[sdiy] Using both OTAs in 13700 as one OTA

[Magnus Danielson]

Aaron,

On 02/04/2016 04:28 AM, Lanterman, Aaron wrote:
> Suppose you only needed one OTA to be a VCA, and you have a 13700 handy, which has two OTAs in it.
>
> Instead of using just one OTA, and leaving the other idle, what about a scheme like this:
>
> Let’s call the OTAs Negator and Positoria.
>
> Take the voltage input Vin and run it through the usual divide-down resistor divider, take that voltage and attach it to the negative input of OTA Negator,  and also attach it to the positive input of OTA Positoria. Tie the other inputs to ground.
>
> Take the current output of OTA Positoria, run it through a resistor of R ohms to ground to generate a voltage, call it V+, and attach that voltage to the positive input of an op amp.
>
> Take the current output of OTA Negator, attach it to the negative input of an op amp, and run a resistor of R ohms from the op-amp output to the negative input.
>
> The op-amp output is
>
> Vout = 2 R gm Vin,
>
> Where gm is the transconductance gain of the OTAs, assuming they’re the same.

I would wire up a normal diffrential op-amp setup there.

> I was thinking this might help cancel out the control signal bleed through, and might also have some SNR advantages. Or, there could be disadvantages that would counteract those possible advantages.

Bleedthrough? Maybe to some degree for the common mode inbalance, but 
lack of nulling biases doesn't help. Trying to create a better OTA bias 
situation would help more, such as both inputs seeing the same impedance 
and then a trimmer for offset compensation helps.

SNR, sure, you doubled the transistor area for input. This gives you 3 
dB improvement.

> Anyway… does this sound like a reasonable thing to do, or a not so good thing to do?

I would trim the sails somewhat differently if I would do it. I would 
rather figure out what a second OTA could do for me in the design.

Cheers,
Magnus