# Difference between current and voltage preamplifier?

1. Jan 21, 2012

### swami.me

I am trying to build a transient photocurrent/photovoltage setup and I was wondering what is the difference between current and voltage preamplifier? Here is a detailed desciption of my setup

I will be providing a a very short laser pulse (5ns) to a photodetector in either open circuit or short circuit conditions and measuring the photovoltage and photocurrent respectively through an oscilloscope by connecting the output of photodetector using a preamplifier and then oscilloscope.

2. Jan 22, 2012

### Staff: Mentor

In general, a current preamp has a low input impedance, maybe ideally zero. (*) A voltage preamp has a high input impedance to present minimal loading of the sensor.

(*) there may be conflicting goals: zero impedance to not affect linearity where the sensor has non-linear impedance, and ideally a matching impedance for best S/N performance.

3. Jan 22, 2012

### Bobbywhy

Photodetector preamplifiers, photovoltaic mode, photoconductive mode, and selecting the right photosensor (phototransistor, photodiode, and a few others) are clearly described here:

http://en.wikipedia.org/wiki/Photodiode

Since your optical input pulse will be only 5nS in duration an ordinary phototransistor would not react fast enough. You will need to use a circuit using an avalanche photodiode that is reversed biased. This complicates the design because you will need something like minus 20 or 30 volts for the bias.

Last edited: Jan 22, 2012
4. Jan 23, 2012

### swami.me

5. Jan 23, 2012

### Bobbywhy

Swami.me, here are some suggestions for your amplifier:

OPA620, OPA648, Texas Instruments
LT1360, Linear Technology
or HA5160, Harris

A ceramic capacitor of 0.1uF should be connected to the power input pin of the amplifier and grounded at a minimum distance.

For high speed/high rise times a Transimpedance amplifier configuration is often used. The time response of your amplifier greatly depends on the time constant of the feedback resistance and its parallel stray capacitance. To minimize the effect of this time constant, connect two or more resistors in series as the feedback resistance so as to disperse the parallel stray capacitance. If a bandwidth of over 100 MHz is desired and if the amplifier has its own internal feedback resistors, using them would help reduce the stray capacitance, reduce the overall circuit size, and suppress the undesired effects of lead inductance of each external component. You may select which of the suggested amplifiers have these on-chip resistors. Another performance enhancement would be to use a ground plane structure where the entire copper surface of the PC board is at ground potential.

Here is a National Semiconductor application note entitled “Design Considerations for a Transimpedance Amplifier: (the link takes you to a TI site, but the app note is National Application Note 1803)