# current source op amp

Discussion in 'General Electronics Chat' started by Zeeus, Jun 24, 2019.

1. ### Zeeus Thread Starter Member

Apr 17, 2019
376
12

Vin is the divider (R5 and R6) voltage

Correct? Iout (current through R7) = Vin/R2

if correct then not working well, Iout changes alot( and output not saturated)

What is TS missing?

Thanks

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2. ### ericgibbs Moderator

Jan 29, 2010
8,126
1,643
hi Z,
What is the purpose of this circuit.?
Are you trying to set the current thru R7 [500R] to a fixed value.?

You say Iout changes a lot, please post some values.
E

3. ### Zeeus Thread Starter Member

Apr 17, 2019
376
12
hi E, Yes through R7..do you have spice? if so, you can change R7 and see current changes
From my calculation (dunno if right : if right then should work) this current is Vin/R2

Will try again later

4. ### ericgibbs Moderator

Jan 29, 2010
8,126
1,643
hi Z,
I do use LTS.
Post some actual values of what Current thru the 500R you require and what you are actually reading.
What is the design range of the R7value and the desired current.?
E

5. ### Zeeus Thread Starter Member

Apr 17, 2019
376
12
Z is expecting the current through R7 (= Vin/R2) to be constant no matter what R (that does not take the output into saturation)..

R7
1k = 518uA
1.2k = 640uA
Expecting the current to be constant as the output voltage is not in saturation for any of this R7

6. ### ericgibbs Moderator

Jan 29, 2010
8,126
1,643
hi,
Did you design this circuit.?

E

7. ### Kjeldgaard Member

Apr 7, 2016
368
128
The resistor R2 is in series with the parallel connection of R5 and R6, and is therefore not equal to R1.

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8. ### ericgibbs Moderator

Jan 29, 2010
8,126
1,643
hi,
The potential divider source was being loaded.
Look at this option.
E

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9. ### Zeeus Thread Starter Member

Apr 17, 2019
376
12
Thanks E,
See now
Can also reduce the divider resistor values by 10 then works, correct?

Please can you explain how the output impedance of u2 becomes low due to feedback?

10. ### Kjeldgaard Member

Apr 7, 2016
368
128
In the original schematic from # 1, there are the following values on the important resistors:
R1 = 1KΩ, R2 = 1KΩ, R5 = 22KΩ and R6 = 1KΩ

Parallel value of R5 and R6 = 956.5 Ω

If we have to have a total resistance corresponding to R1 from the R2, R5 and R6 network,
then R2 must be changed to 1000 Ω - 956.5 Ω = 43.5 Ω

Last edited: Jun 25, 2019
11. ### Zeeus Thread Starter Member

Apr 17, 2019
376
12
Thanks..Missed something?
R3 and (R7||R4).. Important resistors too?

btw, how to make variable resistance R7 vary linearly and show on oscilloscope?
R7 = V / i7
Or possible to look at I as R7 changes?

Thanks

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Last edited: Jun 25, 2019