Photodiode amplifier circuit that detects changes in ambient light
Detects various light sources such as visible light, infrared, and ultraviolet
Increases the output voltage of the circuit by generating current according to light energy.
You may have noticed that the displays and systems inside your car gradually become brighter as the sun sets on your way home from work. In the past, you had to manually adjust the display brightness in cars, but now the brightness is automatically adjusted without driver intervention.

Operational amplifiers are a simple way to control backlights in vehicles to make displays easier for drivers to see and save power. Using operational amplifiers instead of complex light sensors allows for more flexible customization.


Design of operational amplifier with transimpedance configuration
Automotive display systems such as head units, remote displays, clusters, and head-up displays detect ambient illuminance with circuitry and adjust backlight brightness based on light intensity or time of day through ADCs or MCUs. Transimpedance amplifier circuits, more specifically photodiode amplifier circuits, can detect these changes in ambient light and respond accordingly.

The goal of a photodiode amplifier circuit is to convert the low current from a photodiode into a useful voltage. To do this, design the operational amplifier in a transimpedance configuration as shown in the image below. Photodiodes can detect various light sources such as visible light, infrared, and ultraviolet light. When the photodiode is exposed to more light energy, it generates a higher current, which increases the output voltage of the circuit.


Using transimpedance configuration in vehicles
For automotive use, a small bias voltage must be applied to the non-inverting input of the operational amplifier to prevent the output from saturating on the negative power rail when there is no input current.

A resistor divider is often used to bias the non-inverting input above ground with a positive voltage supply. This reverse bias allows the circuit to respond faster, at the expense of lowering the photodiode junction capacitance. Reverse bias also improves high-frequency performance. A feedback capacitor is required to maintain circuit stability.
Photodiode amplifiers allow for flexible customization of the circuit to suit your needs. By strategically selecting automotive grade operational amplifiers or adjusting the components in the circuit, gain can be controlled. This allows for providing appropriate input bias current or adding compensation to correct stability issues.

A wide range of automotive grade operational amplifiers are available to suit each specific application. For example, TI's OPA348-Q1 provides a low photodiode current of 45 mA and operates with an extended bandwidth of 1 MHz.

TI's LM2904-Q1 consists of two independent frequency-compensated operational amplifiers with a gain bandwidth product of 0.7MHz. The operational amplifier provides an economical solution for automatic regulation. It is priced at less than $0.25 in 1,000-unit quantities, less than a quarter of the price of other ICs with the same functionality.

Photodiode amplifier circuits are a simple, yet flexible way to automatically measure the backlight in many automotive systems. TI offers a comprehensive range of operational amplifier products and resources to support circuit design, enabling you to design a customized solution for backlight control.

Using a transimpedance configuration in this way in automotive display systems allows for simple and low-cost automatic backlight control.

This article is based on a contribution from Texas Instruments.