[sdiy] Op amp for saw core VCO

[rsdio at audiobanshee.com]

On Oct 26, 2014, at 1:29 PM, rsdio at audiobanshee.com wrote:
> On Oct 26, 2014, at 12:56 PM, Oren Leavitt <obl64 at ix.netcom.com> wrote:
>> On 10/26/2014 1:29 PM, Tim Ressel wrote:
>>> So your standard sawtooth core VCO with the integrator and discharge FET. I am re-designing an existing product that uses a CA3130 as the integration amp and want to update it. What is a good modern amp for this application? Or if you prefer, what kind of amp is needed here? Precision? High speed? I have used an OP37 and an OPA602 with good results, but I am hoping for an inexpensive alternative.
>> 
>> High input impedance for sure, and a fast slew rate to keep the reset step sharp.
>> I'm with Ian on the OPA134.
> 
> If you're using an external FET for discharge, then I think you don't need to pay for exceptionally fast slew rate.
I thought about this a little more (after gentle inspiration from Ian) and realized that even though the FET is responsible for conducting the current that discharges the capacitor, the op-amp output would still need to follow that voltage change very quickly, and thus the slew rate of the op-amp would still be a potential limiting factor.

Brian