controlled current source functionality question

Thread Starter

yef smith

Joined Aug 2, 2020
756
Hello , I am trying to implement the following circuit in LTSPICE taken from the attached article.
I put an input of 1V amplitude.
however the current range is very little as show in the plot below.
what are the mechanisms to make the swing larger?
LTSPICE file is attached.
Thanks.
1707560521798.png1707560295514.png

1707560731459.png
 

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Thread Starter

yef smith

Joined Aug 2, 2020
756
Hello , I have changed the PMOS as shown below.
However I got the circuit oscillating.
The current on the load inductor is oscillating at the exact amount no matter what input voltage i put into the signal.
Why its not following the input signal?
What causes such behavior?
Thanks.
UPDATED LTSPICE file is attached.
1707564588391.png
1707564488506.png
 

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Alec_t

Joined Sep 17, 2013
14,337
The AD8033 opamp has FET inputs, whereas the Burr-brown 3500B has bipolar inputs. That might account for the different behaviour.
 

Thread Starter

yef smith

Joined Aug 2, 2020
756
Hello Ericgibs, as you can see in the results i need the current on the load to vary as a function of sine.
however as you can see in the plots the load current is almost steady at 8.25mA,what could be done to open the swing amplitude so the 1V sine input will cause 20mA amplitude sine wave?
Thanks.
 

Thread Starter

yef smith

Joined Aug 2, 2020
756
Hello, Ericgibs presented a good fix, however i want to understand the dynamics of opening the swing of the current values because currently its almost constant 8.25mA i want the output to swing pk-pk 20mA.
What could e done to open the swing?
Thanks.
 

Thread Starter

yef smith

Joined Aug 2, 2020
756
Hello ericgibs,Yes +10mA to -10mA is also great.
When i changed R11 and R8 i see that the current values changes however its still very steady.
I need some how to make the output current vary +-10mA .
What is the intuition to achieve it?
Thanks.
1707573064841.png
 

BobTPH

Joined Jun 5, 2013
9,003
When i changed R11 and R8 i see that the current values changes however its still very steady.
Those are not the ones limiting the output current. Look at the circuit again. Which resistors are in series with the output?

The output current is also dependent on the input voltage. Did you increase the input voltage when you tried to change the current?
 

Alec_t

Joined Sep 17, 2013
14,337
As per post #9, change the opamp to AD8031.
Increase R4 and R6 to 100k. Reduce R9 and R10 to 500 Ohms.

Edit:
Connect a 10u cap between the opamp inverting input and ground, to reduce the likelihood of oscillation.
 
Last edited:

Thread Starter

yef smith

Joined Aug 2, 2020
756
Hello ericgibs, i think the Mosfet are saturated thats why i get constant current no matter the voltage ,
how do you reccomend to open it?
Thanks.
 

Thread Starter

yef smith

Joined Aug 2, 2020
756
Hello Ericgibbs, i think its because the mosfets are saturated.Could you recommend something to make the swing larger?
Thanks.
 
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