Electronics > Projects, Designs, and Technical Stuff

Opamp follower and Sallen-Key filter behaving strange

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I made this circuit (edit: for some reason inline attachments don't seem to work… images are attached at the end of the message).

It's part of a bigger project. Input (FILTER) will get signal from a VLF antenna amplifier (mostly 50 Hz mains hum) and output (PC_OUT) will be connected to the PC's sound card. VCC is 5 V (from 78L05).

I tested this circuit and it behaved strange – the output was severely distorted. When tracking down this „strangeness”, I came out with the following oscilloscope screenshot.

Yellow trace is the input (FILTER point) fed from a signal generator. Blue trace is the signal on pin 1 (IC301A's output – a voltage follower that I want to use to create a bias level for inputs of two other opamps).

Pin 1's DC voltage is correct, but it has ripples that correspond to the input signal (they're phase-shifted, most probably because of the RC network in the first, high-pass stage, but they correlate with the input signal's amplitude and frequency).

P301 is shorted during testing, and PC_OUT is disconnected.

Other observations:

• Removing R304 removes the problem
• R303, R304 and R305 were 1 kΩ first (and C302 and C303 47 nF), but it didn't make any difference
• VCC is stable and decoupled (and without any ripple)
• Voltage follower's input (IC301 pin 3) has a nice DC voltage on it

What can be wrong here?

Kim Christensen:
Possible crossover distortion from the LM324... Try putting a 10K resistor from pin 1 of IC301A to ground or VCC.

I was going to say the same thing. The first opamp will have trouble keeping it's output at part rail because of crossover distortion in it's output stage. The output stage driver has to swing 3xVbe to keep the output steady and it might not swing fast enough. It is conceptually the same as having backlash in a mechanical system. Try an opamp that is specified for audio usage.


Unfortunately adding a 10k resistor didn't change anything. Do you recommend some other amplifier that might be better suited for this purpose (and can work from a single 5V supply)? LM2904 would be OK?

And, more importantly, is there a way spot such problems in the future? Is there a parameter of LM324 that would suggest that it will exhibit such crossover?

LM2904 is LM358, a dual LM324. All have this problem and they are they only common opamps with this problem. Sometimes it's documented in the "application notes" section of some datasheets or sometimes it's not, it's a well known quirk at any rate.

Try 1kΩ if 10kΩ doesn't help, or maybe a big-ass electrolytic on the output. Capacitance normally makes opamps unstable, but sometimes really heavy loading with some ESR actually works out in practice, YMMV. Note that a cap will only reduce the glitches, not really fix the problem. However, it's more power efficient than class A biasing the opamp.

As for alternative chips, pretty much anything is better than LM324/358. But most old opamps don't work very well at 5V and modern ones you may not have in stock. Some MCP6004 could be an OK upgrade for LM324.


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