Fig. 1 shows the circuit of a swamped amplifier.
Note that d.c. emitter resistance RE is divide into two parts viz. RE1 and RE2. Only resistance RE2 is bypassed by the capacitor CE while resistance RE1 is not.
This method swamps or minimises the effect of
r′e on the voltage gain without reducing the
voltage gain too much. Now the total a.c. emitter resistance is (r′e+ RE1) instead of
r′e as in a standard
CE amplifier. Therefore, the voltage gain of a amplifier at no-load becomes :
If RE1 ≥ 10 e
r′e , then the effect of e
r′e is almost negligible and the voltage gain is give by ;
Therefore, the voltage gain is essentially independent of e
r′ or it is reasonably stabilised.
Effect of swamping on Zin (base)
. The Zin (base) with RE completely bypassed is Zin (base)= β er′e .
When the emitter resistance is partially bypass, the portion of the resistance that is unbypass (i.e.
RE1) is seen by the a.c. signal and appears in series with e
r′ . Therefore, for swamped amplifier,
Zin (base) = β ( r′e + RE1)