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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.
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Common emitter amplifier

Common emitter amplifierIn 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 conceptsAmplifier gainInput output phaseAc emitter resistanceAc beta Amplifier gainCurrent gainVoltage gainPower gainInput output phase relationshipCurrent 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 resistanceDef: The dynamic resistance of the transister base emitter junction used in voltage and input impedance calculations. formula for this relation is as follows. re=25mv\ie.

AC BetaDef: It is the ratio of ac collector current to ac base current. Formula: =IC /IB

Swamped amplifierDef: an amplifier that uses a partically bypassed emitter resistance to increase ac emitter resistance. Formula for this relation is; Av = rc / re + 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 (re + 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 r and AvDisadvantages of swampingswamp amplifier provides voltage gain that is relatively stable against variation in Hfe. Swamping improves stability but reduces Av.