Electronic Circuit Analysis

Power Amplifiers
153
VCC-::::,VCE=Vy
DC input power = V y' ~
Vmax + Vmin
Vyy = 2
T)
25tV max - V min)x 2
(Vmax + V min
)
50tVmax-Vmi)
= (Vmax + vmiJ
V yy will be the quiescent voltage itself for transformer coupled amplifier. Since there is no DC
voltage drop across the transformer.
If V min = 0, maximum efficiency = 50% for class A transformer coupled amplifier.
For series fed amplifier, V yy = V max = 2 V m
- 25 t v max - Vmin ) OJ: If V - 0 . - 250J:
T) - V /'0. max - ,maXImum T) - /'0
max
Therefore for a transformer coupled amplifier conversion" is twice. (50% compared to 25%
for series fed amplifier)
4.4 Transformer Coupled Amplifier
In AC amplifier circuits, the input AC signal should be coupled to the amplifier and output of the
amplifier should be coupled to the load resistance. The coupling device should be such that, it allows
only the AC signal to the amplifier circuits and blocks the DC components present in the signal
generator, because we are interested in amplifying only AC signals. For this purpose a capacitor can
be used for coupling.
Types of coupling "
1. Capacitor coupled amplifier
4. Direct coupled amplifier
Fig. 4.8 Coupling in amplifier circuits
2. Transformer coupled amplifier 3. RC coupled amplifier
5. Inductor or Tuned amplifier
Es is the AC signal generator and Rs its source resistance. C I and C 2
are the coupling capacitors.
They are chosen such that, for the lowest frequency signal to be amplified, Xc and Xc are
I 2