WeAreAs1 at WeAreAs1 at
Wed Nov 10 07:54:41 CET 1999

In a message dated 11/9/99 8:28:01 PM, you wrote:

<<In the feedback loop of an op-amp this is quite a good

reference. The largest variation in a zener will be the

dynamic resistance (the change in zener voltage from

least current to maximum). By isolating a zener in the

feedback loop of an op-amp the current flow is held

constant. If you consider the positive terminal of an

op-amp connected to ground, and a zener diode

in the negative feedback loop with a resistor to -V.

The op-amp output will servo to whatever value will make

the virtual ground (negative input) the same as

the positive input (ground). That would be the zener voltage

with current controlled by the resistor to -V. If you

make this a resistor and a trimpot (with a value of

10% of the resistor) you can adjust the output

to an exact voltage.>>

This sounds like a useful little circuit configuration.  I wonder why I've 
never seen it in use?  I've never seen it in any synth or musical equipment 
schematics, having read hundreds of them over the last couple of decades.  
Was I not paying close enough attention?

Just to be clear on the circuit, the zener in the feedback loop is oriented 
with its cathode to the opamp's minus input and its anode connected to the 
opamp's output, correct?  Besides the resistor connected between V- and the 
opamp's minus input, and the zener in the feedback path, there are no other 
components, right?  What if you needed to get a negative reference voltage?  
I suppose that you'd connect the resistor to V+, correct?  What then, about 
the diode orientation?  Would you have to reverse it? (with respect to the 
positive voltage reference circuit)

Although it looks like it would work especially well when used as a precision 
voltage reference, could this also be used in a situaton where you might need 
to derive a particular alternate regulated supply voltage, if the current 
demand was low enough?  For instance, if your synth had only +/-15 volt 
supply, but you needed a -5 volt supply for a couple of CEM3340 VCO's, or you 
needed to power some 4066 analog switches with +/- 7.5 volts.  How much 
current do you think such a circuit could reliably source?  Would the type of 
opamp be the determining factor?

Michael Bacich

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