Help with the output (load) current of this differ. amp circuit?

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SandiegoSD

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Hi guys, I'm trying to tackle a precision current source design. This is an application note i found from AD. The one shown above is a simplified version of this:

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And the I(output) = Vref/R1
with Rg1 = Rg2 = Rf1 = Rf2 for the complicated one. Source here:


I'm just trying to start with the simpler one I showed uptop but can't even get past that. Could anyone enlighten me on that?

The source is from Analog Devices titled :
Difference Amplifier Forms Heart of Precision Current Source
 
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Need suggestions for the power supply solution on this design.Ideas?

Hi folks, I'm designing a Milli-ohm measurement test board wrt to this schematic designed by K.Wong.
View attachment 68134

Source : http://www.kerrywong.com/2011/08/14/accurate-milliohm-measurement/
The variation from KW's design in my circuit is that, as you can see on the 1.5mOhm load resistor, I'm trying to generate 500mA. As a result, my ckt ends up using a 10V supply to the pos. input on the differential amp AD8276(the circuit is basically a Widlar(improved Howland) current source), and Vs(pos power supply) needs to be twice of that, so I'm putting 22V.

Before I'm going further, I'm wondering what kind of voltage supply circuit/components would be necessary for this final test board. I'm planning to use wall plugs(120V), so I'd need a converter(possibly an Open Frame converter?) to bring it down to maybe 20~30V, then another component that regulates it to 22V or 24V to feed the AD8276. Also I can see I'd need something(maybe a buck converter) to generate the 10V on the positive input on the amp.
I haven't designed any voltage regulator before and have little experience . Could anyone give me some suggestions on how to achieve the conversion from 120V wall outlets to a fairly stable 22V/10V voltage ? I'd appreciate it if you could name some specific parts or designs that do this.
Thank you.

I've only finished the current source design (left half of KW's ckt)and I've attached the simulation I did in Pspice for the circuit.

View attachment 68135
 
hi,
I would suggest a 7824 , 24V regulator for the +Vcc, it would require at least a 2V to 3Volt higher input to the 7824 regulator, say 27Vdc smoothed.
The 7824 would be dissipating 3*0.5 = 1.5W

The +10Vref could be a TL431 Vref, set for +10V.

Note the AD8276 output current limit of around +10mA, when driving the TIP with an emitter current of 500mA, the TIP must have a gain higher than 50 at 500mA.

E.
 
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Regarding hfe on the transistor in this Widlar/Howland current source?

HI guys, I'm working on this current source design. (Reference: https://www.analog.com/library/analogDialogue/archives/43-09/current_source.html)

For my design I'm trying to bump up the load current(output current) to 500mA range. I did some spice simulation and it seems the Iβ is only 4.74mA (green circle)
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So I'm looking at a gain (DC hfe) of about 500/4.7=~100 on the bjt. I'm putting TIP31A here only temporarily . but it seems like (from the TIP31A datasheet) at 25°C with Ic = 0.5A, the hfe is only around 40~50. (apparently the spice model used in the simulation software isn't that comprehensive)
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My Vce is ~12V while in this graph it's 4V, but it's not that much different (I'd need hfe 100 if Iβ is 4.7mA).
So I'm wondering, should I look for a bjt with large(>100) hfe at 500mA ? Is it true that the larger hfe the merrier for me?
 
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The datasheets from different manufaturers are different.

Who's datasheet did you copy?
I didn't copy the very old datasheet curves from Texas Instruments which show a typical hFE of 70 at 500mA.
Here is the datasheet curve from Fairchild where the typical hFE is 160 at 500mA.
Nobody knows what is the minimum hFE at 500mA.
 
Hfe needs to be high enough to prevent excessive loading of the diff amp output. Hfe=50 will require 10 mA from it. Short circuit output current is ±15mA. You might want to consider a Darlington.
Keep in mind that power dissipation might be an issue.
 
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Hfe needs to be high enough to prevent excessive loading of the diff amp output. You might want to consider a Darlington.
Keep in mind that power dissipation might be an issue.
Why did you mention Widlar in the title? I know who Bob Widlar is, but I never heard of any connection between him and the Howland current pump.
 


