How do reduce droop?

[stafil]

I am using LTspice to simulate a circuit that has a PWM driving a pulled-up sensor using a NPN BJT.

When the PWM truns off I see a droop of about .6Volt.

What's the best way to get rid of it? Should I use a capacitor in parallel to the BJT?

My circuit is:

Code: [Select]

                    +------/\/\/\/\------+
                    |                    |
  PWM source ------NPN                   +--- PWM consumer
                    |                    |
                    |                   12v
                    |                    |
                   GND                  GND



[Dave]

Just because the circuit seems to work in simulation, doesn't mean that it works in reality. Check the current that is flowing through the base of the transistor when the PWM input is high. That amount of current would surely destroy a transistor in the real world.

What you're seeing is the base-collector capacitance dragging the collector below ground when the base voltage starts dropping.
First you need to add a resistor between the PWM source and base and calculate it so it doesn't saturate the transistor too much. You might try adding a Baker clamp to further improve the result.

[stafil]

Just because the circuit seems to work in simulation, doesn't mean that it works in reality. Check the current that is flowing through the base of the transistor when the PWM input is high. That amount of current would surely destroy a transistor in the real world.

What you're seeing is the base-collector capacitance dragging the collector below ground when the base voltage starts dropping.
First you need to add a resistor between the PWM source and base and calculate it so it doesn't saturate the transistor too much. You might try adding a Baker clamp to further improve the result.

Oh ok. In the datasheet 2N3904 was rated 200mA and according to LTSpice the current was from 120mA to -40mA(droop?) so I thought I would be ok. Good that I asked

The Baker Clamp however altered the PWM signal, but adding a 10k Ohm resistor helped a lot, thanks!