Voltage Translation: Can a Capacitor Do It?

[Ravaner]

Hello. I wish to transfer very thin pulses ( 0, 5V, 100 ns) to the base of a transistor which emitter is at -70 V (and base correctly DC polarized). I don't want to use traditionnal translater circuit using npn+pnp transistors, but just a capacitor (supporting the voltage difference). Is there any reason against this kind of idea ?

 

[whome9]

You could also use a 70v Zener diode as a level shifter.

 

[vk6kro]

Normally, if you just fed this through a capacitor, the output would swing negative relative to the base as well as positive.
This is probably not what you want as the transistor would be biased for low standing current.

So, you need a clamping diode. I would try something like this:

The unmarked resistors are the normal bias resistors. This would need adjusting.
R may be necessary to maintain the input waveform shape.
The diode would be a Schottky type for speed and low capacitance.

I haven't tried it, of course, so it may not work or may need adjusting.

Another way would be to use an optocoupler.

 

[Ravaner]

Many, many thanks for your help. This goes in the way to confirm of what I previously suppose.