How to Build a High-Sensitivity Gigahertz Current Amplifier for Photocurrents?

[darkwhite87]

Does anyone know how to build a low noise current amplifer that has wide bandwidth from ~10Hz to ~1Ghz, and together with high sensitivity (can amplify ~nA/pA)

I need one for my project to amplify photocurrent, but am struggling to find any suitable products or design guides...

cheers

 

[f95toli]

Define "low noise"...
The general answer would be that no, it is not possible to build such an amplifier. All transistors that work well at high frequencies (GaAs, InP etc) have very high 1/f noise meaning they are extremely bad at amplifying low frequencies. Also, remember that the Johnson noise will be quite high for an amplifier with 1GHz BW (how high will obviously depend on the source impedance).

 

[darkwhite87]

"low noise" as in i need to amplify something in the order of "pA" in the background level of "nA"...
i would be amplifying a pulse with width in the nanoseconds range, i would guess most of the frequency component are in the high range, so I may be able to withstand a amplifier which is bad at amplifying low frequencies...
the source impedance "photocell" is extremely large ~Gigaohm/square...so I think it is a must to match that to a conventional 50 ohm cable prior to the amplifier..

cheers