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Old 2nd September 2009, 12:19 AM   #1
Mark245 is offline Mark245  United States
diyAudio Member
Join Date: May 2006
Default Capacitor Throwing off Bias Point


I have a question about a simple audio amplifier I am building. It consists of a CE stage connected to a CC stage. Very simple. The CE has a .1uF bypass cap from emitter to ground. The problem is that the Vbb bias voltage at the base of the CC stage is .5v lower than I expect(it should be 3 but it is like 2.5v). When I remove the bypass cap on the CE stage, Vbb become the correct 3v value. Any idea why this is happening?

I also notice that if I connect a 1uF cap accross the power supply rails, this fixes the problem. Why would this help?

Any suggestions appreciated. I can post the schematic if so requested.


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Old 2nd September 2009, 03:52 AM   #2
diyAudio Member
Join Date: Apr 2006
Location: Minnesota
The circuit seems to be unstable without the power supply bypass cap. You should always use the bypass cap to ensure stability.
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Old 2nd September 2009, 11:01 AM   #3
diyAudio Member
Join Date: Sep 2004
Location: South Coast, UK
Your amp is an oscillator!

Removing the CD cap lowers the gain, hence stops oscillating.

No reason to be minimalist on supply caps! Stick in a big one!
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