Are all these three linear current regulators always likely to be stable?

[treez]

Which is likely to be the most stable, A, B or C? (most unlikely to oscillate)
LTspice simulation also attached

 

[FvM]

With a slow OP like LT1006, I would expect all circuit variants to be stable. B and C seem functionally identical.

 

[treez]

C's opamp has much higher gain than B, so C could surely be more likely to go unstable?

 

[FvM]

C's opamp has much higher gain than B

No. Both circuits differ by an OP input series resistor which is effectively meaningless.

Generally, all circuits can be expected to be stable because they are operated with below unity feedback factor and no noticeable external phase lag.

 

[treez]

below unity feedback factor?.....but the gain of B and C is surely 1001/10 = about 100.?

 

[FvM]

It's voltage feedback, so we are talking about voltage gain.

 

[treez]

thanks, its actually a non-inverting opamp though. R(input) is the 10R resistor, and R(feedback) is the 1k, and the vbe.
And the gain of this is more than unity.
I admit its a strange one because the reference is where the input signal would normally be, and the voltage across the R(input) is what's being regulated.?

 

[FvM]

Feedback factor of a voltage mode OP is the ratio of OP output voltage and differential input voltage. The feedback factor sign is always negative for negative feedback. In your circuit you have a emitter follower with voltage gain < 1 between OP output and input, so the feedback factor magnitude is below unity.

 

[KlausST]

Hi,

i prefer the local HF feedback in "A", it makes the circuit stable even with long wires (inductivity) to the LEDs and with influenced EMI.

Klaus