Selecting photodiode for Tranimpedance amplifier
in [Design]
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From: Winfield Hill on 17 Jun 2010 21:13 electro wrote... > > 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. 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. -- Thanks, - Win |