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Simple UA741 and 2N3392 circuit troubleshooting

roboDNA

Jul 22, 2022
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Hi,

I'm learning how to use the UA741CP together with a 2N3392 to get a -5V voltage source.

For some reason, when I breadboard the components, I get 0.60V instead of the expected -5V.

I have created a Falstad circuit simulation and it does work, but my breadboard version does not.

I have checked the resistor values, and pin assignments many times and they seem ok. I tried swapping out the 741 and 2N3392.

Any ideas on what I could be doing wrong?

Here is the simulation:


Thank you!
 

crutschow

May 7, 2021
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Don't see how it can work with no negative voltage source to the transistor emitter.
How do you expect the emitter to go negative without that?
Try adding a 1kΩ resistor from the emitter to -10V.
 

Sunnysky

Jul 15, 2016
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The problem using an NPN with any load towards 0V raises the voltage as Vbe is reverse biased. Using a PNP emitter out instead will sink current.
 

roboDNA

Jul 22, 2022
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Thanks for the info. I'm looking into this now. I was under the impression the - input of the opamp would be at -5V since the + input is at -5V, since the opamp would be attempting to make those 2 inputs equal... and that is where the -5v would originate from...

I'm also curious to discover why my original Falstad simulation worked, showing -5V. I'll report back when I made headway.
 

Harald Kapp

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Nov 17, 2011
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and that is where the -5v would originate from...
You're right in assuming the opamp will try to match the voltages at the + and - inputs. However, there has to be a source for that voltage to come from. In your circuit the only source driving the - input is the NPN transistor which can pull up to near 0 V but not down to -5 V. Try a load resistor, e.g. 1 k, from the emitter of the NPN transistor to -10 V. This provides a path for regulation of the node at the emitter to .-5 V via the opamp.
This load resistor is the answer in @Sunnysky solution provided in post #4, lower image.

That you see -5 V in your simulation may be a simulation artefact depending on how the model for the opamp is constructed internally. I see the same effect using another simulator, namely LTSPICE.
 

roboDNA

Jul 22, 2022
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You're right in assuming the opamp will try to match the voltages at the + and - inputs. However, there has to be a source for that voltage to come from. In your circuit the only source driving the - input is the NPN transistor which can pull up to near 0 V but not down to -5 V. Try a load resistor, e.g. 1 k, from the emitter of the NPN transistor to -10 V. This provides a path for regulation of the node at the emitter to .-5 V via the opamp.
This load resistor is the answer in @Sunnysky solution provided in post #4, lower image.

That you see -5 V in your simulation may be a simulation artefact depending on how the model for the opamp is constructed internally. I see the same effect using another simulator, namely LTSPICE.
I think adding a load to -10V via a 1k resistor makes sense to me now. ( no pun intended ;) Thanks @Sunnysky for the schematics which helped big time.

UPDATE: I tested my circuit using a 1k resistor pulling the emitter down to -10V and everything is working as expected now, thanks again everyone.
 
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