[sdiy] Does this look like phase reversal?

[Roman Sowa]

Youre driving TL074 out of working range, and you know it. Ignoring 
datasheet's limits intentionaly is one way to get into troubles.
P-FET inputs of TL074 have no space to bias properly at such a low 
voltage, so they simply conduct from source to gate, polarized like a 
diode. So this is where you get 0.88V at 100k resistor.

LM324 is designed to work with inputs going down to negative supply, 
TL0x is not. Use LM324 or any rail-to-rail input opamps.

Roman

W dniu 2017-09-07 o 10:23, Guy McCusker pisze:
> Hello,
>
> I'm looking for some help troubleshooting the simplest circuit I've
> built in ages, a CGS28 sequential switch.
> http://www.cgs.synth.net/modules/cgs28_seq_switch.html
>
> There seems to be trouble in the input stage, which is a TL074
> configured as four comparators. I've been poking around the clock
> input (pins 3, 2, 1 on the schematic.)
>
> The TL074 is powered from +12V and 0V. Pin 2 (-ve input) is biased at
> 1.55V and measures fine. With no input CV, pin 3 (+ve input) is at
> 0.88V; I can't see where that is coming from and seems way too high to
> be input bias current with the 100k resistor to ground. (It goes away
> when I remove the IC from the circuit so it's not coming from an
> obvious construction error elsewhere.)
>
> With those inputs, the output on pin 1 sits at about +11V. But it
> should be negative i.e. near 0V, so something is wrong.
>
> When I apply a 0-5V square wave to the input, I can see it clearly at
> pin 3, and the output goes between 11.4V and 11V, with very narrow
> spikes to about 0V at the transition points.
>
> Does this sound like phase reversal in action? Or something else? And
> would that explain the apparently excessive input bias current as well
> as the weird outputs?
>
> I've tried a few different TL074s from different batches, with similar
> results, though the actual output voltages differed a bit. i noticed
> that the TL074 data sheet specifies common mode input down to the
> negative supply + 4 V, so the inputs here are certainly out of range.
>
> Ken Stone originally specified an LM324 here which should be ok with
> input down to 0V. Should I try one of those?
>
> Alternatively I might have messed something else up --- if you have
> any ideas I'd love to hear them.
>
> Annoyingly I won't be able to tinker with this for a couple of days
> but the benefit of your expertise will be much appreciated.
>
> Cheers,
> Guy.
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