Photodiodes are often used as passive elements to detect optical signals and output a current.
When a bias is applied to a photodiode, the current output can be controlled to provide thresholding, linear response, or nonlinear response.
In particular, placing a photodiode in reverse bias or small forward causes the output current to be a linear function of input light intensity.
Simple circuits with photodiode bias can be used to gather sensitive optical measurements.
Your next optical system will need some kind of detector, and photodiodes are a standard detector element used to convert input light into an electrical signal. A photodiode is just like any other diode in that a voltage can be applied across the terminals. However, it’s also a light detector, and the input light in a photodiode generates a current.
When designing a circuit to collect and measure the current from a photodiode, it should be obvious that the current can also be controlled by applying some photodiode bias. The photodiode bias you use will determine how the device responds to input light, i.e., whether the output current is linear or nonlinear. When you need to integrate your photodiode into a standard detection and measurement circuit (i.e., with an amplifier) here’s how you can ensure you get the desired linear or nonlinear output.
How Photodiode Bias Affects Optical Response
To better understand why you might want to apply photodiode bias in an optical detector circuit, take a look at the standard diode equation with additional current generated by input light with optical power of P. In the equation shown below, the total output current is just the sum of the photocurrent, equal to PR, where R is the photodiode’s responsivity (measured in A/W). The remaining quantities in the photodiode have their regular meaning in the standard diode current equation.
Photodiode current equation.
Note that all photodiodes have some input optical intensity at which nonlinear effects begin to become apparent, and the above equation assumes we are operating with a linear optical response but a nonlinear electrical response.
The input optical power generates some electrons in the photodiode junction which then move away from the junction in reverse bias (thus the negative sign). By applying some bias, you can increase or decrease the reverse bias current. There are three regimes where the photodiode bias can be used to control the output current and how the photodiode responds to input light:
V ~ VB (breakdown mode). This is normally used in avalanche photodiodes to detect very low-level optical signals. By taking advantage of gain from impact ionization, the diode can output a very large current when a weak optical signal is received by the photodiode, but the photodiode response is nonlinear
0 < V < -VB (photoconductive mode). In this region, the capacitance seen by electrical signals leaving the photodiode is lower, so the device can operate with a faster response. The drawback is that the dark current is larger (i.e., the noise floor is higher). Fast photodiodes that need to detect very fast optical pulses are normally used in this range of photodiode bias. The output current is also highly linear.
Small positive V (photovoltaic mode). In this region, the time-domain response is slower due to capacitance, but the input light from the photodiode will still be in the linear regime. This region is also used to develop a load line for the device such that the output has maximum current at a particular photodiode bias.
How to Apply Photodiode Bias
When you’re designing a photodiode circuit, the application of bias does not depend on the way in which the signal from a photodiode is collected and read. The typical way to use a photodiode with bias, whether with discrete components or in an integrated array, is to use a transimpedance amplifier. A typical arrangement is shown in the circuit below.
Photodiode circuit in photoconductive mode and applied bias.
In the above circuit, the applied bias puts the photodiode in reverse bias, so we are operating in photoconductive mode. This circuit could also be used with high reverse and an avalanche photodiode, which would provide high gain for detecting low-level optical signals. To run the circuit in forward bias, simply flip the voltage source around.
Ensuring Linear Output Current
Even though a photodiode is an optically linear circuit, it is also an electrically nonlinear circuit. Because of this, a photodiode needs to be designed by simulating a load line for the photodiode circuit. By adjusting the value of the resistor, you can extract a range of input light intensity values where the output current is linear.
To do this, you need a SPICE simulator with SPICE subcircuit models for your components. The important simulation involved here is to sweep the value of the photodiode bias and simulate the output current for different input light powers. This also requires placing a parallel current source on the photodiode circuit to simulate the device’s responsivity and input optical power. To learn more about this process, read this article for guidelines on these simulations.
When you need to design an optical detector circuit, you should use the best PCB layout and design software to lay out your board and ensure you apply the correct photodiode bias. Allegro PCB Editor includes all the PCB layout tools you need to capture your photodiode circuit, place components in a PCB layout, and verify the position of optical elements in 3D. You’ll also have access to advanced design verification tools and field solver utilities to analyze the behavior of your high-speed and high-frequency electronics.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.