Re: balanced photodiode

sperelat wrote:
Thank you.

Anyway..it's not my homework :) , and I did not know that they have

app note! I learned from you that companies often provide app note of
their product .

Though I have nevert taken analog electronics course before,my
supervisor expect me

to get the bandwidth in one day. T T..(I'm newcomer to electro-optical
experiment--;)

I strongly agree with getting the Hamamatsu and Burr Brown app notes as it's difficult to post schematics, equations, and graphs in a Usenet group.
But I'll try to give the main pointers.
Consider a photodiode to a current source shunted by a capacitor.
It also has a small series resistance, large shunt resistance plus a small series inductance. As frequency is increased the series resistance and inductance become increasingly important.

For the most part the the shunt capacitance is determined by the area of the photodiode and the magnitude of the current source is linear with the light (uv/vis/ir flux) at a given wavelength. The bandwidth of the circuit is determined mostly by the time constant of diode's capacitance and the effective input impedance of the amplifier circuit.
Whatever amplifier circuit is use, it's overall effect must be considered. Transimpedance amplifier circuits have a low input impedance which gives them relatively wide bandwidth, but it's certainly not zero impedance.

To increase bandwidth you can:
Choose a photodiode with less area and hence less capacitance.
Reduce the amplifier circuit gain (not the op-amp's open loop gain) which lowers it's input impedance and perhaps add subsequent gain stages if you need the overall gain.

Reverse bias the photodiode. That decreases its capacitance some but adds an additional noise source.

You may be able increase the light flux to improve signal to noise and allow operating the amplifier at lower gain.

Smaller diodes obviously have a smaller collecting area. Use optics
to focus light onto the diode's active surface. Use other optical
techniques to improve collection efficiency such as anti-reflective coatings.

Tiny photodiodes are commonly used at gigabit data rates in fiber optic circuits. I've used a 2 millimeter diameter (relatively huge) photodiodes to measure light from 10th magnitude stars with 1% precision though the circuit bandwidth was around 1/10 hz in that application.

You haven't told us your application or what bandwidth and sensitivity you need. You'll need to build your circuit and actually measure it's bandwidth. The stray capacitance around the op-amp is usually important to the overall performance of real circuits and it's a pain to try to calculate. What's the capacitance of the amplifier's feedback resistor? It does matter.

If you need a large light collecting area, high sensitivity, and wide bandwidth consider using a photomultiplier. They don't have that nasty shunt capacitance to deal with.
.

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