~100khz sinudodial signal superimposed on the output of a common emitter stage - diyAudio
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Old 19th October 2012, 05:06 PM   #1
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Default ~100khz sinudodial signal superimposed on the output of a common emitter stage

No waveforms as I was too tired last night to grab a screenshot, but basically I have a common emitter stage with a gain of 10, (RC = 10k, RE = 1k) on a diy pcb I made (along with other components such as active crossovers etc). The input is fine (no superimposed sinusodial) but the output at the collector has a small (about 50mV) sinusodial on the output. The gain looks fine (Vout/Vin) ~= 10 and it tracks when I adjust the input trimpot. The output is unloaded (I have not soldered on the output stage yet)

What could the possible reasons be? I swear I have seen this before and corrected it but can't remember what it was.
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Old 19th October 2012, 05:16 PM   #2
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Could be RF being imposed on the waveform. Try spaning way out wtih the scope and adjust so you can estimate or measure the frequency of the offending signal. Also try turning things off around the work area like floresent lights and wi-fi devices ect.
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Old 19th October 2012, 05:25 PM   #3
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Originally Posted by firechief View Post
Could be RF being imposed on the waveform. Try spaning way out wtih the scope and adjust so you can estimate or measure the frequency of the offending signal. Also try turning things off around the work area like floresent lights and wi-fi devices ect.
As stated in the title its about 100k (more precisely about 110k). I'll try turning stuff off and see if that help but lets brainstorm for now since I'm currently at work.

I'm going to build this stage on a breadboard and see if this signal still persist. What would that eliminate if it's gone? The power supply?

In what other cases could this happen?
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Old 19th October 2012, 07:41 PM   #4
Elvee is online now Elvee  Belgium
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Originally Posted by Fusion916 View Post
In what other cases could this happen?
You will first need to post a complete and accurate schematic of the circuit and its surroundings, it is impossible to guess exactly what's going on otherwise.
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Old 19th October 2012, 07:49 PM   #5
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You will first need to post a complete and accurate schematic of the circuit and its surroundings, it is impossible to guess exactly what's going on otherwise.
Yikes... That is not going to happen until I get home.

One note is that there are 4 opamps (2 8 pin dip packages) on board and every input and output of each opamp is clean, along with the input of the bjt. So is there was an RF or power supply issues it should show up elsewhere.

I must just have something connected wrong in the circuit.
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Old 19th October 2012, 09:54 PM   #6
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Also, DC voltages are fine, just sinusoid riding on top of it.
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Old 20th October 2012, 12:40 AM   #7
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Ok, figured it out, for some reason my negative rail has ~120khz noise on it about 400mVpp. Investigating now.
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Old 20th October 2012, 12:50 AM   #8
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Originally Posted by Fusion916 View Post
Ok, figured it out, for some reason my negative rail has ~120khz noise on it about 400mVpp. Investigating now.
LM337 regulator?
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Old 20th October 2012, 12:57 AM   #9
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LM337 regulator?
nte967

Unloaded there is no ripple. Not capacitor ripple because the noise is 120k and not 120hz. I think it has something to do with the diodes.
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Old 20th October 2012, 01:11 AM   #10
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Ok I needed to add a cap at the output of the diode string. Strange, not sure why.
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