I always wondered how my TV’s remote control worked as a child. Once I got my first camera phone, I realized my phone’s camera was sensitive to the infrared LED on most remote controls, and that the TV must have the same type of detector. This is where a photodiode circuit comes into play and allows the TV to receive and decode the modulated infrared signal from a remote.
Photodiodes have a number of uses besides changing the volume on your television. Whatever application you are working with, you’ll need to understand how a photodiode generates some current and how to design a circuit that uses this current.
How Does a Photodiode Work?
In the simplest sense, photodiodes are photoelectrical elements, meaning they convert light to electricity. They can also be driven with a voltage to produce a small amount of current in the absence of a light source. Once light hits the photodiode, the current increases significantly, similar to a solar cell. If you look at the current-voltage curve for a photodiode, it has the same shape as the current-voltage curve for a solar cell. In this way, a photodiode acts like a simple light sensor, where the sensitivity can be controlled with an applied voltage.
When a photodiode is driven with some voltage in the absence of illumination, it will behave like a typical rectifying diode. At low applied bias, there is a small amount of saturation current (called the dark current) that will flow through the device. Just like a typical diode, a photodiode has some breakdown voltage in reverse bias.
Once a photodiode is illuminated, the behavior of the device depends on whether there is a bias applied to the photodiode, as well as the polarity. The IV curve for the device will scale with the applied voltage. A photodiode has two modes of operation: photovoltaic mode (when run in forward bias) and photodiode mode (when run in reverse bias). In both cases, the current can be calculated with the following equation:
where P is the optical power of the light source (W), and R is the responsivity of the photodiode (A/W). Note that the remaining parameters in this equation have the same meaning as those in a typical rectifying diode.
Selecting a Photodiode for Your PCB
There are a number of operational parameters to consider when selecting a photodiode for your PCB. Here are some of the most important photodiode parameters to consider:
Responsivity: this is a measure of the current produced per Watt of optical power that falls on the device. Responsivity is actually a spectrum, meaning that it is a function of the wavelength of incident light. This is the range of wavelengths to which the photodiode will produce a current. As an example, silicon photodiodes are typically sensitive to wavelengths ranging from approximately 200 nm (UV) to approximately 1100 nm (IR). The responsivity will vary throughout this wavelength range.
Shunt resistance: this is the slope of the current-voltage curve of the photodiode at zero applied bias. An ideal photodiode should have infinite shunt resistance, but the actual shunt resistance can range from 10’s to 1000’s of mega Ohms, thus the best photodiodes have the highest shunt resistance.
Saturation current: this is related to the shunt resistance and is equal to the current when run at reverse bias (between 0 V and the reverse breakdown voltage). The shunt resistance is normally defined with reference to a 10 mV source voltage and the saturation current using Ohm’s law.
Terminal capacitance: this determines the transient response time of a photodiode. When combined with the load resistance, the rise time is equal to the RC time constant.
Responsivity temperature coefficient: as the temperature of a photodiode changes, the responsivity at specific wavelengths will also change. The peak in the responsivity spectrum will exhibit a redshift as temperature increases due to band narrowing.
Designing a Photodiode Circuit
There are many applications for photodiodes, and the exact circuit you need to build will depend on the desired application. One of the simplest photodiode circuits is a light sensor. This involves connecting the output from the photodiode to the inverting input of an operational amplifier, and a feedback resistor is connected between the output and the input on the op-amp. This essentially converts the current from the photodiode to a voltage, where the output voltage is equal to the photodiode current multiplied by the feedback resistance. In this case, you do not need to place the photodiode under bias.
A simple photodiode circuit with an amplifier
Note that a photodiode can also be used to receive digital data encoded in modulated optical pulses. This is typically done by applying pulse width modulation or amplitude modulation to the light source. In the case of pulse width modulation, you will need to account for the frequency limitations of your photodiode and amplifier as this limits the maximum data rate. The frequency limitation of a photodiode is related to its response time (see terminal capacitance above). The maximum response frequency is typically taken as the knee frequency for a digital pulse with a particular rise time. This frequency is equal to 0.35/(response time).
A certain type of photodiode called an avalanche photodiode is specifically designed to operate at a reverse bias that is near the reverse breakdown voltage. In effect, this multiplies the number of photo-generated carriers, producing some electrical gain during illumination. This type of photodiode is useful when working with weak optical signals and is more durable than simply running a standard photodiode near the reverse breakdown voltage.
In order to design a photodiode circuit easier next time, place all analog circuitry away from (noisy) digital circuits. Follow up by keeping the planes that supply voltage to the analog and digital circuitry separated. These planes are generally connected somewhere in the chassis or on the board, but keeping them separate keeps the noise away from analog circuitry.
Designing systems that use photodiodes as switches or sensors is easy when you use the right PCB layout and design software with a full suite of design tools. Allegro PCB Designer includes the features you need to layout boards photodiodes. You can also import your board into Cadence’s analysis tools to simulate and analyze the behavior of your photodiode circuit.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.
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