# Jfet as HV resistor

I want a voltage controlled resistor for high voltages for a project I'm working on and using a common emitter/source transistor to shunt current into ground in order to get a desired voltage is not working well in my circuits because it is generating its own current. I need a true, end to end voltage controlled variable resistor with input voltage through the top and load on the bottom.

I've heard Jfets can act as resistors but in my simulations the source voltage just seems to copy whatever the voltage is on the gate. What am I doing wrong? Can you not use a jfet as a end to end resistor?

It should be noted that I plan on using a digital feedback loop to negate any non-linearity and that I will use a SIC Jfet for the high voltages.

To clarify, I effectively need a drop in resistor replacement that I can control with voltage

• "...in order to get a desired voltage" - if the goal is to get a particular voltage, why do you need a voltage controlled resistance? – Bruce Abbott Feb 22 '16 at 3:20
• FETs (or BJTs, for that matter) do not act as resistors, i.e. Is is not going to be proportional to Vsd, although the name "transistor" is combined of the words "transfer" and "resistor". A transistor is much more like a controlled current source than a resistor. FET can be used as a voltage-controlled resistor for small-signal applications, but that is not what you need. It sounds like you're trying to do something very simple if a very roundabout way. – biggvsdiccvs Feb 22 '16 at 3:44
• I've heard Jfets can act as resistors but in my simulations the source voltage just seems to copy whatever the voltage is on the gate. What am I doing wrong? Actually, nothing. With the load tied from source to ground, The FET is a source follower. Look it up... Questions: 1. What's your source voltage, your load resistance, and what kind of voltage variation do you need across the load? 2. Does your load have to be high-side driven or can it be low-side driven? 3. How about adding a schematic to your post so we'll all be on the same page. 4. are we talking MOSFETs or JFETs? – EM Fields Feb 22 '16 at 10:15
• @BruceAbbott: If the goal is to drop a particular voltage across, or pass a particular current through the load, using a fixed voltage source, then a variable resistance in series with the load and the source will allow that to happen and a voltage variable resistance will allow it to be done automagically, with feedback, which the OP plans to use. – EM Fields Feb 22 '16 at 10:24
• Sorry I should have been a little more clear, effectively what I need is a drop in resistor replacement, not necessarily a voltage control. – coinmaster Feb 22 '16 at 13:30