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

[Uwe Schueler]

hi Damian
> ... I'm surprised, ... that people don't develop their own ICs as hybrid
> boards... I'm surprised that in a world with billions of bright
> minds there isn't even ONE project doing one or the other...
belong to the other group ( not so bright minds) and have done it ;-)
a thick film hybrid audio attenuator,3 decades ago...but let's talk  
about today:
I still have  2 reels = 6000pcs of BC846BDW1T1 (dual npn in SOT-363)
thanks to ebay the price for all of them was lower than for 1 pcs. of these
obsolete vintage matched duals ! So I'd like to make a small DIL module with
2..4 of them + 0805 NTC on the top side an some 1206 heating resistors  
on the back side. The remaining problem is how to select them ? :) But  
that's a new topic...


> 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!
I do not suggest to replace the Fontana-bridge with a NIC. The  
original app. was
compensating a parasitic Z, but for Synth DIY/audio stuff this might be not so
important, as Phil mentioned. He said there is no benefit in using a Fontana-
bridge instead of a simple opamp + transistor current source.I  
answered such a current source could also be done by NIC and I don't  
see an advantage of a Fontana-bridge in such application like  
generating linear voltage ramps in VCOs,
ADSRs...
In my NIC picture Zp is controlled by a voltage source with impedance Zvs=0,
so Zp and Zs ARE still parallel. All impedances Z1,2,3,p,S stay in the  
original positions, so no "manipulation" there - but I agree,  
combining the diffamp with the opamp and amputating the Vin pin is a  
severe manipulation :-)
cheers u~