[sdiy] OTA offset in Transient Generator

[François CHAUVET]

Hello all, 

This is my first post here... I am testing a “transient generator”, that is,
basically, an attack-decay envelope generator. The classical circuit has the
“feature” that the level at which the “attack” segment ends is largely
arbitrary, and not worth being made a voltage-controlled parameter. 

So I chose to implement the smoother function of time, namely y(t) =
t.exp(-t), up to some time constants of course. The differential equation is
easy to obtain, and the resulting circuit is a simple second-order one, very
similar to a SVF but with resonance factor = 2. 

Everything goes okay when I use “ideal” OTAs (in LTSpice -- I have no lab
currently, did I mention?). But when I use “real” OTAs, be it CA3080s or
LM13700s, they seem to have an intrinsic offset that prevents my envelope
from going to zero at the end of the “release” phase. 

Does anyone know if this is real, or just a simulation approximation? Since
the circuit is necessarily DC-coupled, this offset cannot be trimmed off
(sadly). 

Thanx in advance for any advice, 

-- françois

 


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