transistor emmiter capacitors

[Kevin Weddle]

I've seen some circuits that use a capacitor in parallel with an emitter resistor. The RC time constant needs to be short compared to the period of the input, or signal distortion will occur. Is this a real consideration of circuit design, or does it not really matter in most circumstaces?

 

[audioguru2]

An emitter resistor adds negative feedback to a common-emitter transistor.
If the emitter resistor is bypassed then the gain and distortion are increased.

If the time constant is short then only high frequencies have boosted gain.

 

[Kevin Weddle]

I don't see how the term "negative feedback" can be applied to an emitter resistor. 

 

[audioguru2]

I don't see how the term "negative feedback" can be applied to an emitter resistor. 

Example:
The input signal causes the base to go positive. If there is an emitter resistor then the emitter voltage also goes positive which reduces the effective input signal level. That is negative feedback.

If the emitter resistor is bypassed with a capacitor then high frequencies will not have the negative feedback and therefore will have the gain boosted.
If the emitter resistor has a pretty big bypass capacitor then all audio frequencies will be boosted but DC will have full negative feedback.

 

[sarath1]

Hi
If i can employ two negative feedback in amplifier?
What is the result?

 

[audioguru2]

If i can employ two negative feedback in amplifier?
What is the result?

Yes.
Use an unbypassed emitter resistor and use a base resistor fed from the collector and an input resistor that feeds the signal to the base.
They both reduce gain and distortion.
The first one increases the input impedance.
The second one reduces the input impedance.View attachment 40554