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

[cheater00 .]

Hi Phil,

On Sun, May 5, 2013 at 12:05 PM, Phil Macphail
<phil.macphail at liivatera.com> wrote:
> 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.

That reference does come from a country that don't even remotely have
the capability of manufacturing electronics, and most likely don't
have any means of really marketing the idea.. The Chinese crank out a
lot of papers. I wouldn't put a lot of thought into the pickup rate of
this idea.

> 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!

I like your points. But (with my limited knowledge of semiconductors)
it looks like the z terminal (where the capacitor goes) has got a
low-impedance connection to the negative voltage supply but not to the
positive. And it kind of looks like i_n and i_p have a high-impedance
path connecting them So I don't know why the CMRR has to suffer? My
logic is very fuzzy, I know (I should apply for AI research..)


> 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.
>>> _______________________________________________
>>> Synth-diy mailing list
>>> Synth-diy at dropmix.xs4all.nl
>>> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>>
>>
>>
>>--
>>/Ove
>>
>>Blog: <http://blog.gg8.se/>
>>
>>"Here is Evergreen City. Evergreen is the color of green forever."
>>_______________________________________________
>>Synth-diy mailing list
>>Synth-diy at dropmix.xs4all.nl
>>http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>
>