Dealing with bjt seems like a lot more trouble than necessary. I think I could use a N channel MOSFET in place of that current amplifying transistor and it should (as a source follower) function the same as the emitter follower constructed with the bjt in this circuit ?? It does give me 500mA in simulation. (that BUZ21 is temporary, not final choice)
Also I dialed down the voltages so i guess there's less burden in terms of heat, but I see heat sinks are definitely required here
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A Mosfet needs a gate voltage that is much higher than its source voltage for it to completely turn on. So it needs a boost-the-voltage circuit to drive its gate to the higher voltage so that it works as a source-follower.

The BUZ21 and most other Mosfets need the gate to be 10V higher than the source for it to completely turn on.
But it might work when the total supply is 10V and the maximum output is only 5V at 0.5A.
 
Just a comment from me. I have some experience with the Howland current pump.

Sense voltage across R2, the 1.5 mOhm resistor will be very small, so low input offset error is required from U2, the AD8603. If you substitute this part number, make sure that the substitute has low input offset voltage specified. I don't know if it is still true, but op-amps with bipolar input transistors had superior input offset voltage spec than FET input op-amps.

You might also consider eliminating U2 altogether. The value of Vref inside U1 is 40K. There is no reason why the output of R2, the 1.5 milliOhm resistor, would need buffering in order to drive a 40K load. The circuit will still maintain high precision without help from U2.
 
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I agree with you about eliminating the op amp.
I'm not sure the 1.5mΩ resistor is a sense resistor. It is in series with the current output. I'm wondering why the OP would want to sense the current into ground. The current (500mA) is set by the input voltage (5V) divided by the 10Ω resistor.
It appears to me that the circuit is oscillating. Note that each probe point lists the frequency as ≈27.5kHz. This doesn't surprise me. Most op amps with external large-geometry source followers are prone to instability due to the huge gate capacitance.
 
Back in post #6, the OP mentions that the 1.5mOhm resistor is the load. So that is the unknown resistance that the circuit is intended to measure.
 
ChrisP58 said:
Back in post #6, the OP mentions that the 1.5mOhm resistor is the load. So that is the unknown resistance that the circuit is intended to measure.
Click to expand...
OK. My memory doesn't work after a week.

I tried simulating the circuit. It has a little ringing on fast edges, with the accompanying peaking in the frequency domain, but it is not close to oscillation. of course, hardware could be different.
I did notice that the OP's reported 27.5kHz was extremely low level, so it might be some sort of artifact.
 
Oh nvmd..I was looking at this page explaining different current sources. It calls that circuit"Widlar Op-Amp Current Source for Grounded Load" and it looks just like an improved Howland current source with negative input to the inverting input. so i wasn't sure.

Thank you for reminding me that. This is a problem and instead of dodging the lack of high hfe by using a MOSFET, I think i should check out Darlington as Ron suggested.
Also I plan to use both 2.5V and 5V at the +IN of AD8276, and in the case of 2.5V, even a Logic Level MOSFET (Rds specified at Vgs=5V) probably wouldn't work.
I think a Darlington like this 2N6038 rated at 40W will do.
Also I had to loosen the tolerance to 1% on that 5Ω resistor to find one that meets the power requirement.oh well. i can address that in my calculations after I measure the actual resistance.
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Solid. I got rid of the buffer.
++++++++++++++++++++
I have a question related to power Regulation for the said circuit.
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In the sketch I get 24V from a noisy power supply, the LM2940T is a linear regulator (LDO) that outs 10V, and the three LM4040 are Shunt Voltage Reference rated at 5V 2.5V and 1V(this one not a LM4040).
I'm not so certain about connecting voltage references to the output of a linear regulator (I'm worried about currents and input impedance issues)
Can someone tell me whether it's ok to do this?
 
The input voltage does not appear on the gate of the MOSFET (or the base of the transistor). The op amp drives the gate to whatever voltage is required to cause the input voltage to appear across the 10Ω resistor. Audioguru's concern was that vcc=10V to the op amp might be marginal for adequate gate drive. He wrote:
But it might work when the total supply is 10V and the maximum output is only 5V at 0.5A.
Click to expand...
I think that it would work. The simulation worked well.
It's good that you got rid of the op amp, because its output can only go down to about 10mV (maybe a little lower), while it would need to go down to 0.75mV to follow the input.
 
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