Photodiode Amplifier Noise Effects

I don't think a bit of noise on the photodiode bais voltage is much of a problem. When you remove the photodiode try putting in a capacitor whose value is equal to the capacitance of the biased photodiode and meause the noise again.

George Herold

Thanks George,

Interesting.

Subbing in a 10pF NPO cap (the max capacitance of the PD at -5V as per the datasheet) yields a somewhat reduced level of about 140nV/RtHz.

That seems to indicate to me that some of the objectionable signal is, in fact, coming from the bias supply, and some is from some other unanticipated effect of connecting the InGaAs photodiode assembly to the cascode amplifier, correct?

Why don't you remove supply and add ground, and see what happens.

greg

This is very interesting to me. Please post results.

greg

Yup.

I suggest you read and go through the noise analysis of the photodiode TIA circuit. There is 'noise gain' in the circuit that is due to the photodiode capacitance. The idea behind Phil H's cascode input is to reduce this effect. I've not done any noise analysis of the TIA circuit with a cascode input. (I've dreamed of building this circuit, but as of yet have not had the free time or application that needed it.) You might estimate the impedance of the 5pF at 75 MHz and compare this to the impedance of the cascode. (Are you running any DC bias current through the cascode?) I think you can drive the cascode harder and reduce it's impedance.

I'm not sure why you are focused on the bias supply. The purpose of the bias supply is to reduce the capacitance of the photodiode. A little noise on the bias voltage will mean a bit of fluctuation in the photodioe capacitance, but this should hardly effect the noise at all. (Try using a noisier voltage bias and see if this increases your output noise.)

Did you build this photodiode preamp yourself?

George Herold

George,

Grounding the bias end of the 'fake' photodiode (the 10pF cap) yields a similar noise figure (138nV/rtHz). Essentially this is AC noise

Yes, Phil's circuit is meant tor educe the noise gain effect of Cd by isolating it from the TIA via the cascode amplifier. The Cd is then replaced by the (hopefully) smaller Cs of the cascode amp's collector.

This was the point of removing the photodiode from it (there's not much I can do about the Cs of the cascode, outside of replacing the transistor). By simply removing the photodiode what I have left is the cascode (biased on) plus the TIA, amplifying it's own noise plus the input from the cascode.

Without the photodiode gives me: TIA noise + cascode noise

Installing the photodiode should give me: TIA noise + cascode noise + (photodiode NEP * responsivity) + Bias supply noise

The NEP on the photodiode yields sub nV/RtHz after the TIA, so all I have left is the bias supply I wanted to see if there was anything I could do on the very front end to quiet things down a little in the 30-60MHz region.

You can bias the cascode and reduce the re of the emitter (which I have done). The amp is biased to 417uA, which seems like a lot, but I needed that to get the response flatness over the BW I required. The effects of this should be included as described above.

I am focused on the bias supply because that seems to be all that is left (as far as I know) for a noise source. Noise on the photodiode bias will be divided across the impedance of the bias voltage filter, the photodiode (362 ohms), and the cascode emitter (.

My astrophysicist buddy suggested thermocouple effects as well (I'll try that next with a hunk of wire and some cold spray)

Schematic plus layout, yes. Fab + assembly done by board house.

Whitedog, My copy of Phil's book is at home, but I'll look later and see if he does any noise analysis in it. (Better yet he may log on and and you can hear it right from the horses mouth. My guess (and this is really a guess mind you) is that you are seeing the TIA op-amp noise coupled to the output when you put either the photodiode (PD) or equivalent capacitor in the circuit. You would certainly see the same type of thing if you removed the PD from a 'traditional' TIA without the cascode front end. (Less noise without the PD and more with PD or equivalent capacitor.)

You should be able to 'filter the piss' out of the bias supply. A simple RC should be more than adequate. If you are really worried stick in the 'capacitor multiplier' circuit also in Phil's book. I've used this circuit to reduce 15 Volt power supply noise to the 1 nV/ rtHz level.

What opamp are you using in the TIA? What level of photocurrent are you trying to sense? Can you increase the bias voltage on the PD?

"Schematic plus layout, yes. Fab + assembly done by board house."

Ahh I'm jealous... Well someday I hope to build a fast photodiode front end of my own.

George Herold

(I'd forget the thermocouple effects. This can give you some DC offsets, but I've never heard of it affecting the noise at 30-60 MHz.)

ias

ide quoted text -

Whitedog, I was thinking of this on the ride home. Your numbers suggest that you're using an opamp with a BJT front end. (The 1-2 pA/rtHz of current noise through the 35 k ohm resistor is your 80 nV/rtHz. of 'preamp' noise.) You really need to stick an FET opamp your circuit.

If your supplies are only +/- 5 Volts there is a "fast FET" from analog you might try. (If you bias every thing right you might be able to run these from 0 to +10 V)

George

Oops! Do not share these.

I read the thread. Separate the photodiode bias supply from the TIA supply. "Yellow wire" it if you have to.