I am having issues with a 'high' voltage level shifter I am creating. I need to drive a 28Vpp waveform with specific timings ( <.1us rise time, 1us pulse width), converting from the 3.3V logic that comes out of my microprocessor. I currently have the circuit below:
Which produces the following waveform:
Great! However, I will need to need drive a fairly long cable. When I add 100pf of capacitance to the output:
I get the following waveform:
This waveform doesn't meet my requirements of having a rise time of less than 0.1us. What can I do to overcome this? I have tried replacing the transistor based circuit with an LM139 comparator, but that was still susceptible to capacitance. Are there ICs available that could fix my issue? I can handle about 4us of prop delay if needed. Thanks!
Best Answer
Since you're getting the output from the collector, the output impedance of the whole circuit will be the R3, 1k. It's quite high to drive a capacitive load, because if you connect one then it will form a low-pass filter with a cutoff frequency of \$f_c = 1/(2\pi \ 1000 \ C_L)\$. For a 100pF load, the cutoff frequency will be around 1.6MHz. That's why you cannot meet the rise-time requirement.
The output impedance should be low. Extremely low. You can try the following circuit:
simulate this circuit – Schematic created using CircuitLab
The first stage is a translator and the output is a push-pull output stage having extremely low output impedance.