[sdiy] improved dynamic range diffpair-vca

[Magnus Danielson]

From: np <np at planetarc.de>
Subject: [sdiy] improved dynamic range diffpair-vca
Date: Tue, 24 Apr 2007 23:08:18 +0200
Message-ID: <462E71C2.8060001 at planetarc.de>

Hi Nils,

> Hi list, and hello to some old-timers who remember me.. I've been away 
> from the list for several years....

It's been a long time! Welcome back!

> Is anyone familiar with this circuit here:
> 
> http://www5b.biglobe.ne.jp/~houshu/synth/Vca0302.gif
> 
> It's a long tailed pair with two extra transistors working as diodes. 
> Not the same configuration as the linearization diodes we have in some 
> OTA's. I gave it a try using a CA3046 transistor array, and it just 
> magically works. I can drive the input up to 50mVpp with low distortion. 
> Nearly twice as much as without the extra trannies.
> 
> To me that looks almost like a free meal. Where's the drawback?
> 
> And could someone please shed some light on why these "diodes" work and 
> what they do? They somehow linearize the input for sure, but I have no 
> clue what exactly is going on here.

It is actually quite simple. You require twice as much Vbe to acheive the same
current, the resulting arctanh function thus scale the input differently.
That's what's happening. The Ice current is the same for both diodes.

The obvious drawback is that you require additional trannies. You also want
them to be arrays such as 3046 for thermal coupling.

6 dB more is good. However, how can we make it take towards 10-20 Vpp signals?
How about some clever Vbe/Ice scaling or something? Such that Instead of doing
+ 1 diode-voltage you get + 200 or something. I am thinking in terms of tossing
in some resistors for the additional transistors. Using that and some
linearization and you should have a large-signal linear solution in your hands.

Cheers,
Magnus