Variable Frequency Current Source ?

[alin_im]

Hey guys,

Can I use a function generator as a variable frequency current source ?

If yes, how ?

I want to model a photodiode to test a transimpedance amplifier and I need a variable frequency current source  and a capacitor in parallel. The frequency rage is between 10 - 100 MHz and the current between 10 - 100 uA

Alin I.

[alm]

Sure, a voltage source in in series with a sufficiently large resistor (compared to the load) behaves like a current source (Thevenin/Norton). However, given the frequencies involved and the currents, I see problems.

To have a decent amplitude accuracy at 100 MHz, you would want your load impedance as seen from the generator to be close to 50 Ohms. This would require a transimpedance amplifier input impedance well below 5 Ohm, but would require voltages down to 500 uV. And obviously the capacitor in parallel will screw up your frequency response (depending on the value). So I doubt it would work particularly well.

Maybe if you calibrate your system over its frequency range. And regulation will not be amazing due to the low source impedance. Maybe coupling the signal through a transformer could improve matching and regulation?

[alin_im]

My generator and my input impedance are 50 ohm, I was thinking of using a 1V peak to peak and a 10 kohm in series to obtain 100 uA current source, do you think it would work ?

[alm]

Only if your cables are extremely short or you do not care about voltage accuracy. Try it: connecting the generator to a 10 kOhm resistor with a coax cable. Then vary the frequency and observe the voltage at the 10 kOhm resistor with a 10x scope probe. The behavior will be very similar to an unterminated transmission line, since the impedance of the 10 kOhm resistor is very large compared to the 50 Ohm impedance of the generator and cable.

[Dave]

You could always terminate the coax with 50R right next to the input of your DUT and just connect a 10k resistor from the 50R terminator and DUT input. The additional 10k termination is barely going to affect the 50 ohm termination.

[alm]

True, that would definitely help the termination issue.