Electronics > Beginners
Correctly stabilize an opamp current source
ZeroResistance:
The circuit I tried is attached. Its the simple and ubiquitous op amp current source with a BJT.
Except for the op-amp I used a faster one TL072 powered @ 12V and BJT BD139.
The current sense resistor is 1ohm. When I give 1V at the input I was expecting around 1A at he output but I get around 0.5 to 0.6A.
I cannot find the max output current of the TL072, but I beleive the opamp cannot source the base current required for correct operation of the current source.
I tried also with a darlington transistor TIP122 in the same setup but the output oscillates.
I would have loved to use a mosfet and I did try with the STP55NF06 but again the output oscillates. For the mosfet I tried using additional components as given here https://www.electronicsweekly.com/blogs/engineer-in-wonderland/current-sink-stability-2015-10/, an additional resistor(tried 1K and 10K) and capacitor (tried both 100pf and 1nF) but that wouldn't stop the oscillation.
The above circuits all had the drain / collector powered by 12V.
I have assembled the circuit on a breadboard.
My questions are
1. For the BJT version would I just need another transistor stage (like a 2N3904) in between the BD139 and opamp can safely source that amount of base current?
2. For the Mosfet version, how do stabilize the op amp and prevent it from oscillation?
StillTrying:
just need another transistor stage (like a 2N3904) in between the BD139
Yes, making your own emitter follower often works. 2N3904's collector to V+, and add a 820 or 1k, emitter to GND.
TL072 isn't single supply.
ZeroResistance:
--- Quote from: StillTrying on April 24, 2019, 08:47:16 am ---
TL072 isn't single supply.
--- End quote ---
Ok, I missed that point! However I had even checked with an MCP602 @ 5V supply. However could not test a Mosfet with his since the Mosfet needs the output to swing higher that 5V. Will try with a Emitter follower stage in between.
iMo:
Here is a sim with MCP6001, switching 100mA/1A.
The proper style of wiring is essential in order to suppress the oscillation as well.
PS: even single rail, rail to rail opapms cannot go down to 0V. Thus I would not expect to get something below 30mA current (0.03V).
ZeroResistance:
--- Quote from: imo on April 24, 2019, 09:39:06 am ---Here is a sim with MCP6001, switching 100mA/1A.
The proper style of wiring is essential in order to suppress the oscillation as well.
PS: even single rail, rail to rail opapms cannot go down to 0V. Thus I would not expect to get something below 30mA current (0.03V).
--- End quote ---
Great sims! However I can't seem to get it work in a practical setting.
I don't have a MCP6001 so was checking with a MCP602.
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