[sdiy] op amp based filter

[JH.]

This will work indeed.
Run the opamp with rather high gain (set with feedback resistor divider) to 
bring the bandwidth down, and divide-down the input signal with a similar 
resistor divider to get unity gain overall.

No idea why you would want to do this with a 741, or any other opamp of 
fixed bandwidth - it only makes a fixed filter at the price of high noise.
But use an quad opamp with programmable bias current for its input stage 
(like the now obsolete LM346), and you can build an OTA-based LPF without 
using OTAs. The opamp's first two stages act as OTA, the internal 
comensation cap acts as integration cap, and the opamp's output stage acst 
like the unity gain buffer in the classic OTA filter.

This is still noisy, tends to have a limited lower cutoff range because of 
the tiny, tiny internal capacitors, but has been used in some Inatlian and 
Russian synthesizer circuits AFAIK.

Basically, I know of 3 variants to get to the same basic OTA-filter 
structure:
1) Using an OTA and an extra buffer (the usual way)
2) Using a Norton amplifier (LM3900) and an extra input stage for variable 
gm (ARP did this)
2) Using an opamp that already has variable gm for its input stage (LM346)

JH.


----- Original Message ----- 
From: "cheater cheater" <cheater00 at gmail.com>
To: "synth-diy" <synth-diy at dropmix.xs4all.nl>
Sent: Saturday, July 24, 2010 6:07 AM
Subject: [sdiy] op amp based filter


> Hi guys,
> if I took an op amp with a very low GBW, and set the gain so that the
> bandwidth becomes the cutoff frequency I need, could I make a low pass
> filter out of that? I would need to make up for the gain before and
> after that op amp (with op amps too, but ones that are much higher
> bandwidth of course), and I would probably have to chain several such
> modules (say 4-8) one after another to get a strong effect, but is the
> basic idea correct?
>
> The only 741 datasheets I could find mentioned a GBW of 4MHz, so
> that's a bit too much - to have a cutoff of 4 Hz you'd need to have
> gain of 92 dB, which could be a bit too much to ask of the system
> without drowning in noise. I've noticed Digikey has a lot of op amps
> with a very low GBW, even down to 2.7 kHz.
>
> Any ideas are welcome!
>
> Cheers,
> D.
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