Common emitter amplifier

In electronics, a common-emitter amplifier is one of three basic single-stage bipolar- junction-transistor(BJT) amplifier topologies, typically used as a voltage amplifier.

Ac concepts

• Amplifier gain• Input output phase• Ac emitter resistance• Ac beta

Amplifier gain

• Current gain• Voltage gain• Power gain

Input output phase relationship

• Current phase: The input and output currents

are in phase.• Voltage phase: The input and output voltages

are 180 degree out of phase.

The ac emitter resistance

• Def: The dynamic resistance of the transister

base emitter junction used in voltage and input impedance calculations.

formula for this relation is as follows. R’E=25MV\IE.

AC Beta

Def: It is the ratio of ac collector current to ac

base current. Formula: Β=ΔIC /ΔIB

Swamped amplifier

• Def: an amplifier that uses a partically bypassed

emitter resistance to increase ac emitter resistance.

Formula for this relation is; Av = rc / r’e + re

Effect of swamping on Zin

The input of impedence of a transistor base to equal beta times the ac resistance of the emitter. The ac resistance is equal to (r;e + re) for the swamped amplifier.

Formula for this relation is ; Zin(base)= hfe (r’e + re).

Diagram

The effect of swamping on voltage gain stability

• The voltage gain of common emitter amplifier can be affected change in temperature. The dc current gain of transistors(Hfe) varies with temperatutre.

• Any change in hfe can affect the value of Ie and therefore the values of ré and Av

Disadvantages of swamping

• swamp amplifier provides voltage gain that is relatively stable against variation in Hfe.

• Swamping improves stability but reduces Av.