Electronic Circuit Analysis

128 Electronic Circuit Analysis
Now consider 3 such stages cascaded as shown in Fig. 3.29. R is the series resistor and hie is the
transistor low frequency input impedance. h re of the BJT is small and so it can be neglected.
U2 U2 L
U2
e 1
is the input voltage
Fig. 3.29 Three stage cascaded amplifier.
. e l
"-J e l
Ibl= (R+hie)-R
Since hie of the BJT is small compared to R.
RL is a 50 n load for the circuit.
The voltage gain gets multiplied for each stage. So for n-stages,
eo = (n. ic) (~L ) = n. Po ib (~L )
n. Po. e!. RL
2R
where Po is the low frequency current gain (P) of the BJT.
n = number of cascaded stages.
The Voltage Gain for 'm' cascaded stages is, (y'G) m
(V. G)m =
[
no RL' po]m
2R
Typically, a single stage will have a 3-db frequency of200 MHzs in high frequency amplifiers.