LM741 as a Transimpedance Amplifier
Motivation
W1VLF’s optical rig uses a ready-made transimpedance amplifier based on a low noise op-amp which has its output fed to a coaxial cable connected to a SDR dongle. I don’t have any great op-amps on hand, but I’d still like something to get up and running with a receiver.
I have several LM741 op-amp ICs from when I was in University, so I thought I’d try to design around that.
Research
TI and John Caldwell have published a photodiode amplifier reference design guide: “TI Designs - Precision: Verified Design – 1MHz, Single-Supply, Photodiode Amplifier Reference Design” from November 2014. This guide was extemely useful for helping predict the performance characteristics and identifying likely shortcomings of designs based on which op-amp and photodiode are used. Critical values that I needed to find for my parts were:
- Op Amp’s Unity Gain Bandwidth
- Op Amp’s Common Mode and Differential Mode input capacitance
- Photodiode’s minimum and maximum currents based on illumantion
- Photodiode’s capacitance under the likely reverse-bias voltage (depending on the op-amp’s supply voltage and output range)
The photodiode I currently have on-hand is for IR remote receivers, and it is an obsolete part from Panasonic, PN323. Digikey helpfully provides the datasheet for it.
TI’s datasheet for the LM741 was also handy, though it didn’t provide input capacitance values. The reference design guide suggests a value of 10pF is likely to be near the value of the sum of the differential and half of the common mode capacitance.
The LM741 probably has pretty small input capacitance, judging by its block diagram, so I’ll use those default values for now.
Preliminary Design
I chose to target a bandwidth of around 12KHz, in order to match the bandwidth of my pi-pico synthesizer/transmitter. The signals of interest are going to be between 200 and 3000 Hz, so this should be plenty of bandwidth.
In order to make best use of the output range of the op-amp, the feedback resistor is chosen such that it hits a maximum output voltage when the photodiode is conducting the maximum light-stimulated current. For 55uA, I determined a value of 68K would work well. This will probably clip under intense illumination, but intense illumination is an unlikely scenario. for my use-case.
In order to filter out the high frequency components and stabilize the amplifier, I’m using a feedback capacitor with a value of 200pF
There is a stability check based on the capacitances and the op amp’s unity gain bandwidth. With a 1MHz unity gain bandwidth for the LM741, and a 12KHz pole for the feedback path, I am well within the stable operating range.
Simulation
In order to determine likely values for input biasing and output filtering, I set up a simulation in the circuitjs simulator.
It has a simulation model for the LM741 op amp, though I don’t know how accurate it is for this use case.
It is worth noting that a 5V and 0V supply voltage to the LM741 is not within its recommended operating parameters, but I need very little output current and I don’t need much range of output voltages, either. The output is going to a SDR (or maybe directly to a soundcard?).
Built circuit
I built the circuit as described in the simulator, but using a fixed feedback resistance of 68k ohms and a 1nF feedback capacitor. I used the output of the op-amp to drive a piezo buzzer and tested the reception of IR remote signals, as well as the signals from my own IR transmitter circuits.
It’s capable of driving the piezo quite strongly. Because I’m hoping to connect this to an SDR or soundcard, I’ll have to consider protection circuitry.
The signals come through cleanly, as long as I set the bias voltage on the non-inverting input correctly. This will be a useful tool for debugging my synthesizer.
I also tested this with a magnifying glass to focus onto the photodiode and I could hear the buzzing from a remote pointed at it from about 5 meters away. Considering I need considerably less voltage for driving a soundcard’s line-in, I suspect this apparatus would work pretty well for far greater distances.