Photodiode amplifier - version 2

A revised version of the circuit and PCB for a photodiode amplifier, to be used in PDH-locking (Pound-Drever-Hall) as well as RAM-nulling (residual amplitude modulation) in a laser experiment I am doing. The changes compared to the first prototype are:

  • The required bandwidth and gain is not easy to achieve in one stage, so there's a second stage of amplification after the transimpedance amplifier.
  • I'm suspicious of the noise caused by the switched-mode powersupply, as well as the DC2DC converter, of the previous design. So this circuit has just +/-5 V regulators and can be driven from a regular (known good) +/-12 V lab powersupply (or even two 9 V batteries).

Here is a schematic and simulation results produced with the free version of NI Multisim from Analog Devices. The design is for roughly 1 MOhm of transimpedance gain in total, here split between 7 kV/A transimpedance gain, and 144 V/V for the non-inverting second op-amp. At 1 kV/A of transimpedance gain a 5 uW optical signal at 633 nm (HeNe laser!) that produces a 2 uA photocurrent will result in a 2 V output signal. The AC analysis shows very slight gain-peaking for the transimpedance-stage (red trace) and a -3 dB bandwidth of >3 MHz overall (green trace).

pd_amp_2013feb8 pd_amp_2013feb8_ac_sim

The first op-amp used in the transimpedance stage only needs to have a bandwidth slightly exceeding the transimpedance gain bandwidth (the feedback resistor R1 together with the compensating cap C1, the capacitance of the photodiode C2, and the input-capacitance (not shown) of the op-amp form an RC low-pass filter). The AD8597 is marketed as "ultralow distortion/noise" and is fast enough (10 MHz). The second non-inverting op-amp needs a high gain-bandwidth-product (GBP) since we are amplifying ~100-fold here. The ADA4817 has a small-signal bandwidth of 1 GHz and GBP~400 MHz, so should work OK here.

A voltage of only 14 mV over the transimpedance-resistor is not ideal. The Johnson noise (which in principle a good designer can control/minimize) in the resistor will dominate over the shot noise (which we cannot avoid) in the optical signal. For shot-noise limited performance the rule of thumb is to make the voltage drop at least 51 mV (which will make Johnson and shot noise equal). Without tricks however that is not possible as here we have both a weak signal (2 uA of photocurrent), we want a high gain (1 kV/A in total), and we want to go fast (~3 MHz bandwidth)! If you relax any of those requirements (more power, less gain, slower response) it is straightforward to build a shot-noise limited amplifier in one or two stages.

The PCB, fresh from the mill:


Far right is a 3-pin TO-18 socket for the photodiode. Right-middle are the two op-amps with their feedback-resistors/caps, as well as two de-coupling caps for both +5V and -5V. Left-middle are 7805 and 7905 voltage regulators, and the BNC output-connector is far left. All the surface mount components are mounted on the top layer of the board, while the through-hole components are bottom-mounted. Resistors and caps are 1206-size. This PCB should fit the earlier enclosures I turned on the lathe.

Hopefully I will have time to assemble and test one or two of these next week. I should measure the actual frequency-response and compare it with the simulated one.

LPKF Protomat S91 PCB-mill

Update: here's a picture of how the original spindle looks like.

By popular demand, some pictures of the modified LPKF Protomat S91 PCB-mill (featured here). The spindle assembly on this mill has been re-built. The original has an LPKF spindle motor and a solenoid for pushing/pulling the spindle up/down along the Z-axis. This modification uses a Proxxon spindle and an air-cylinder for the Z-movement.



These two pictures shows the spindle from the front. Pressurized air is input to the valve which routes it either to output A or B. This pushes the air cylinder either to the UP or DOWN position. The cylinder pushes on an aluminium plate to which the spindle motor is attached. The moving plate is guided by a linear bearing. A screw at the top of the linear bearing allows adjustment of the Z-depth of the DOWN position. A spring at the top also helps with pushing up the spindle.



This picture shows the cutter. A vacuum cleaner attaches to they grey tube, and sucks away all chips produced during drilling and milling. A cylindrical cover or "door" around the spindle (now open for tool change) is rotated shut when the machine runs.


This shows the electrical connections. The modification of the spindle involves connecting a cable from the second connector from the right to a custom-built relay box. Otherwise the connections are as on a standard machine.



This shows the relay box. The cable from the base of the machine is used to control three On/Off devices: vacuum-cleaner on/off, spindle-motor on/off, and Z-axis up/down. The spindle-motor and vacuum cleaner connect to standard AC-mains sockets. The Z-axis up/down control signal is connected to the air-valve on the spindle assembly.


Some additional views. Note how small the required Z-movement is.


These pictures show the air-cylinder.


PCB Milling

We have a 1994 LPKF Protomat S91 PCB mill in the lab for making prototype PCBs. Here it was used to cut a circular part (not a PCB) which was first drawn in CorelDraw, then saved in HPGL format, and then opened in BoardMaster which is the program that controls the mill through a quirky serial protocol. I think the original LPKF design has a solenoid for the up/down z-movement of the tool. The solenoid would become unreliable during a long run, because it was getting very hot, so on our mill it has been replaced with a more reliable pneumatic cylinder. The spindle is a Proxxon hand-tool, and tool changes are manual.