I designed a new photodiode amplifier. Click on the image for a larger view:
And an associated printed circuit board:
The printed circuit board is available to purchase from OSH Park if you like.
I will update this post after I receive the PCB and test the circuit.
The first part of the schematic is the power supply. VIN is +15V.
The L78L12 supplies regulated +12V to bias the photodiode. The RLC lumped element filter that follows ensures a low noise voltage source for the photodiode bias.
The filter quickly drops below the noise floor above a couple MHz. This is theoretical. Actual performance won’t be quite as nice.
The 47 Ohm resistors thrown into the filter will not measurably affect the photodiode response. The DC power will be no more than a few milliwatts maximum.
The AC power will probably be below the microwatt level, I’ll guess around -40dBm. Again, I’ll update this when I have some results.
The next section I’ll talk about is the photodiode itself and the amplifier.
The photodiode is an OPF432. It comes in a handy package that has an ST style fiber optic connector so you don’t have to have any bulky external mechanical parts to couple light into the diode.
The photodiode is reverse biased at +12V. According to the datasheet this should provide a rise time of about 8 microseconds. I believe this refers to the time constant. The datasheet claims this part can be used up to 100 MHz which is a period of 10ns.
The signal rises in half the period which is 5ns. If the datasheet rise time is accurate at 8 microseconds, I believe the AC output amplitude (peak) of a 100 MHz modulation will be:
Current = (5ns/8us)*50A/W = .03125 A/W
A/W is amps per watt of input power. In my application the input amplitude may be approximately 10uW. Multiplying:
31.25mA/W * 10uW = 312.5nW
This sounds very low, but let’s convert to units of dBm (decibel milliwatts):
P = 10*log10(312.5nW)+30 = -35 dBm
The GALI-74 is an MMIC that conveniently provides a 50-ohm matched input and output with +25dB of gain from low frequencies up to well above the design goal of 100 MHz.
The output will be at about -10 dBm which corresponds to an amplitude of approximately 70 millivolts into a 50 ohm load.
The final section of the schematic provides a low pass filtered output which indicates the DC amplitude of the incoming light.