Using JFETs as variable resistors

[cipi-cips]

siliconix fet data book

I have a problem I need to build common emitter amplifire but instead of regular resistors that creates common emitter amplifer I need to put JFET as variable resistor



I think it is something like this but I am not sure, I tested this and it is not working lots of oscilation at output.

At the output there should be a signal that is amplified and there if you attach some voltage on the JFET (all 4) you should get variable amplifire

 

[LvW]

Re: JFET

Here is my recommendation:
Before using JFETs as resistors in an amplifier you should examine the principle and restrictions of using JFETs for such a purpose.
For example, the operating range is limited to the ohmic region and the source potential should be constant and not signal dependent (preferrably of zero volts).
It is clear from your circuitry (right) that it cannot work.

 

[FvM]

Re: JFET

At best, a JFET in ohmic region can be regarded as an approximation of a resistor for sufficient low signal voltage. A linearization circuit can extend the signal voltage range, reducing the influence of the additonal quadratic term that's generally determining the FET characteristic.

For the shown circuit, I need a plausible motivation not to dismiss it as pure nonsense.

 

[flatulent]

Re: JFET

One common circuit uses a jfet as a current source load for a common source amplifier jfet. I have seen this circuit used in a high performance phono amplifier many years ago.

 

[LvW]

Re: JFET

At first, you really should know and clarify which properties of the gain stage have to be changed via JFET - and over which tuning range:
gain or bias point or input impedance or quiet current ? Or all of them ?

 

[cipi-cips]

Re: JFET

well hmm the problem is that I got this project and I need to build it and all what I get all explanations I have I wrote it in first post, so I don't know I wanna hear your ideas ??

 

[LvW]

Re: JFET

well hmm the problem is that I got this project and I need to build it and all what I get all explanations I have I wrote it in first post, so I don't know I wanna hear your ideas ??

OK, there was somebody who gave you this project - but he should have given you a clear advice what to do !
Most important question: Are you required to replace all resistors or only some ?
Because it makes really no sense to replace each of the 4 resistors by JFETs - independent on the question if this would be technically possible.

 

[cipi-cips]

Re: JFET

yes the problem is that I need to replace all four resistors so I realy dont know what to do I have tryed with only one FET and I manage to get good results but with all 4 hard

 

[LvW]

Re: JFET

OK, if you are required to use 4 JFETs this is, in principle, possible. Perhaps it is a rather good exercise to study the properties of JFETs.
But there are some points to pay attention to:

1.) Both JFETs in the collector and in the emitter path have to work as dynamic resistors. That means they have to cope with ac signals and must cause only minor distortions. Consequence: operation in the quasi-linear region of the JFET output characteristic. This restriction limits their operating voltage range remarkably..
And pay attention to connect the source pin to a fixed potential (ground resp. Vdd).

2.) Both JFETs which are used for biasing the BJT can be operated also outside their linear region since they are used only as "static resistors" - which means they only have to realize a fixed dc voltage-to-current ratio.

3.) My recommendation: Use any simulation program at first.

 

[FvM]

Re: JFET

I still miss a plausible motivation for the circuit. The FETs should serve a purpose, but I don't see any possible.

In most cases (or should I say in any case?) the non-linear FET properties would dominate the linear (resistive). So you basically can modify e. g. Ugs or better (Ugs+Ugd)/2 of each FET and it results in a modified amplifier bias point. But it doesn't behave really like a resistor.

I guess, the idea could be to create a demonstration circuit, that visualizes the effect of resistance dimensioning in a transistor amplifier. I once designed a demonstration circuit for a lab I had to supervize, but it had simple pluggable resistors (and an on-chip thermostat for the transistor, additionally). If the circuit purpose is of this kind, digital potentiometers or analog switches may help, but FET as variable resistors aren't a good solution, I think. Also paired LDR with a resistance controlling feedback circuit may be a solution.

 

[Eugen_E]

Re: JFET

The circuit in the attachment - the worst designed by me
It doesn't make any sense, Q1 is almost saturated, the input impedance is low because of the negative feedback, it distorts at high input amplitude, etc. For small signal the voltage amplification is about 15.

For your particular application you should find out which resistors should be replaced with JFET. Egg: collector resistor replaced by jfet configured as current source. Emitter resistor replaced by jfet controlled resistor to control the amplification depending on gate voltage, etc.

 

[FvM]

Re: JFET

The usable voltage range for a FET as programmable resistor is a few 100 mV up to a few volt, depending on the intendended level of linearity respectively distortions. By keeping Vg=(Vds+Vgs)/2 constant, the quadratic term in FET charateristic can be eliminated, creating a symmetric behaviour. A simple linearization circuit for a ground referenced FET is like this:

The voltage/current characteristic with Vg as a parameter. The resistance is said to follow a hyperbolic function Rdson•Vgsoff/(Vgsoff-Vg).


To control a floating FET resistor, a differential amplifier would be needed.

P.S.: I didn't mention explicitely, that a JFET with a high Vgsoff is preferable for a maximum usable voltage range. BF245C isn't bad in this regard. However, individual Vgsoff (and Rdson) variations have to be considered and would require individual adjustment for each variable resistor. This is the other fact apart from limited voltage range, that practically restricts application of this technique.

I also didn't show the characteristics for a FET without linearization. You can believe that it's simply bad and inappropriate for most applications..

 

[FvM]

Re: JFET

To alllow a comparison, I also plotted the simulation of BF245C voltage/current characteristic without linearization. Vgs is identical to Vg in linearized circuit (-0.5 to -5.5 V)

 

[cipi-cips]

Re: JFET

I have tryed to design the circuit in simulation program (Multisim Electronics Workbanch ) but I've failed hmm does anyone of you know how to make the circuit, I have tryed with both schema that you posted ???

 

[FvM]

Re: JFET

I wasn't using Multisim, but it should basically do as well. What problem did you experience? Posting the *.ms10 file may help others to duplicate the simulation.

 

[cipi-cips]

Re: JFET

First I have draw a circuit like Eugen_M posted and I got errors that circuit is invalid hmm then I realised that when you disconnect connection between ground and gate from FET the circuit is working (no errors) but at the output I got same signal as input, then I have drawn circuit with FET's like you wrote with 1M resistors and power supply for all FET's and I didn't get anything at output ?

 

[FvM]

Re: JFET

The circuits can be simulated in any SPICE simulator, e. g. PSPICE, Multisim. Possible problems as you reported are not related to the specific circuit, I think.

My FET circuit with linearization is only showing the variable resistor element with it's control voltage. It may be used for the ground referenced FET's as is, but would need a differential amplifier if used floating respectively Vdd referenced for the two upper FETs. A final circuit has to consider the source of control voltages.

The simulation showed however, that the linear range is limited, so the supply voltage shouldn't be too high. I still doubt, if the circuit is meaningful at all.

 

[cipi-cips]

Re: JFET

hmmm odd, well I rely don't have any idea how to create this circuit I mean how to build that amplifier with FET's as resistors, hmmm hmm what do you think do you have any idea how it should at lees look like ??

 

[LvW]

Re: JFET

.... I still doubt, if the circuit is meaningful at all.

I completely agree with FvM as far as a practical meaning of the circuit is concerned.
However, I think the intention behind this project is probably to become familiar with FETs and their characteristics as well as with some restrictions and specific application hints. And, up to now, the discussion has shown that it works.

 

[FvM]

Re: JFET