op amp as schmitt trigger problem

[mike page]

Hi all.

I'm designing a table top synth that runs on 9v single supply.
It needs a high audio rate 'oscillator in', and the circuitry down stream requires a nice >7v+ noise free square to work properly.

I can get it working by using an op amp as a comparator then a 40106 to clean it up, but ideally i'd like to use the other half of the op amp as a schmitt trigger.

Ive followed schematics applying positive feedback & tried lots of different resistor values and lots of different op amps, but nothing seems to work like the 40106. any advice?

Cheers

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[elektrouwe]

hi Mike,you probably need a rail-to-rail opamp, or at least one swinging close to GND. A TL072 will not work, maybe a LM358 or TLC272.
But if you wantl to clock a CMOS counter, these can be too slow.
I would use a LM393 or similar comparator and add positive feedback to make it a Schmitt Trigger (you don't need an extra one)

[dk]

Just out of curiosity, why would you want the second op-amp in the chain to be a Schmitt trigger? Isn't the point of adding hysteresis to avoid the first comparator from improperly changing state on a non-digital input signal? The output of your first comparator should be (close to) digital, in which case the second op-amp shouldn't be seeing a signal that would otherwise require hysteresis... unless there is something else at play?

Is the high audio rate oscillator also running off your single supply? If so, and the output is unipolar, you could just run straight into a 40106 without using an op-amp... unless you're set on using the op-amp and not using a 40106...

[mike page]

Cheers for the replies. My thinking was the comparator would boost any signal to near rail to rail, then a schmitt trigger would give it a nice clean up.

after a little more experimenting i found a simple single transistor buffer does the job just fine!