phasers

[Martin Czech]

SNIP
> of the first inverter (Va), and then compared these voltage
> to the those of the other inverters (Vb, Vc, Vd,...).
> 
> What I found was that at output voltages lower than approx.
> 3V, matching was fairly bad. With Va = 2V, for example, one
> of the other outputs was at around 1.1V, and another at 2.3V.
> 
> Then, as I increased the reference voltage above 3.5V, all
> outputs seemed to be in perfect match, all the way up to
> around 14V, where the op-amp's input range starts to affect
> the circuit.
> 
SNIP
 
Very good, i like measurements!

Well, my first thought is that the single MOST in those hex inverters
are certainly not made for matching, i.e. no common centroid layout
etc. One of the most critical parameters to match is FET threshold
voltage. I know: in the early days of MOST technology the threshold
voltage was very difficult to design into the process, and it is still
one of the most critical parameters to watch in a modern fab. It could
be that the threshold voltage is in the range of 2-3 V.  This would
mean that applying voltages <3V would have lead the FETs to
subthreshold behaviour, in this case matching is very difficult, even
if it is "built in". Far away from the threshold these effects are not
of so much importance, the matching gets better, now it is beta that
has the most influence on current.  Maybe this is an explanation for
your measurement results.

m.c.

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