Ferrite bead model and real behaviour

Bias does dramatically reduce transit time, but it's mostly by widening the depletion zone. In the undepleted region, carriers have to propagate by diffusion, which is slow. It does change the spectral response and responsivity a little, but it's a second order effect caused by reducing recombination.

Higher photocurrents also improve the responsivity by a percent or so by saturating the recombination centres. (Interestingly, putting a positive voltage on top of the oxide helps too, by repelling carriers from the bottom of the oxide, where there are a lot of traps.)

The guy that thought up those names should have been boiled in oil. The number of people who expect photodiodes run at reverse bias to behave like photoconductors would blow your mind. (They don't.)

I use quad cells that way to take ratios. On my shelf I have a box that I built in 1990 that uses a Ge quad cell and a dual op amp to make a

You only need a dual op amp because if you have the diodes arranged as

-----

then if you form the differences A = (V1-V4) and B = (V2-V3), then

X = B-A

and

Y = A+B.

You wire one meter and a resistor between A and B for X, and two resistors to one side of the meter and ground to the other side for Y.

Cheers

Phil Hobbs

Usually what I care about is that the eN*C noise goes down by 17 dB.

Cheers

Phil Hobbs

Different worlds. Whenever I've had a PD circuit, I've always been pushing the speed limit more than the noise limit.

fortunately there is a wide spectrum of beads to choose from that span from lower ranges with higher mu and higher microwave ranges with lower mu and lower ESR from the Nickel content.

Speed doesn't always change at the same rate as capacitance--after a certain point most PDs are limited by carrier diffusion in the epi or the back contact region.

Cheers

Phil Hobbs

I'm usually designing cheap-enough circuits that it's limited by the PD capacitance -- it's the usual PD current into an op-amp, which then needs to be compensated for the PD capacitance to the bias supply's ground, and the higher the PD capacitance, the lower the op-amp pole gets.

I'm sure there's better ways -- I've just never needed to go there.

Bootstrap cascode! See Phil's book.

Just the cascode can isolate the PD capacitance from the opamp, for maybe 3 cents.

Hey, I know! What if we use a negative-capacitance circuit to cancel out the PD's capacitance? ;-)

Tim

Yay! That'll fix it >:-} ...Jim Thompson

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