[sdiy] Simulating SSM2164 circuits with LTspice

[David G. Dixon]

> * SSM2164 VCA cell
> .SUBCKT SSM2164 IN OUT CV
> * The two transistors in the input's differential pair have the same base
> * current, hence the same Vbe. The second transistor is referenced to
> * ground so IN is virtually grounded.
> V1 IN 0 0
> * Models 5k impedance for the CV input
> R2 CV 0 5k
> * Stability network
> R3 IN 1 560
> C1 1 0 560p
> * Current gain expression
> G1 OUT 0 VALUE={-I(V1)*PWR(10, -V(CV) * 1.5)}
> .ENDS

I'm no SPICE expert, but that looks about right.  I defined the current with
exponential gain as a "nonlinear dependent source" in Multisim, which
achieves the same thing.  The input and output are automatically at ground,
and I just added a 5k resistor from VC to ground for the input impedance.

The one thing is doesn't do is freak out if it is not terminating at virtual
ground, like the real 2164 does.  Hence, you will still have to remember
this when using 2164 in simulated designs.  Also, don't forget to actually
add the stability components to your real circuits!