[sdiy] NPN pair VCF?

[Aaron Lanterman]

On Apr 21, 2009, at 12:07 PM, np wrote:

> Dan Snazelle wrote:
>> been noticing lately that just about every VCF out there uses a PNP  
>> pair.
>> is it possible to use an npn pair in a vcf?
>>
> I think that has to do with the fact that all the current controlled  
> gain elements (OTA's ect). are controlled from a current referenced  
> from ground or the negative supply.
>
> These inputs are current-suckers, and you need a current-source to  
> feed them. That can only be done with a PNP.
>
> You could use a NPN based expo-converter and turn it into a current  
> source via a current-mirror, but this way you'll end up having a PNP  
> pair in the current mirror, so nothing is gained.

If you use a basic four-transistor discrete OTA - Jurgen Haible's 2040- 
inspired circuit & the MOTM version of it come to mind - those want a  
control current drawn out of them, so for them you can use an NPN pair.

> For non OTA based filters such as a transistor ladder it should be  
> possible to use NPN pairs by just turning the filter upside down.  
> Haven't tried though.

Most transistor ladder filters I've seen use NPN pairs to begin with.

I have seen one with PNPs before, but I can't remember where now.

- Aaron