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

[cheater00 .]

Hi Uwe,

On Sun, May 5, 2013 at 1:06 PM, Uwe Schueler
<uwe.schueler at uni-tuebingen.de> wrote:
>> Will this circuit be any good for anything but high frequency stuff?
>
> I'm afraid you are right; modern (>90's) OTA/Gm-cell circuits have been
> developed in CMOS for high speed low power LSI chips. Bad chance to see
> a 14pin LMxxxx audio-CDTA ever.

Regarding CDTAs, OTAs, Current Conveyors, and other cool analogue
building blocks.. I'm surprised, with the miniaturization we have
nowadays, and how inexpensive pick&place is in china (even for small
runs!) that people don't develop their own ICs as hybrid boards. Is
there seriously a reason for this not to be happening? I understand
that it's a bit difficult to match things, but that's a hurdle, not an
impossibility... I'm surprised that in a world with billions of bright
minds there isn't even ONE project doing one or the other. And we're
talking about audio applications here... so exactness is much less
important than being able to play around with the circuit.

>> Re: Fontana bridge. Why would you need that? It seems like the purpose
>> of the Fontana bridge over the usual opamp/transistor and resistor...
>
> why an external transistor for a current source ? If you don't need it as
> expo and currents are <20mA, opamp-only current sources are nice.
> I often us a NIC as current source for sawtooth/triangle oscillators,
> because with 1 opamp and 4 resistors you can get a current source
> plus a buffered signal output !
> BTW: for me the Fontana bridge looks just like a NIC variant:
> https://picasaweb.google.com/Kulturgueterschuppen/Tech#5874424814629940642
> Can someone explain the benefit of a 2-opamp fontana-bridge over a 1 opamp
> NIC (driven by source with Ri=0 )? ( in a few words please, I have a
> higher-math-allergy ;-) )

Interesting point about the Fontana being bridge, however I seriously
don't think this is how it works! Z_p is parasitic impedance, it's
always in parallel with Z_s! You can't just manipulate the circuit
like that.. :-) So yeah, the Fontana Bridge is similar to a negative
impedance converter, but cannot be replaced by one in the way you
suggest!

Cheers,
Damian