Voltage Translation: Can a Capacitor Do It?

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Discussion Overview

The discussion centers on the feasibility of using a capacitor to transfer voltage pulses to the base of a transistor with a significantly lower voltage emitter. Participants explore alternative methods for voltage translation without traditional transistor level shifters.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant proposes using a capacitor to transfer thin voltage pulses (0, 5V, 100 ns) to a transistor base at -70V, questioning the viability of this approach.
  • Another participant suggests using a 70V Zener diode as a level shifter instead of a capacitor.
  • A third participant notes that using just a capacitor could result in negative voltage swings at the output, which may not be desirable for the transistor's biasing, and recommends adding a clamping diode, specifically a Schottky type for speed and low capacitance.
  • This participant also mentions the possibility of using an optocoupler as an alternative method.
  • A later reply expresses gratitude for the suggestions, indicating that they align with the original poster's prior assumptions.

Areas of Agreement / Disagreement

Participants present multiple competing views on the best method for voltage translation, with no consensus reached on the effectiveness of using a capacitor alone.

Contextual Notes

There are unresolved considerations regarding the specific circuit adjustments needed for biasing and waveform shaping, as well as the potential limitations of each proposed method.

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

clamp diode.PNG


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

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