W
Walter Harley
- Jan 1, 1970
- 0
Something I was working on recently got me wondering:
Suppose I want to have an adjustable resistance, that will be linear (that
is, ohmic) for AC voltages less than 1Vp and audio frequencies. The
resistance needs to vary from about 50k to 5MEG, controlled by some other
(analog) voltage or current. The relationship between control signal and
resistance does not need to be particularly linear or repeatable from part
to part.
So far, easy. Now, the slightly-harder part: the power supply is a pair of
3V lithium cells, and to maximize battery life it would be desirable to draw
< 100uA or so.
If it weren't for the current spec, I could use an FET optocoupler like an
H11F3, I think (not sure if they're ohmic up to 1V at lower resistances, but
the datasheet seems to suggest that at these high resistances they'd be
okay). But to get one of those down to 50k takes >1mA through the LED.
With higher voltage rails I could just use a JFET. But with 3V rails,
wouldn't the Vgs, and the unit-to-unit variation in Vgs, bite me? With only
2V between Vss and max negative signal I'm not sure I can keep the channel
from turning on.
I suppose one idea would be to use a charge pump to generate a more-negative
voltage rail. Or are there JFETs that will work with these low voltages,
without needing to be individually selected for threshold voltage? Or maybe
a low-power MOSFET? Any other ideas?
Thanks for any suggestions!
-walter
Suppose I want to have an adjustable resistance, that will be linear (that
is, ohmic) for AC voltages less than 1Vp and audio frequencies. The
resistance needs to vary from about 50k to 5MEG, controlled by some other
(analog) voltage or current. The relationship between control signal and
resistance does not need to be particularly linear or repeatable from part
to part.
So far, easy. Now, the slightly-harder part: the power supply is a pair of
3V lithium cells, and to maximize battery life it would be desirable to draw
< 100uA or so.
If it weren't for the current spec, I could use an FET optocoupler like an
H11F3, I think (not sure if they're ohmic up to 1V at lower resistances, but
the datasheet seems to suggest that at these high resistances they'd be
okay). But to get one of those down to 50k takes >1mA through the LED.
With higher voltage rails I could just use a JFET. But with 3V rails,
wouldn't the Vgs, and the unit-to-unit variation in Vgs, bite me? With only
2V between Vss and max negative signal I'm not sure I can keep the channel
from turning on.
I suppose one idea would be to use a charge pump to generate a more-negative
voltage rail. Or are there JFETs that will work with these low voltages,
without needing to be individually selected for threshold voltage? Or maybe
a low-power MOSFET? Any other ideas?
Thanks for any suggestions!
-walter