be achieved. But this can be done using a transformer.
[ :~ ) = Turns ratio of transfonner
N 1 = Number of turns on primary side.
N2 = Number of turns on secondary side.
R, is much larger than RL" Therefore [~~) > I or the transfonner that should be used should
be a Stepdown transformer. Therefore the output voltage at the secondary of the transformer will be
much smaller compared to the input voltage since stepping down action is taking place. [This is the .
case with class A and class B power amplifiers in the case of lab experiment]. But the current
amplification will be there and because of Z matching, maximum power will be transferred to the
load. Similar to resistive capacitor coupled amplifier, we have the frequency response which depends
upon the inductance of the primary and secondary. The transformer on the primary side is chosen
such that the sotirce resistance of the generator matches with the input Z of the amplifier circuit.
Transformer coupled amplifiers are used in low audio frequency range only because at higher
frequencies the XL of the transformer will be large and so the gain falls.
Another advantage with the transformer coupled amplifiers is the AC current passing through
the load resistance RL results in only wastage of power, since we are interested in only AC output