[sdiy] OTA as Voltage Controlled Resistor

[Scott Gravenhorst]

music.maker at gte.net wrote:
>
>"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) 

OOPS -- one from V+ to out, one from out to V- ...  sorry...

>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
>________________________________________________________________________
>-- Scott Gravenhorst
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-- ScottG
________________________________________________________________________
-- Scott Gravenhorst
-- FPGA MIDI Synth Info: jovianpyx.dyndns.org:8080/public/FPGA_synth/
-- FatMan Mods Etc.: jovianpyx.dyndns.org:8080/public/fatman/
-- Some Random Electronics Bits: jovianpyx.dyndns.org:8080/public/electronics/
-- When the going gets tough, the tough use the command line.