AW: super nice transconductance amplifiers

[Haible Juergen]

	>I presume a lot of people are familiar with the CA3080,CA3060
and the 
	>LM13700/LM13600.  Does anyone know of any OTA's that have GOOD
signal 
	>to noise and have linearizing diodes like the 13700's?  I have
always 
	>had problems getting better than about 80dB s/n out of the
13700 and 
	>never really worried about noise on the CA3080's since I always
drove 
	>the input pair way out of linearity.  The linearizing diode
input 
	>does several amazing feats that I need, but no OTA's that I
have ever 
	>seen are all that great.  It must be a REAL transconductance
amp 
	>also, not one of the AD voltage controlled gain blocks.  It
must 
	>have continuously variable gain, not peicewise continuous or
stepped 
	>gain.  

80dB sounds much for a 13700 ... I thought it was slightly less.

Have you tried the CA3280? It has linearizing diodes, but in a better
circuit than the 13700.

I found the old SSM2020 VCAs very pleasant sounding (they had
linearizing diodes), but they are long gone.

The SSM2024's which are still available, don't have diodes (as far as I
know), but they are less noisy than a 13700 *with* diodes. So you get
a decent SNR *plus* soft clipping, which does it for me.

You might also try to build your own ota and linearizing diodes with low
noise transistor arrays. One pair for the for the diodes, one for the
variable gm pair. Omit the current mirror stuff and use an opamp
circuit for the back end, if you like.
(I saw circuits which used transistor arrays as linearizing diodes
in OB-X, ARP Quadra and EMS Polysynthi, so it should work.
But I haven't tried it myself yet.)

Paul Schreiber sells these 8pin - dual VCA chips from CEM.
I am quite sure that they also have gilbert input stages.

JH.