Driving a PNP transistor from an NPN transistor?

[Pranshu Maan]

Ok, there was a mix-up because R1 in my original schematic was in a different position from the new one, you meant R1 of the original schematic and I thought of the new schematic.
Anyway what you mean is this and would work fine, the difference would be about 8mW for R2.

View attachment 56042

Alex

Hello Alex,
I have one question regarding this circuit. Simulation shows that Q point for PNP does not remain in saturation region for temperature below zero degree. Hence, if the requirement states the operating temperature less than zero degree C then we may not use this configuration for switch application. Any idea how to stabilize Q point in saturation region for this circuit ?
Thank You

 

[FvM]

Simulation shows that Q point for PNP does not remain in saturation region for temperature below zero degree.

Can you be sure about the result? Datasheets don't even show typical temperature characteristics of BC560.

I would think that a forced B of about 4 assures saturation at quite low temperatures. But to be safe by design, you may want to chose a switch transistor with specified saturation behaviour.

 

[Pranshu Maan]

Can you be sure about the result? Datasheets don't even show typical temperature characteristics of BC560.

I would think that a forced B of about 4 assures saturation at quite low temperatures. But to be safe by design, you may want to chose a switch transistor with specified saturation behaviour.

Hello FvM,

I am using BCP53 from infineon.

Circuit was simulated in ORCAD @-40 degree C.

Structure is as follow:

NPN driving base of PNP through 2.95k resistor. Base of NPN connected to microcontroller pin through base resistor. Power supply of 8V connected at the emitter of PNP and load (LM317) connected at the collector of PNP.

 

[Pranshu Maan]

Ok, there was a mix-up because R1 in my original schematic was in a different position from the new one, you meant R1 of the original schematic and I thought of the new schematic.
Anyway what you mean is this and would work fine, the difference would be about 8mW for R2.

View attachment 56042

Alex

Hello Alex,
I do not think that the resistor R2 is really necessary to mount. If the base current of the PNP transistor is set to zero by turning off the transistor Q1, Q2 will be in cut-off region. Some designers say that placing the resistor R2 helps Q2 to turn off completely.
But according to me this should be optional.

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[FvM]

I do not think that the resistor R2 is really necessary to mount. If the base current of the PNP transistor is set to zero by turning off the transistor Q1, Q2 will be in cut-off region. Some designers say that placing the resistor R2 helps Q2 to turn off completely.
But according to me this should be optional.

It's only optional for bad designs.

 

[Pranshu Maan]

It's only optional for bad designs.

Thank you FvM.
You are right. R2 will ensure proper turning off of the transistor irrespective of the leakage current.

 

[Pranshu Maan]

Hi All
I need your feedback for the circuit shown below.

Q131 is used to invert the logic i.e. I am trying to activate the circuit when output of microcontroller pin goes low.
Two parallel resistor have been used in the collector of Q130 inorder to keep the power dissipation within the limit.
V153 is the battery and the aim of this ciruit is to switch the supply to the load represented by 100K.
Later I plan to replace 100K by LM317 used as a constant current source.

Please let me know is the circuit ok .