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PMOS Turn ON using BJT

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hithesh123

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I need a ckt to turn on PMOS. Came up with the ckt in attachment.

The ckt works in simulator, but I am not sure about turning on BJT when its collector is just floating. will it work.
The BJT base will be controlled by a microprocessor.
 

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umesh49

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Your input voltage should be at source and output should be at drain. Q2 collector needs to be connected at gate with R4 between source and gate. Base of Q4 needs to be drive high to make FET ON. Why this R2 and R5?
 

hithesh123

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R2 is load and R5 is current sense resistor.

Great. R4 between source and gate will pull up the gate to Vcc.

How much should R4 be?
 
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umesh49

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A 10k value for R4 should be good choice. Keep the R6 to be 1k.
 

KerimF

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A floating gate is surely not recommended.
Perhaps on the simulator, a large resistance has been added between 'source' and 'gate'.
Did you measure the gate voltage? If it is 5V when Q3 is off then there must be a sort of pull-up resistor in the model.
If it is my circuit, I let R4=100 Ohms and I add Rgs=1K in parallel with the sourde and gate.

Kerim

Edited:
Sorry, I just noticed that the source is connected to the load and not to 5V. Please forget what I said ;)
 

goldsmith

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hello friends
At first step you should use a resistor from vi , to collector ( with large value ) and for more safety it is better to you , using a resistor in parallel with GS junction.
Good luck
Goldsmith
 

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