capacitorcurrent-sensingelectrontransconductance
I want to collect charge from an electron beam source. For that, I'm proposing to put a floating capacitor with its bottom terminal floating and directly exposed to the electron beam gun. The top terminal would be connected to a transconductance amplifier.
Do you think that it will work? What considerations or modifications would you suggest?
The circuit, as drawn, does not work.
Furthermore, even if it did work, it is doing so by horribly abusing the transistors base-emitter junction diode, which can potentially damage the transistor.
It's worth noting that it is possible to make the circuit work. As far as I can tell, the person drawing the diagram simply forgot a diode:
The pull-up resistor also needs to be somewhat smaller, in order to turn Q1 on hard enough that the generated current waveform is appropriately symmetric. It's 10K in the above image, but 1K works much better.
Don't use 2N2222, use 2N7000 MOSFETs (that motor must be small) and it will be much easier. Try this:
You will have to tweak the value of R5 to fine tune the timing. D2 is there to avoid C1 discharging through anything that isn't R5, and to charge C1 as fast as possible. The time constant between R5 and C1 determines the turn-off time.
This circuit could be improved by doing a harder switching action on M1 to lower switching losses, but if it is not switched on-off very frequently, it will work fine.
Oh, and dont forget about the flywheel diode D3, the motor inductive spike can be very nasty.
Best Answer
It will be probably much easier without the capacitor. Shining the e-beam on the steady potential electrode will generate a much more stable current reading using a conventional transimpedance amp.
The problem with the capacitor terminal is that it will charge; and eventually this charge will deflect the e-beam from hitting the electrode.
Another issue is that you will generate secondary electrons emission losses from your target, so if you want to measure the beam current accurately, the target should have a hollow geometry with the beam entering through a hole. That way, stray emission will be recaptured.
Depending on the beam intensity, target heating can become an issue.