Electronic Circuit Analysis

Tuned Amplifiers - I
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5.3.1 Equivalent Circuit on the Output Side of the I Stage
RI is the input resistance of the II stage.
Ro is the output resistance of the I stage amplifier.
II
Fig. 5.8 Equivalent circuit
The input IZI of the common emitter amplifier circuits will be less. So the output impedance of
the circuit being coupled to one common emitter amplifier, should also have low IZI for impedance
matching and to get maximum power transfer. So in order to reduce the impedance of the LC
resonant circuit, to match the low IZI of the common emitter circuit, tapping is made in the LC tuned
circuit. Tapped single tuned circuits are us~d in such applications.
5.3.2 Expression for 'Inductance' for Maximum Power Transfer
Let the tapping point divide the impedance into two parts LI and L 2 .
Let LI = nL so that L2 = (1 - n)
Writing Kirchoff's Voltage Law (KVL)
VI = jroL . II - jro (L2 + M) 12
o = -jro (LI + M) II + (R j + jroL 2 ) 12
Where M is the mutual inductance between LI and L 2 . Solving equations 1 and 2,
..... (1)
..... (2)
VI {Ri + jroL 2 )
I = ---:----"-"--:--..,...:::--~
I
jroL (Ri + jroL 2 )+00 2 (L2 + M f
..... (3)
Hence the IZI offered by the coil along with input resistance ~i of the next stage is
..... (4)
..... (5)
But roL2 much less than R i .
As R i
, the input resistance of transistor circuit II stage is KQ and much greater than roL2
..... (6)