[sdiy] Instability in an OTA-C LP cell

[Paul Perry]

TL072 can lock up - phase reverses when the input common mode
limit is exceeded. See data sheet.

paul perry Melbourne Australia

----- Original Message ----- 
From: "Olivier Gillet" Subject: [sdiy] Instability in an OTA-C LP cell


> Hi all,
>
> Today I observed something strange with an OTA-C low-pass cell I was
> playing with (half LM13700 + half TL072 as a buffer ; 220/10k at the
> input ; 1nF ; powered by +/- 5V). For large inputs (2.5V pp, -> 55mV
> pp at the OTA input), and high cutoff values (> 10kHz), the filter
> "crashes" in a state where the output is either the positive or
> negative clipping value of the op-amp (a bit less than 4V for the
> TL072). There's no resonance - only one 1-pole cell.
>
> I accidentally found something that appears to fix the problem without
> impacting the sound - a 47k series resistor at the non-inverting input
> of the op-amp (values below 20k only increase the cutoff frequency at
> which the problem starts to occur):
> http://dl.dropbox.com/u/612135/ota_buffer.png
>
> Has anybody any idea about what's going on? Why would the fix work (or
> appear to work)?
>
> Thanks!
> Olivier
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