Help with 4-20mA Analysis/Design

[MyElectronsFellOut]

Hi guys, Could anyone point me in the right direction here.

Im trying to get to grips with a current loop design. Specifically, the MOSFET analysis part. I've looked at a few online schematics and have tried to design a simple version, if there is such a thing.

My idea is to use the Op-amp to amplify the input signal to output a coresponding voltage at the gate, that would generate the 4-20mA across Rp.  My problem thus far is in analysing the MOSFET datasheet and applying the formula to workout the Construction Parameter (k) given as k = Id/(Vgs-Vgsth)^2
Am I right in assuming that once I have k, I can workout the Vgs required to generate the 4-20mA signal? If so, is Id = Idon, Idss or do I pick an Ids-Vgs pair of numbers from the transfer characteristic graph,  which only plots for a Vds of 5V....

[T3sl4co1l]

MOSFET parameters vary all over the place, and as you note, are nonlinear anyway.

What you need is to move the feedback resistor, from the op-amp output, to the MOSFET source.  And connect a common ground from the bottom of the source resistor.  If this 4-20 transmitter must be isolated, you need an isolated supply for the loop source (32V) and opamp, and signal isolation (to the op-amp input).

Tim

[dannyf]

Am I right in assuming that once I have k, I can workout the Vgs required to generate the 4-20mA signal?

Yes, except that 1) those parameters vary from device to device; and 2) they are, in this particular setup, dependent on what you attach tot he circuit as load.

What you need is a closed-loop circuit.

[MyElectronsFellOut]

MOSFET parameters vary all over the place, and as you note, are nonlinear anyway.

What you need is to move the feedback resistor, from the op-amp output, to the MOSFET source.  And connect a common ground from the bottom of the source resistor.  If this 4-20 transmitter must be isolated, you need an isolated supply for the loop source (32V) and opamp, and signal isolation (to the op-amp input).

Tim

What does putting the feedback resistor on the Source side do?

So I assumed that the (K) value would be a constant, but taking values of Id and Vgs from the transfer characteristic graph (I took 5 points from 2 - 10A), I got different values of K, though they were in the same ball park.
I took an average of them and used the result in equation Id=K(Vgs-Vgsth)^2 to work out Vgs for obtaining 4mA and 20mA,  If my calcs were right, the difference required in Vg is only around 57mV.  1.6 - 1.657V. 

Vgsth is typically 1.6V according to the data sheet so the 1.6 sounds about right, but the range of Vg required is a bit small to work with...

[ajb]

What does putting the feedback resistor on the Source side do?

Uses the resistor as a current sense element.  If the bottom of the resistor is at ground, then the voltage at the top of the resistor will be equal to the current through the resistor times its resistance.  You'll need to set the value of the resistor and the gain of the opamp to create a suitable relationship between input voltage to the op amp and output current, taking into account how much compliance voltage you require for the 4-20mA loop. 

If for some reason the resistor cannot be referenced to ground you'll need to use a differential amplifier or another trick to translate the voltage across the resistor to a ground referenced signal.

[T3sl4co1l]

For more on why direct drive won't work, check out Vgs(th) min/max limits in the datasheet, and the tempco of both Vgs(th) and K.

For more on why feedback does work, read up on op-amps, negative feedback, current sense (shunt) resistors, and heck, good old fashioned nodal analysis and stuff if you need the practice!

Tim

[MyElectronsFellOut]

For more on why direct drive won't work, check out Vgs(th) min/max limits in the datasheet, and the tempco of both Vgs(th) and K.

Tim

From the datasheet...

VGS(th):                                Gate Threshold Voltage:    VDS = VGS, ID = 250 µA                                                                       Min:  1V           Typ:   1.6V         Max:  2 V
Delta VGS(th) / Delta TJ:       Gate Threshold Voltage Temp. Coefficient ID = 250 µA, Referenced to 25 oC:                                                     -4 mV /oC               

I have attached the Transfer Characteristic Curve below.

Tell me if I'm reading this correct then.  for the setup I currently have.
The hotter the Tjuntion, the lower Vgs required to for the MOSFET to start conducting?
Also, as I want to create a 4-20mA ouput, then going by the transfer characteristic curve, I would only need a very small Vgs range to get the required mA output? (which would lead me to think my calculations weren't to far off).

But this is a problem, as I want to operate this MOSFET in Sat mode where Vds doesn't affect the mA output, but Vgs can vary from say, 5-20V, and produce the required 4-20mA acorss the Drain Source.

[T3sl4co1l]

Exactly.  So if Vgs(th) is varying on the order of volts, and you need a difference at the gate of maybe 40 to 80mV, and you can't know which is required -- it's impossible!  So there must be a better way.

Tim