4 FET Wheatstone Bridge - Opinions Please

[Trent]

Hello,

I am currently working with a Wheatstone bridge containing 1 JFET to sense static fields, and the circuit is working. However, having 4 sensors in a bridge gives a signal 4 times as big. I think I have found a way to connect 4 JFETs so they are all operated by a single antenna wire and their resistances will (hopefully) rise and fall appropriately to give me a larger voltage drop across the bridge.

You'll notice that there are no bias resistors in the circuit. This is common in many static sensing circuits so I kept up the tradition. The FETs are simply acting as voltage controlled resistors anyway, passing DC only. There is no AC signals involved.

Brief explanation of operation:
The gate of T1 senses a positive voltage, for example, so it becomes more conductive. The drain side of T1 becomes more negative so the gates of T3 and T4 become negative and they conduct less. Meanwhile, T2 gate is more positive than T3 and T4 so it conducts more, just like T1. And vice-versa.
The result is 4 changing resistances instead of one so more signal output.

The FET connected to the antenna is outdoors with only the source and drain wires coming indoors. The rest of the circuit can be built indoors and connected to my current circuit.

I would like some opinions about whether or not you think this circuit would work. If not, where did I go wrong?

Thanks

 

[(*steve*)]

T2 and T3 look like simple current sources givint a current of Idss for the FETs (which, incidentally, can vary significantly between devices).

T4 is essentially acting as a diode since the gate is almost certainly forward biased.

T1 is biased similarly to T2 and T3, however the voltage seen by the antenna will vary that up and down.

I would think that this circuit would be somewhat unstable and very dependant on the individual device characteristics. I'm not sure what effect T4 has, but I don't think it's necessarily what you want. I think you could replace T3 and T4 with resistors, and I'd be looking at placing resistors in the source of the other 2 FETs to give a more predictable current (and one which you could then trim).

 

[Trent]

Thanks Steve for your prompt reply.

I'm not concerned with variations between devices, only that the resistance of T1 and T2 go up and down together and the resistance of T3 and T4 change opposite to T1 and T2.

The voltage on the antenna is supposed to go up and down by small amounts and change the resistance of all 4 FETs.

The purpose of this circuit is to get away from using resistors. I have a working circuit now with just T1 as the only FET. But if the resistors can vary, like T1, the bridge output will be greater.
I don't need a predictable current, as it is constantly changing throughout the day. Also, adding bias resistors will increase the total resistance of the bridge, and it is most sensitive to small signals picked up by the antenna when the bridge has a small resistance.

As far as stability goes, as long as I get a constant DC current without unwanted fluctuations, I'll be happy.
I know the circuit is not perfect, but I think it will give me a bigger signal than a bridge with 1 FET and 3 resistors.

 

[(*steve*)]

What gives you a larger signal is T3 acting as a constant current source.

Insert a trimpot in the source lead of T3 so that one end connects to the source, and the other to the gate and then on to the drain of T1. Choose T3 such that it has a higher Idss than T1 (either by measuring them or by choosing a different part with an intrinsically higher Idss). You can then remove the other 2 trimpots.

T2 and T4 should just be replaced with resistors

 

[Trent]

Please bear with me, I'm still trying to understand how these FETs are operating. I'm new at working with transistors.

So, are you saying my explanation of how a sensed voltage affects each FET is wrong? I've copied my explanation from the first post here for your convenience:
"The gate of T1 senses a positive voltage, for example, so it becomes more conductive. The drain side of T1 becomes more negative so the gates of T3 and T4 become negative and they conduct less. Meanwhile, T2 gate is more positive than T3 and T4 so it conducts more, just like T1. And vice-versa.
The result is 4 changing resistances instead of one so more signal output."

When T1 and T2 conduct more, more current goes through the meter. If T3 and T4 simultaneously conduct less, even more current goes through the meter. A negative charge on the antenna makes T1 and T2 conduct less and T3 and T4 conduct more, so the current flow is opposite.

I don't expect all the FETs to react to the same extent, but I thought there would be some changes in the right directions.

I've checked this using the Wheatstone Bridge calculator I found online at:
http://www.daycounter.com/Calculators/WheatStone-Bridge/WheatStone-Bridge-Calculator.phtml

I don't want a constant current source in the circuit. The current is supposed to change with the changing antenna voltage. T1 and T2 work together and T3 and T4 must work opposite.

I admit the T2 gate connection isn't the best, but why do you say replace T4 with a resistor? If its gate is at the same potential as T3 gate, then the resistance will vary in the same direction as T3 like it should, and both work opposite to T1. Right?

Thanks

 

[Trent]

Version 2

Steve,

Shown here is version 2 of the circuit. I've included some trimpots and a source resistor for T4. Is this any better?
I've never had to figure out how to bias transistors. As I've said, I'm very new at this.

Now that I have these trimpots to vary gate potentials, how do I adjust them? Adjust each for max current?

You mentioned in your first post that you thought the circuit would be unstable. Please explain how it would be unstable, what would I see?

Thanks

 

[(*steve*)]

You obviously understand this far better than I do.

You now have 2 constant current sources and the possibility of a forward biased gate on T4.

You also have way too many controls.