[sdiy] OpAmp Output Current Question

[Scott Gravenhorst]

"Czech Martin" <Martin.Czech at Micronas.com> wrote:
>I have the english version of Art of Electronics.
>I think I'll find that circuit.
>Tomorrow.
>
>Can you understand spice ckt?

I like jalapeño peppers, does that count?  Umm.  no spice here.

Sorry to be a dim bulb... (not enough current)

And I also realized that my cs.gif schematic won't work either,
something has to push current into the input and that being an
opamp again will have the same problems as the original circuit.

>
>m.c.
>
>> -----Original Message-----
>> From: owner-synth-diy at dropmix.xs4all.nl
>> [mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of Scott 
>> Gravenhorst
>> Sent: Montag, 12. Januar 2004 15:45
>> To: synth-diy at dropmix.xs4all.nl
>> Subject: RE: [sdiy] OpAmp Output Current Question
>> 
>> 
>> "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...
>> 
>> =========================================================  
>> - Where merit is not rewarded, excellence fades.
>> - Hydrogen is pointless without solar.
>> - What good are laws that only lawyers understand?
>> - The media's credibility should always be questioned.
>> - Governments do nothing well, save collect taxes.
>> 
>> -- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
>> -- Linux Rex         | RedWebMail by RedStarWare
>> -- FatMan: home1.gte.net/res0658s/FatMan/
>> -- NonFatMan: home1.gte.net/res0658s/electronics/
>> -- Master of Arcane and Hidden Knowledge.
>> 
>> 
>

=========================================================  
- Where merit is not rewarded, excellence fades.
- Hydrogen is pointless without solar.
- What good are laws that only lawyers understand?
- The media's credibility should always be questioned.
- Governments do nothing well, save collect taxes.

-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
-- Linux Rex         | RedWebMail by RedStarWare
-- FatMan: home1.gte.net/res0658s/FatMan/
-- NonFatMan: home1.gte.net/res0658s/electronics/
-- Master of Arcane and Hidden Knowledge.