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Guest SM2GXN

Hi audiguru!

I know that the gain changes when the emitter resistor is bypassed and how it affect the frequency response of the amplifier. But you forgot to comment on my thoughts about the voltage divider :)
Can you figure out any parallell to the formula Rc/Re and the formula for a bypassed emitter resistor. What I meant was this, as the reactance gets smaller for the frequency to be amplified and Rc still got same value the total reactance the signal see is a very low emitter resistor though not pure resistive.
Then Rc/Xc :) I know it isn't that easy but is there any relationship at all?
It take some time for me to write all this en english and some of it can be difficult to understand, just let me know if there is anything you dont unerstand.

Bjorn

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Hi Guys,
An expert at The Amplifier Institute:
http://dialspace.dial.pipex.com/prod/dialspace/town/pipexdsl/q/aqpl43/ampins/ampins.htm
agrees with my discussion about the beta of transistors:

BETA.
Beta is not a fundamental property of a BJT. Never design circuits that depend on beta. Unless of course you're making a transistor tester...

Here are some of the factors that affect beta. This should convince you that it is a shifty and untrustworthy parameter.

Beta varies with Ic.
First it rises as Ic increases, usually reaching a broad peak, then it falls off as Ic continues to increase.
Beta increases with temperature. This seems to be very little known. Most things, like leakage currents, get worse as temperature increases, so this makes a nice change.
Beta is lower for high-current transistor types.
Beta is lower for high Vceo transistor types.
Beta varies widely between nominally identical examples of the same transistor type.

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Guest SM2GXN

Hi audioguru!

Sorry for being stupid I must have been blind, all I asked for was in your post with the distortion.gif
Thing was that I've mixed emitter resistor RE and as you call it built in re so we can say that gain without bypassing RE is gain= RC/RE+re and with bypass cap gain=RC/RE.
Hopefully I got this right now  ;D
Thank you audioguru for once again clarifying in a easy way.

Bjorn

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so we can say that gain without bypassing RE is gain= RC/RE+re and with bypass cap gain=RC/RE.

Hi Bjorn,
You almost got it right. The gain with RE bypassed is RC/Re. ;D

You can also use the emitter resistor's bypass cap as a treble-boost highpass filter. The frequency at which the capacitor's reactance equals the value of the emitter resistor the gain will be up 3dB, with 20dB for each decade of higher frequencies until the gain reaches RC/Re.
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Guest SM2GXN

;D yes my fingers made RE instead of re.
Do you mean voltage gain 6dB?
I finally found a link in my bookmarks that could be of interest to those who has little or no knowledge about transistors.
audioguru when you got time please check the link and let me know what you think, I find that the content of this link is a good tutorial.
Here it is: http://www.ee.nuigalway.ie/subjects/ee208/asd/notes/

bjorn

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Guest SM2GXN

Yes ofcourse didn't think of that, thanks!
Once up on a time I could calculate most of this but as the years pass (getting older) some things just fade away ;D
Now days almost every experiment and new design has to be looked up in a book or by passing a new topic here  :) somtimes even the simplest circuit become a pitfall.

Bjorn

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