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[SOLVED] High side Mosfet Driver

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themaccabee

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High side Mosfet Driver

Hi,

I need to drive a MOSFET to generate a constant current source,




the idea is to drive the MOSFET in linear region to obtain a set current corresponding to the VOLTAGE Value .
Is this configuration ok?
Is npn transistor can be replaced by an n channel mosfet?Is there any advantage in using mosfet instead of npn transistor
Thanks
Regards
 

Prototyp_V1.0

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No, it wont work. The net feedback is positive. If you exchange positive and negative opamp input, it should work.

But I cannot understand why adding a BJT. It just complicates the circuit.

The circuit will work if you remove the BJT and feed directly from opamp output to gate of mosfet. Dependent of type opamp, you can probably get rid of all resistors but load.

Also 5V voltage may be too low for the opamp to work properly. Also a negative voltage supply may be neccesary if your opamp doesn't have rail to rail output.
 

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uoficowboy

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No, it wont work. The net feedback is positive. If you exchange positive and negative opamp input, it should work.

But I cannot understand why adding a BJT. It just complicates the circuit.

The circuit will work if you remove the BJT and feed directly from opamp output to gate of mosfet. Dependent of type opamp, you can probably get rid of all resistors but load.

Also 5V voltage may be too low for the opamp to work properly. Also a negative voltage supply may be neccesary if your opamp doesn't have rail to rail output.

The way I'm looking at it makes it seem like it should be stable. If it were an N-FET on top, then yes the feedback would need to be inverted. But it's a P-FET and the output of the OA is being inverted by the BJT. So I think it's OK.

However I agree that there doesn't seem to be a lot of reason to have the BJT there. I think he could remove the BJT, drive the P-FET directly, switch the inputs of the OA, and he'd be all set. A frequency compensation circuit would be good to have too.
 

FvM

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But it's a P-FET and the output of the OA is being inverted by the BJT. So I think it's OK.
Yes, it's a P-FET symbol, but connected with source and drain reversed.
 

Prototyp_V1.0

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Yes, it's a P-FET symbol, but connected with source and drain reversed.
Sure it is. My eyes must have been very dryed out yesturday.

Agree. In theory, the circuit should work.
 

themaccabee

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I replaced the transistor with a MOSFET. Not because that the transistor is not working..but due to other stability issues in the circuit..I think transistor is a more faster than a MOSFET when controlled by a variable voltage in this fashion..
Thanks again for all the help
:)
 

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