I am trying to build an inverting charge pump capable of supplying ~80-100mA at the second stage. Since the PWM signal must source the current, I decided on using a push-pull stage based on this article, which advertises the following schematic:
I build this circuit (only two charge stages; that's enough in my case), but am using Vin = 12v, PWM = 5V, as well as BC327/337 transistors due to the higher current load.
However, loading the circuit causes the transistors to get very hot with the advertised R1/R2 and the CE driver stage. I tinkered with those values but was not able to get a satisfying result. The Vbe of the upper transistor does not allow it into cut-off (I think that's the problem).
I have since looked at different push-pull designs, class AB and D amplifier stages and their biasing using both resistors and diodes, as well as other schematics, but have been unable to come up with a satisfying result.
1. How can I get a clean 0-12V square wave at the collector junctions of Q1/Q3
2. Is there a better way to drive this charge pump with a square wave
I will likely use paralleled LMC7660 drivers in the final circuit, but now I would really like to understand why this isn't working and getting it working
As suggested, I have tried separate CE drive stages for each transistor, but the base voltage of the top PNP (Q2, red trace) appears to be misbehaving. The NPN base voltage trace of Q1, green appears ok: