[sdiy] Using both OTAs in 13700 as one OTA

[Michael E Caloroso]

Watch the offsets at the input pins.  They vary with iBias and can't
be trimmed out.

OTAs are noisy.  Paralleling them would make a noisier system, it
would not be differential.

MC

On 2/4/16, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:
> 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
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