Capacitor Throwing off Bias Point

Mark245 · 2nd September 2009, 12:19 AM

Hey,

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.

Thanks,

Mark

sawreyrw · 2nd September 2009, 03:52 AM

The circuit seems to be unstable without the power supply bypass cap. You should always use the bypass cap to ensure stability.

cliffforrest · 2nd September 2009, 11:01 AM

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!

2nd September 2009, 12:19 AM	#1
Mark245 diyAudio Member Join Date: May 2006	Capacitor Throwing off Bias Point Hey, 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. Thanks, Mark

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2nd September 2009, 03:52 AM	#2
sawreyrw 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.

2nd September 2009, 11:01 AM	#3
cliffforrest diyAudio Member Join Date: Sep 2004 Location: 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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