General Information on photodiodes and transimpedance amplifiers
Photodiodes convert light to current. There are two basic ways of operating a photodiode. They are photovoltaic (unbiased) and photoconductive (biased). There are advantages and disadvantages of both circuits. Although an unbiased photodiode's dark current is very low, its speed is very slow and its capacitance high. A biased photodiode's dark current is higher than an unbiased circuit, but its speed is faster and capacitance lower. High capacitance causes high noise in the amplifier, and a biased circuit's dark current can cause high offsets in amplifiers with high gain. Therefore, these are very important to parameters to consider when designing and amplifier with a photodiode.
Since the photodiode's output is current, it needs to be converted into a voltage for most applications. The most common current to voltage converter circuit is the transimpedance amplifier. The transimpedance amplifier is popular because of its low input impedance, linearity, large dynamic, low output impedance, and good bandwidth.
A transimpedance amplifier is an operational amplifier with a feedback resistor and usually has a feedback capacitor. These feedback components are connected between the operational amplifiers inverting input and its output. The photodiode is also connected to the inverting input. The operational amplifier's non-inverting input is connected to ground.
View Schematics of Circuits The current to voltage conversion gain is equal to the photodiode's photo generated current multiplied by the transimpedance amplifier's feedback resistor value, excluding offsets.The two spreadsheet files described below simplify the design process and have all the information needed to quickly design a photodiode / transimpedance amplifier.
Description of Spreadsheet Model
There are two circuit model Excel spreadsheet files, one for modeling unbiased, and one for modeling biased photodiodes connected to a transimpedance amplifiers. Click on the underlined view sheet links as you read to view examples of the unbiased spreadsheet file's sheets, or open the spreadsheets right now by going to the download section at bottom of the page. You are welcome to save these two files on your computer.
View Sheet 1 Sheet one of the spreadsheets is an input sheet. View Sheet 2 Sheet Two is the calculation sheet that calculates all the major characteristics of a transimpedance/photodiode circuit. It includes maximum offset, noise, sensitivity, rise time, and bandwidth. It determines if the design will be stable. It also determines the feedback capacitance for a 5% and 25% output overshoot for a given feedback resistance. View Sheet 3 Sheet Three has a table of over 100 operational amplifiers to choose from. The amplifier's characteristics can be used to sort the table using Excel's sort command.Spreadsheet Model Instructions
Select the biased or unbiased spreadsheet models.
These instructions are for the unbiased spreadsheet. The biased spreadsheet is very similar.
View model schematic on the bottom of Sheet One of the spreadsheet.
View Sheet 1 Select a photodiode from the Advanced Photonix website characteristics in cells C10 to C14 of Sheet One.Enter the feedback resistor and capacitor values in cells F17 and F18 respectively. Remember that most resistors have about 0.25 pF of capacitance so be sure to add that to your feedback capacitance. If the designer isn't sure what feedback resistor value to use, just pick 100 kohms to start, it can be optimized later.
Enter the part number of the operational amplifier from Sheet Three in cell C9 of Sheet One and its characteristics will automatically appear in cells C10 to C15 of Sheet One.
View Sheet 1 & View Sheet 3 For non-biased photodiode circuits only FET and CMOS input amplifiers should be selected. Large value feedback resistor circuits also require these types of amplifiers. For biased circuits a bipolar operational amplifier can also be considered especially for Avalanche photodiodes. Bipolar amplifiers are usually only used with feedback resistors of 50k ohms and lower, because the output offsets caused by their high input bias current. Bipolar amplifiers have lower input voltage noise, but higher input current noise FET amplifiers. They are typically much faster than FET and CMOS type amplifiers and maybe considered for high-speed circuitry. Try the different amplifiers, feedback components, and photodiodes and see how they perform on Sheet Two. View Sheet 2The Maximizing Bandwidth Approximations Section on the bottom of Sheet Two is used to optimize the circuit for speed.
View Sheet 2 Feedback capacitance values for an overshoot of 5% and 25% are calculated. The 5% overshoot capacitance value is the smallest capacitance recommended in a production circuit. A capacitance value for a 25% can be used, but the devices will need to be tested for overshoot because the component tolerances may cause the circuit to have excessive ringing and instability. Remember the faster the circuit is the more noise it will have and less stable it will be; therefore don't design the circuit for more bandwidth than is required.The biased spreadsheet is basically the same except you enter dark current instead of shunt resistance, and some of the calculations are different.
That's basically all you need to know to design transimpedance amplifier circuits. One final note, power supply bypass capacitors recommended by the operational amplifier manufacturer should always be used and are not shown in any of the model or circuitry schematics.