Supply power to a opamp

Thread Starter

Mihaii

Joined Sep 5, 2017
6
hello,
I need to make an transconductance opamp.I am using this design however i dont know how i need to power it.i have at my disposal an lm741 or a stm601. On V+ i will have 5V and i would like to have the same on V-,in order to iload=V-/R2.What opamp should i use and how should i power it?(thank you)
upload_2017-9-5_18-11-0.png
 

Thread Starter

Mihaii

Joined Sep 5, 2017
6
i will use a range of 0-5V on V+.I thought the current through the load is independent of what the load is.For testing im using an 100ohm resistor but it will be a laser diode.the current range i want to be from 0 to 5V/150ohm
 

WBahn

Joined Mar 31, 2012
29,978
Are you clear on the distinction between the V+ and V- inputs and the power supply inputs to the opamp? These are NOT the same thing.
 

bertus

Joined Apr 5, 2008
22,270
Hello,

You say that you will have a range upto 5 volts on 150 Ohms.
This implies a current of 5 Volts / 150 Ohms = 0.03333 A = 33.33 mA.
As far as I know the 741 can deliver max 25 mA.

Bertus
 

crutschow

Joined Mar 14, 2008
34,285
Below is the LTspice simulation of a constant current circuit using a 741 with a transistor buffer to generate 33mA into the diode load, D1.
I reduced the value of R2 to give more headroom for the diode forward voltage.
0-1V input gives 0-33mA through the diode.

upload_2017-9-5_12-52-27.png
 

Thread Starter

Mihaii

Joined Sep 5, 2017
6
Below is the LTspice simulation of a constant current circuit using a 741 with a transistor buffer to generate 33mA into the diode load, D1.
I reduced the value of R2 to give more headroom for the diode forward voltage.
0-1V input gives 0-33mA through the diode.

View attachment 134368
Thank you very much for taking the time and doing this i very much apreciate.To give you a bigger scope of the project i would like to vary the current that runs through the diode from 0 to 33mA by varying the V+(basically to vary the output optical power of the laser diode) and i need it to be propotional(the current and the voltage and in turn the optical power) do you thing with my curent design its possible?
 

wayneh

Joined Sep 9, 2010
17,496
I need to make an transconductance opamp.
Do you? There is such a device already commercially available, such as LM13700 from TI.

But it sounds like you just want to make an adjustable constant current LED driver? Any old opamp can work for that but using one that can sense down to the ground power rail makes things easier. LM358 is one, but there are many others.
 

crutschow

Joined Mar 14, 2008
34,285
basically to vary the output optical power of the laser diode) and i need it to be propotional(the current and the voltage and in turn the optical power
That sentence is not at all clear.
You are saying the optical power needs to be proportional to the optical power which makes no sense.

Is it that you want to measure the optical power with a sensor and then adjust the current to keep the optical power constant?
 

crutschow

Joined Mar 14, 2008
34,285
Oh, so you just want to do their homework for them and then let them flounder later when they aren't prepared to use what they didn't learn as a consequence.
If this were just a circuit design problem I would agree with you, but this appears to be part of a larger project and I was giving them some help with the circuit to control their laser, for which they already had the basic design.
It's still up to them to understand and explain the circuit operation.

I think they will still have plenty to learn before they finish the project. :rolleyes:
 

Thread Starter

Mihaii

Joined Sep 5, 2017
6
That sentence is not at all clear.
You are saying the optical power needs to be proportional to the optical power which makes no sense.

Is it that you want to measure the optical power with a sensor and then adjust the current to keep the optical power constant?
yes this is what i would like to do, and thank you for understanding.
 

Thread Starter

Mihaii

Joined Sep 5, 2017
6
If this were just a circuit design problem I would agree with you, but this appears to be part of a larger project and I was giving them some help with the circuit to control their laser, for which they already had the basic design.
It's still up to them to understand and explain the circuit operation.

I think they will still have plenty to learn before they finish the project. :rolleyes:
If this were just a circuit design problem I would agree with you, but this appears to be part of a larger project and I was giving them some help with the circuit to control their laser, for which they already had the basic design.
It's still up to them to understand and explain the circuit operation.

I think they will still have plenty to learn before they finish the project. :rolleyes:
 
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