Hello experts,
I am working on a circuit that will amplify pulse signals to +/-45V range to drive a capacitive load. The load is of a range that is less than 5uF. The output buffer stage is BJT push pull pair class A. I am having trouble to design a circuit that can limit the output current within a range of 0-2A with 100mA step. Obviously the output current limit should be controlable by a controlled voltage or current source, but not hard coded by sensing resistor that control the Vbe of the current limit BJT.
What I am doing right now is using a high CM input instrument amplifier to isolate the voltage drop on the current limiting sensing resistor. The REF input of the INA is connected with a controlled voltage source. The output of the INA is then connected with the Gate of a MOSFET. The MOSFET's D is connected to the base of BJT at the push pull out put stage and the MOSFT's S is connected with the GND. By control the voltage that is input into REF pin I can shift the INA's output, hence to control the Vgs. There is one N-MOSFET and P-MOSFET each for the push pull output current limit control.
The circuit did not work out quite well by simulation. The current limiting range is too small (300mA-800mA) and the positive and negative current controll is not symetrical. Linearity is not good as well. I am looking for some helps from this forum but did not get any luck to find a hint by reading the articles there. Can somebody give me a hand out there?
Thanks
-yan