# Audio Amp Build Questions

Discussion in 'General Electronics Chat' started by Joster, Dec 5, 2013.

1. ### Joster Thread Starter Member

Jun 12, 2013
95
1
Hi all,

I have some questions about what the bypass cap on the emitter leg is doing to the gain as well as looking for tips on a power amplifier section.

http://youtu.be/KWNWnegza7Y

Cheers!

Joe

2. ### Jony130 AAC Fanatic!

Feb 17, 2009
4,174
1,188
Without Ce capacitor the gain is equal to
Av ≈ (Rc||RL)/(Re) But if you add Ce capacitor into the circuit the voltage gain will increase to
Av = gm*(Rc||RL)
where
gm - transconductance = Ic/26mV ≈ 40*Ic

Have you check you DC operation point without input signal? Can you show as result?

Last edited: Dec 5, 2013
3. ### LvW Active Member

Jun 13, 2013
674
100
To complete the story about the bypass capacitor Ce:

Ce is not only influencing the amount of gain - it also has influence on the QUALITY of gain. This means the following:
Without Ce we have signal feedback due to Re, which offers some advantages:
* gain value less suszeptible to BJT tolerances and temperature changes,
* better signal quality (less distortions) due to linearizing effect of feedback
* larger signal input resistance.

4. ### Joster Thread Starter Member

Jun 12, 2013
95
1
Thanks! I will use that formula for my circuit and calculate it out. Someone told me that the voltage gain is determined by Rc/Re. Is this true?

5. ### Jony130 AAC Fanatic!

Feb 17, 2009
4,174
1,188
Yest it's true but only when:
- Collector resistance (Rc) is much smaller than the load resistance (RL).
- Emitter resistance (Re) is much larger then 26mV/Ic.
- And we don't use Ce capacitor (bypass cap on the emitter leg) in the circuit.

Also try to read this post
and the rest of this thread also.

Last edited: Dec 5, 2013
6. ### Shagas Active Member

May 13, 2013
802
74
Don't you mean it the other way around ?
When RL is very large in respect to RC then we can consider that the gain is close to Rc/Re .
In fact it's trully Rc/Re when we haven't connected a load.

Correct me if i'm wrong.

7. ### Jony130 AAC Fanatic!

Feb 17, 2009
4,174
1,188
Yes you are absolutely right, I made a stupid mistake. Thanks! I've updated the original post.

8. ### LvW Active Member

Jun 13, 2013
674
100
This simplified expression shows you that for sufficient negative signal feedback the gain is primarily determined NOT by the active device but by the feedback elements.
This case is comparable to the gain expression -R2/R1 for an operational amplifier used as an inverting amplifier.