[sdiy] OTA to voltage-control resonance?

[Antti Huovilainen]

On Mon, 31 Aug 2009, cheater cheater wrote:

> Thanks a lot Antti, that was very helpful of you. I can now understand
> the papers mentioned here better.
>
> D.
>
> On Mon, Aug 31, 2009 at 12:14 PM, Antti Huovilainen<ajhuovil at cc.hut.fi> wrote:
>>
>> [...]
>>
>> The opamp makes sure that output of second OTA half is always equal input
>> signal. Since the outputs of both OTA halves are linearly related, the
>> circuit output must be linearly scaled version of the input signal.

I simulated the approach in spice and plotted the results.
47k/220 input divider, 12k load resistance, ~1mA reference current for 
first OTA, 33pF extra compensation capacitor for the opamp.

Here's 1 kHz 10V input:
1x gain   4.8% vs 0.04%
0.5x      5.2% vs 0.7%
0.1x      5.6% vs 1.2%
0.01x     5.7% vs 1.4%

and with 1 kHz 5V input:
1x gain   1.3% vs 0.005%
0.5x      1.5% vs 0.2%
0.1x      1.6% vs 0.3%
0.01x     1.6% vs 0.4%

With real devices the results may not be as good as the matching is not 
exact. It's obvious that changing Ictrl does reduce the effectiveness of 
the method significantly although it's still an option worth considering.

Antti

"No boom today. Boom tomorrow. There's always a boom tomorrow"
   -- Lt. Cmdr. Ivanova