[sdiy] Capacitors in Moog ladder filter

[Antti Huovilainen]

On Tue, 27 Apr 2004, Czech Martin wrote:

> It was stated several times on this list, that different
> capcitor sizes will lead to different dc bias of the ladder,
> if tuned to the same frequency.

The control current will be adjusted, but the DC bias provided thru the
resistor string will vary very little (millivolts).

> Since the ladder and diff amp at the top will behave nonlinear
> for large signals, this can make a difference.

I've done analysis of the ladder circuit (expect a paper on it "soon"). If
the transistors are assumed ideal (infinite beta, no early effect,
perfectly matched), the control current & capacitors will only affect
tuning of the ladder. Non-ideal transistors may affect this, but I don't
know how much.

I can't say offhand about the effect on the differential amp, but IIRC the
ladder output voltage was independent of the control current (except due
to filtering of course). If you look at the DC case, local feedback within
each stage will cause the differential voltage over each stage to be same
as the voltage in the input differential amp.

Antti

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