[sdiy] WP LM3900 VCF

[JH.]

Look at it the other way round:

The LM3900 is a pnp input opamp, only that the
input transistor pair is missing. 8-)

So you have to add it externally - at the benefit of full freedom
over the input transistors' tail current.

Now look at the typical OTA + cap + buffer filter stage.
It's just an ordinary opamp with phase compensation cap,
and with a variable bias current for its input pair.

In fact, there has been an integrated quad opamp with
variable biasing and internal compensation, the LM346,
which has been used as a 4pole "OTA" VCF in Farfisa
synthesizers.

It's all the same thing, really. (Except that the 346's
bias current must be soooo small that you don't really
get low cutoff frequencies with good performance.)

JH.


> It's a swindle, really!
> I once spent ages tryng to see how a Norton amp 
> could be made into a filter <blush>
> The LM3900 is just used as a current-voltage converter, it's 
> matched pairs of 390s's that is doing the actual VCF
> so far as I can see, looking at this schematic
> (thanks again to Don Tillman)
> http://www.till.com/arptech/pdf/4072.pdf
> 
> paul perry (up past his bedtime in Melbourne Australia)
>