adjustable current/voltage limiter

[eigenroot]

Hi everyone,

I am trying to measure the long tail photocurrent decay of a photodiode with a current amplifier, but the initial part of the decay which I am not interested is giving me trouble since it can easily overload the amplifier or oscilloscope if I crank up the amplifier gain (the original discussion is at https://www.edaboard.com/threads/264508/). Originally I used an anlog switch to filter out the part I don't want, but the charge injection introduces a new disturbance.

Someone suggest me use a limiter, which passes all current below a certain value \[I_0\]but floors any current greater than \[I_0\] to \[I_0\]. However, after a quick search, most RF limiters are voltage limiters, and the voltage cap is determined by the diode turn-on voltage which is not a very adjustable parameter. Is there an adjustable current limiter I can use? Adjustable voltage limiter is also okay although I prefer current limiter.

Thanks.

 

[jiripolivka]

As you do not give any real details it is difficult to advise. A simple current limiter can be built from a FET, with its gate connected to the load and a variable resistor connected between FET source and load. By adjusting this resistor, the device limits \the current quite well, from uA to tens of mA.

 

[chuckey]

Thinking out loud :- If you have an OP amp with a bias of say .5mV on its positive input, if the negative input goes more negative then +.5mV, then the output swings to +Vcc. You could use this huge voltage change to forward bias a diode to shunt out the signal path. Repeat this arrangement but with the inputs changed over and a different op amp/diode. Then by altering the fixed +.5mV, you effectively alter the voltage at which the attenuation occurs.
Frank