Common Emitter Amplifier - AC Impedance in

It's often recommended to improve the AC gain of a common emitter amplifier by putting a bypass capacitor paralell to Re, thus effectively decreasing the Ze for the signal and increasing the gain.

My question is: Won't this also lower the Z input (AC) for the amplifier? If Ze(AC) is only 100Ω or so, won't that lower Zin(AC) to around 10k (assuming a β of 100)?

Hello,

Lowering Ze will lower Zin, that's life.

Adding a capacitor across the emitter resistor increases the voltage gain by removing the negative feedback provided by the emitter resistor. Then the distortion becomes extremely high at fairly high levels.

well audioguru sums it up the bypass capacitor is chosen to reduce the i/p impedance at signal levels hence giving more gain but its value should be carefully chosen.its not preffered where you need very good signal.not a popular technique.if you want a large voltage gain use a follower instead else use class b amp.

My calculation of the input impedance of the 2N3904 was double the amount shown on its datasheet. I was wrong and used the much higher DC current gain, hFE instead of the lower AC current gain, hfe.

Low-Cost Digital Echo/Reverb Microphone Preamplifier Schematic Diagram

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khathorzhe said:

hI!!!!!!!

do u hav a schematic diagram of Low-Cost Digital Echo/Reverb Microphone Preamplifier?

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Torben