[sdiy] OpAmp Output Current Question

[Scott Gravenhorst]

"Czech Martin" <Martin.Czech at micronas.com> wrote:
>Feeding large currents with an op amp can cause
>trouble on the supply, the more the higher the frequency.
>That way you test your PSRR of the rest of the circuit,
>but I do not know if this intended.

I've abandoned the idea of using an opamp summing node to do
this.  From what I see in spec sheets, this isn't practical
at 20 ma.

>A better idea would be to use a noisy supply for those
>currents, and let the op amp just control it.
>The classical one transistor current source circuit.

I'm afraid my lack of an EE puts me at a disadvantage. 
Looking at A of E, pp 73, there is a one transistor current
source shown, but the load is in the collector circuit (which
is where I had it to start with but moved it to the emitter
curcuit to gain linearity).  I'm assuming then that I can
drive the base with an opamp and sense the current through
the emitter resistor?

>It is also linear, since the current is measured as resistor
>voltage drop. 

We must be considering different configurations, what I see
is that the load current (where I would put the LEDs) is only
the collector current, but the emitter resistor measures  Ic
+ Ie.  Where can I see a schematic such as you describe?  Or
are we talking about this: 

http://home1.gte.net/res0658s/cs.gif   ?

A darlington for better linearity?

>Since the gain arround the op amp is slightly
>lower then 1 there is no stability problem.
>
>Your circuit uses the transistor as current gain element.
>Additional gain.
>Could give stability trouble, depending on bias point.
>
>m.c. 
>

And I thought this was simple...

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