I have a TIA with 1.5kohm of gain and 10GHz BW. I have to select a Photodiode for this TIA. I was wondering how do I determine the upper limit on photodiode junction capacitance when selecting the photodiode.
Thank You
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The feedback resistor and the capacitance form a low pass within the closed loop. The amplifier has an open loop response curve.
Two things set the limit on capacitance.
(1) The total gain around the loop must be greater than one at the desired system bandwidth.
(2) The phase shift around the loop should not exceed 135 degrees at the point where the total gain is one.
it
As small as possible, Better yet hire Phil Hobbs as a consultant.
George H.
You'll have a hard time connecting a discrete pin diode to a packaged tia, at least if you want 10 GHz bandwidth. This stuff is usually wire-bonded. It's easier to buy someone's packaged pin-tia in a can. The telecom parts are built in volume and are fairly cheap.
John
That sort of TIA isn't a general-purpose part--it will have been designed with specific photodiodes in mind, as well as specific construction techniques.
I strongly suggest asking the manufacturer and listening closely to what they say--making a 10 GHz TIA work right is not something that happens by accident.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
You've been given some good advice, but here's the relevant formula. Common resistor-feedback TIA amplifiers need some feedback capacitance for stability, to cancel the feedback- resistor / summing-junction capacitance pole. The scene is often expressed as a maximum Rf Cf bandwidth, due to Rf Cin and f_T. The important parts are the Rf Cin noise-gain and the opamp rolloff to f_T, giving us a geometric mean for fc. More or less, fc = sqrt [f_T * 1 / (2pi Rf Cin)]. We can rearrange this to get Cin < f_T / 2pi Rf fc^2.
You have Rf = 1.5k, fc = 10GHz, and let's guess f_T = 30GHz. Just a guess. That gives us Cin < 0.03pF. That's a damn small capacitance, which means (1) a very small back-biased detector die, and (2) no budget for wiring capacitance. It may be that the amplifier has Rf lower than 1.5k, followed by some gain. That would help. Not an easy scene. That's why most receivers in that bandwidth are sold as integrated units. And most of the discrete TIA dies died out.
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Thanks,
- Win
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