[sdiy] Forget the OTA, find me a CDTA!

[Phil Macphail]

I think the CDTA has been made obsolete by the op-amp. The first reference
cited on Wikipedia is ten years old - long enough to find its way into
designs if it was of any use. The use of replica currents (one for
feedback, one for feed-forward) isn't desirable for a low-power solution.
At fine process geometries the differing voltages on the output nodes will
result in poor current matching so linearity will be compromised, and the
grounded capacitor will be a disaster for common-mode rejection so don't
expect to find it an your cellphone anytime soon. Far better to use
switched resistors in a conventional op-amp design, where the higher
open-loop gain will improve the common-mode rejection.
Of course none of this is relevant to synthesiser circuits, but there is
nothing the circuit offers that a gm-C filter can't do better if you
choose your impedances well. It should be possible to emulate the CDTA
cell with both OTA's in an LM13700 if you put the capacitor on the
feedback path if you really wanted to try it.

Sorry for the negativity, I'll make my next post more positive, promise!




On 05/05/2013 03:48, "Ove Ridé" <nitro2k01 at gmail.com> wrote:

>Will this circuit be any good for anything but high frequency stuff?
>Is there any information of how the circuit is constructed inside?
>
>Re: Fontana bridge. Why would you need that? It seems like the purpose
>of the Fontana bridge over the usual opamp/transistor and resistor way
>of generating currents, is to deal with impedance issues in the load
>or the wires leading up to the load. I don't see how this would be
>necessary for audio frequencies, on a short stretch of PCB trace,
>going into a rather well behaved piece of silicon.
>
>On 4 May 2013 11:34, cheater00 . <cheater00 at gmail.com> wrote:
>> Hey guys,
>> just stumbled upon this little novelty item. Apparently the
>> not-available-anymore OTA has been obsoleted by the not-available-yet
>> CDTA:
>>
>> 
>>http://en.wikipedia.org/wiki/Current_differencing_transconductance_amplif
>>ier
>>
>> 
>>http://www.edn.com/design/analog/4405810/The-Current-Differencing-Transco
>>nductance-Amplifier--CDTA-
>>
>> It seems like it's making a case for itself, so who knows.. it might
>> find use in future applications and start mass production.
>>
>> In other news, I've come across this simple-looking voltage-to-current
>> converter based off two op-amps:
>> http://en.wikipedia.org/wiki/Fontana_bridge
>> it's supposed to be a new development.
>>
>> That's it for this episode of the Jetsons!
>>
>> Cheers,
>> D.
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>
>
>
>-- 
>/Ove
>
>Blog: <http://blog.gg8.se/>
>
>"Here is Evergreen City. Evergreen is the color of green forever."
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