[sdiy] OTA as Voltage Controlled Resistor

[Scott Gravenhorst]

"Lanterman, Aaron" <lanterma at ece.gatech.edu> wrote:
>
>On Apr 24, 2013, at 3:33 PM, Justin Owen <juzowen at gmail.com> wrote:
>
>> The LM13700 data sheet shows a schem (Fig. 10) for a 'Floating 
>Voltage Controlled Resistor' - this seems to be a double-ended 
>circuit as opposed to the FET-based circuits (and the the LM13700 
>circuit in Fig. 9) which (from memory) are single-ended. > > My 
>questions... > > 1) Can the double-ended circuit be used to 
>replace a standard discrete resistor - i.e. is either 'end' 
>replacing a leg/pin of a resistor? 
>
>
>I've had students off-and-on over the years experiment with the 
>double-ended version. It never worked out quite the way they 
>hoped. 

Interesting.  I have to wonder if there's a way to analyze this particular
circuit to understand what applications can use it.  What I see (with my non-EE
eyes) is a bridge amplifier with variable gain controlled by Iabc of the two
amplifiers.  The "resistance" supplied would appear to me (again with my non-EE
eyes) to be coming from the fact that Iabc is controlling the output ampifiers'
output current, that is, the "resistance" is being created by the fact that
current flow through the OTA output transistors (a totem-pole so to speak) is
limited or controlled by Iabc.  But the totem-pole output structure is not just
a single transistor, it is two (one from V+ to out, one from out to ground) with
each OTA operating as a sort of potentiometer.  As such, I see this circuit as
not really "floating resistor", but sort of a simulation in some respects of
that.  Is my dual potentiometer view of this a possible analysis method?  I have
a feeling that the circuit works best when small currents are needed.  This
would keep the resistance of each totem-pole transistor to be close to the
resistance of it's "paired" transistor.  As such, I would expect a rather small
range of usefulness.

For the third time, I am not an EE, what I wrote above could be total
unmitigated bullshit so please be kind.  (c:

-- ScottG
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-- Scott Gravenhorst
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