[sdiy] VCO Linear Voltage

[ASSI]

On Thursday 19 November 2009, David G. Dixon wrote:
> Am I missing something here?

Possibly this:
http://www.ece.mcgill.ca/~grober4/ROBERTS/PUBLICATIONS/JOURNALS/1990/Current_Conveyor_History.pdf
...or maybe you got confused as to what the inputs and outputs are in 
this case.

To summarize, a current conveyor has three terminals: X, Y and Z.  X 
admits any current (input impedance is ideally zero) while following 
the voltage at terminal Y (in other words, the voltage difference X-Y 
is ideally zero).  Y does not admit any current (input impedance is 
ideally infinite).  Current impressed at X is conveyed to Z with 
ideally infinite output impedance (i.e. the current does not depend on 
the voltage at Z).

This can be viewed as either an ideal MOS or bipolar transistor with:
X := S := E
Y := G := B
Z := D := C
where the G-S or B-E voltage is zero, the G or B current is zero and 
the S and D respectively the E and C currents are exactly the same 
(possibly of a different sign depending on how you count them) 
independent of the potential at D or C.

So, the 2164 would send the current into X, Y would be grounded to 
keep X at ground also and Z would be connected to the LM13700 and be 
biased at whatever voltage suits that part of the circuit (in bipolar 
transistor speak: a common base amplifier).  You would want a 
realization that allows for bipolar currents to be conveyed, of 
course.  You will find that an opamp with a differential output can be 
easily configured to do exactly this (the output current is inverted).  
More intriguingly you will find exactly the configuration you are 
looking for in the disguise of the Burr-Brown (now TI) "diamond 
transistor" OTA, albeit it needs a smaller supply voltage than you are 
working with.  It's advertised as a voltage controlled current source, 
hence the OTA moniker, but it is in fact a venerable current conveyor 
(see SBOA072 or the circuit diagram for the OPA660, which is much more 
clear than the equation waffle in the data sheets).


Achim.
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