HF/VHF photodiode amplifier


Recently at work I was tasked with determining the frequency modulation of a laser beam. I designed and tested the above circuit in a couple hours.

I tested the circuit first with no photodiode in place using my DG8SAQ vector network analyzer — a very low cost network analyzer based on the highly respected (by me) AD9852 DDS chip.

The circuit showed >+20dB gain from about 4 MHz to the cutoff frequency of the RLP-137. Hooray for small victories!

For the photodiode, I had no choice because I just had one laying around here that I could find the part number for: a Melles Griot 13DSI001. This particular part has a responsivity of 0.45 Amps/Watt at 830nm, breakdown voltage of 60, and 10pF capacitance.

A photodiode is typically reverse biased as you see above, with a positive voltage on the cathode. In my case I don’t care very much about what voltage I’m using as long as I do not exceed the breakdown voltage (more about that coming up.)

As light becomes incident on the diode junction, carriers are created that allow current to flow from the high potential to the low potential. This holds true for high frequencies; a high frequency alternating current is allowed through the device.

When a current source is terminated into a resistor we get a voltage source having an impedance of the resistor value. The photodiode is terminated into a 50 ohm resistor, hence we have a fairly decent 50 ohm source that is converting light modulation into RF of a convenient impedance. The maximum frequency (as I understand) is limited by the 10pF intrinsic capacitance of D1, combined with the load resistance, R3. This gives a cutoff frequency of 1/(2*Pi*RC) of about 300 MHz. I may not have needed the lowpass filter (RLP-137), however with my shoddy perf-board construction I was worried about the GALI-74 potentially oscillating in the GHz range.

The 1k resistor R1 is to limit the maximum current that can pass through the photodiode. Combined with C3, this provides filtering from feedback or noise that could come through VCC.

I’d be happy to see your comments below if you have corrections or advice.

MMIC photodiode amplifier
MMIC photodiode amplifier