Precision current source single Mosfet or twin transistor setup?

[Overspeed]

Hello

In the aim to built a precision current source 100 mA I have two solutions ( I try to learn )

• one Mosfet as the circuit linked ( from Eevblog forum ) this circuit is a 1mA circuit
  two transistors ( PNP / NPN ) as the circuit linked ( from M. S. Mrozowski design )

This question is a general question on the circuit design so what is the advantage of the both solution

both are voltage driven Op Amp source tested working with proven results.

Purpose is to built a 100 mA circuit for low resistance measurement , I agree than BS170 can be not enough , perhaps a paralleling Mosfet setup can solve the problem or a bigger Mosfet ? Mosfet do not need gate current tuning too.

[Kleinstein]

Both ways can work. Which one works better also depends on how the voltage at the resistor is measured as they have a different common mode voltage for the DUT. Another point could be if one needs extra protection in case there is an external voltage - this could be tricky in the 2nd version.

With 100 mA the leakage current at the MOSFET gate is usually not that critical.
One the other hand 2.5 V would be a bit high for the reference resistor to handle the power.

[PCB.Wiz]

This question is a general question on the circuit design so what is the advantage of the both solution

The second design has the unknown part in the feedback, both terminals floating, and is less ideal for measurement.
The first design is a current sink, not source, but maybe you are ok with +ve voltage to one terminal ?

I’d suggest using a power Pmos, to allow grounded load, and a power spread resistor if you must operate from the 14V shown. Add some clamp diodes, in case the unknown element has some inductance.

[Dr. Frank]

Here's my Precision Current Source recycling project:

https://www.eevblog.com/forum/metrology/recycling-of-precision-current-source-noise-reduction-for-low-burden-shunts/msg1432410/#msg1432410

I use a BS170 for its low bias current with an NPN power transistor in a Darlington configuration, up to 100mA.

Frank

[Overspeed]

Both ways can work. Which one works better also depends on how the voltage at the resistor is measured as they have a different common mode voltage for the DUT. Another point could be if one needs extra protection in case there is an external voltage - this could be tricky in the 2nd version.

With 100 mA the leakage current at the MOSFET gate is usually not that critical.
One the other hand 2.5 V would be a bit high for the reference resistor to handle the power.

Hello
Thanks for your answer
Eevblog YT video 577 1 amp source use a RFP3055 which  is equivalent to the new IRLZ14 I link a screen print with a drain to source resistance at 0.2 Ohm

On the Mosfet circuit 1 mA there is no resistor on the gate , normally there is resistor to control ( limit) the current gate ) I have added one on my PCB in process
Regarding the twin NPN/PNP I link the pdf with the complete schematic , the potentiometer are used because that not matched pair

Regarding the 2.5V in the ref resistor , Vishay propose 8 W foil resistor as the VPR220 with low tempco , could be possible to adjust the ref voltage to match the real resistance value ? VPR from Vishay even with 0.5 tolerance are quite accurate in value .

Regards
OS

[Overspeed]

This question is a general question on the circuit design so what is the advantage of the both solution

The second design has the unknown part in the feedback, both terminals floating, and is less ideal for measurement.
The first design is a current sink, not source, but maybe you are ok with +ve voltage to one terminal ?

I’d suggest using a power Pmos, to allow grounded load, and a power spread resistor if you must operate from the 14V shown. Add some clamp diodes, in case the unknown element has some inductance.

hello

I have linked a complete schematic in PDF in my Kleinstein answer

the designer of this circuit claims impressive performances but unmatched transistor request a lot of adjustements compare to a Mosfet 

Regards
OS

Regards
OS

[Overspeed]

Here's my Precision Current Source recycling project:

https://www.eevblog.com/forum/metrology/recycling-of-precision-current-source-noise-reduction-for-low-burden-shunts/msg1432410/#msg1432410

I use a BS170 for its low bias current with an NPN power transistor in a Darlington configuration, up to 100mA.

Frank

Hello
Thanks for the link , use a small Mosfet to drive a bigger transistor is an elegant solution and I suppose by using a quite big transistor ( 1.35 amp) for a quit low current shall allow a negligible thermal problem .

My goal is to built an affordable circuit to compete with a costly Keithley 6220 current source as I have no budget to purchase a $$$$ Keithley and also I need only a DC 100 mA source

Regards
OS