So, my considerations are for operating a mosfet in linear mode, I need a voltage controlled resistor or some rigging of a mosfet which scales predictably in ohmic mode. Has anyone had any experience with rigging mosfets in this manner?
@jadaro
There are numerous ways of doing this, but they require lots of components,
potentially dual polarity stable power supplies, and some are not efficient
uses of electric power - all of these characteristics are inconsistent with
the design of the basic JT JouleThief. The best bet probably is some sort
of bipolar transistor series regulator.
Here's the thing -
these things should not stop you from setting up a
system that will behave as you want...and if what the system does,
looks good to you, you can reduce that operation of the system to simpler
components in a separate experimental step.
Too many experimenters don't do this, they want to combine learning
how a circuit operates with greatly reduce component setup, and then
they can't do both sides of this simultaneously. I hate complexity too,
but I hate end unit complexity and not intermediate development complexity.
So what I am saying is learn how control the circuit and battery charger
to do what you want separately from reducing component complexity. I'm
telling you, I think you are correct about this, and I think you can do it, and
maybe make an actual contribution on here.
---
The methods for linearizing a MOSFET are:
Putting a negative feedback-loop operational amplifier in front
of the MOSFET that has sufficient accuracy to deal with the mosfet
voltage gain of about one million. Operating a transistor in linear
gain mode is not efficient at all when the device is in intermediate
part of the transfer curve. Bipolar transistor are easier to control
because they have less numerical gain...but their gain is in the current
domain (I) and you often need to convert that to the voltage or
power domain to be useful.
If you need output signals with bandwidth of 30Khz or less, look up
"MOSFET switching amplifiers". These synthesize the required voltage
by switching on and off at very high frequencies. Because the mosfet
is either on or off all the time, power efficiency is increased.
A "digital potentiometer" is an easy way to get what you want but
you will need to design a digital front-end for it.
There are a couple of IC amps designed for ACG operation - Automatic
Gain Control, where a voltage input can act as a "volume control"
on the output. But most of the time VFC Voltage Frequency Control of
an astable oscillator is all that is required and even the lowly NE555 has
that. (on pin5 - Control Voltage)
:S:MarkSCoffman