Electronic Circuit Analysis

Power Amplifiers 157
In the low frequency range shunting effect of Lp will reduce the effective load resistance.
Lower 3db frequency is reached when,
2 1t fl Lp = R
or
R
II = 21tL P
where R is the parallel combination of f- and [~l2. RL
oe N2
V
I
Fig. 4.12 Class A power amplifier circuit
The transformer coupled amplifier circuit is similar to a circuit, like this. Instead of having
coupling capacitors C I
and C 2 , we have transformer, coupling. The primary of transformer T2 acts as
R.c. C 2
across R2 helps in making Emitter of transistor at the ground point for AC. Because of C E ,
emitter is at ground potential for AC. Therefore secondary voltage of transformer is applied between
base and emitter of transistor.
4.5 Transformer Coupled Audio Amplifier
Audio amplifier
Video
R.F
Classification of Radio Waves.
40Hz to 20 KHz
5-SMHz
20KHz
Very low frequency(VLF) 10-30 K Hz
Low frequency (LF) 30-300 K Hz
Medium frequency (MF) 300-3,000 K Hz
\
High frequens;y (HF) 3-30 MHz
VHF
30-300 MHz
Ultra high frequency (UHF) 300-3000 MHz
Super high frequency (SHF) 3000-30,000 MHz
An amplifying system usually consists of several stages in cascade. The input and intermediate
stages operate in a small signal c1ass-A mode. Their function is to amplify the small excitation
to a large value to drive the final device. This output stage feeds a transducer such as CRT, loud