transistor as diode clamp

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fm101

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I tried the following circuit simulation in Proteus where transistor is used clamp the input voltage -2.5V to 2.5V to 0V to 5V but I am not getting the expected output voltage as shown in the graph. Why is it not working and how do I choose the capacitor and resistor values?

1.jpg

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3.jpg
 

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FvM
I understand the problem differently.

You want to clamp negative voltage but post #1 circuit clamps positive voltage. Wrong transistor polarity.

After flipping the polarity you'll get -0.7/4.3 V pulse. If you want ideal 0/5V output, a more complex circuit is required.
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I could be wrong, but it seems to me people are confusing clamping with clipping. A circuit that changes a -2.5V/+2.5V signal to 0V/5V is a clamp. A circuit that limits excursions beyond +/-2.5V is a clipper.
 

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For minimum offset of a simple clamp, use a large (e.g. 1A) Schottky diode instead of a transistor:
The capacitor value you need depends upon the input frequency and the output load resistance.

 
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