PNP as a clamp

[tronicman]

hello guys,
i just want a piece of help, this PNP is used as a clamp and im strugling to calculate or to determine the clamp voltage at which the Vout will be clamped ,
thnx for helping me,

 

[crutschow]

The clamp will be approximately the PNP base-emitter voltage (0.6-0.7V) plus the 5V base voltage.

 

[Easy peasy]

5.7 volts + the Ib.R drop in the base resistor, R3

 

[danadakk]

 

[Easy peasy]

you'll always get good clamping for a 1k source resistance ....!

 

[tronicman]

View attachment 165988

im very thankful for this detailed response

--- Updated Dec 3, 2020 ---

thank you guys

 

[FvM]

Silcon transistors have a typical BE reverse breakdown voltage of 7V. The transistor model in post #4 still shows no reverse breakdown at 14V, very unrealistic.

 

[danadakk]

Silcon transistors have a typical BE reverse breakdown voltage of 7V. The transistor model in post #4 still shows no reverse breakdown at 14V, very unrealistic.

Excellent point, so I strapped a 4.7V zener to simulate the Vebo rating (datasheet I looked at
was 5.0 V) problem. Clearly spice model incomplete. As you can see clamp results very poor.





If I raise input R to 10 K -



Regards, Dana.