Phototransistor Basics: Types and Applications

This article covers phototransistor basics, including different types and their applications.

What is a Phototransistor?

• A phototransistor is a transistor controlled by exposure to light. It’s similar to a photodiode controlling an ordinary bipolar transistor.
• It can be either a bipolar transistor or a field-effect transistor (FET).
• The body is encased in resin or plastic or encased in a metal shell with a window.

• Figure 1 depicts the most common variant of phototransistor, the Bipolar NPN.
• Examples: Optek/TT Electronics OP506A, Vishay TEKT5400S, Vishay BPW17N etc.

Phototransistor Applications

Phototransistors have several applications:

• Light measurement or as a light-sensitive switch.
• Its output can be interfaced with a microcontroller containing an ADC (Analog-to-Digital Converter).
• To achieve a clear ON/OFF signal, it can be interfaced to the input of a logic gate containing a Schmitt trigger or processed by a comparator.
• Optocouplers contain a phototransistor activated by an internal LED device. This allows switching current while isolating one part of the circuit from another.

Types of Phototransistors

Here are the different types of phototransistors based on their construction and principles of operation:

• BJT Phototransistor: In the absence of light, a bipolar phototransistor permits a leakage current between the collector and emitter of 100 nA or less. When exposed to light, it can conduct up to 50mA. This differentiates it from a photodiode, which cannot pass as much current.

• FET Phototransistor: A phototransistor has two leads that connect internally with its collector and emitter (or source and drain in a FET). The base of the transistor (or gate in a FET) responds to light and controls the flow of current between the leads.

Other Phototransistor Types or Variants

Due to advancements in surface mount technologies, these variants are widely available, though through-hole packages are also common. Phototransistors are often encapsulated like LEDs and are also available with a flat surface. Flat surface variants are sensitive to light from any direction in front of it.

• Some phototransistor variants provide a base or gate connection. This allows the application of bias current to prevent low amplitude light levels from triggering the phototransistor.

• Photodarlington: Similar to a Darlington transistor, it is made of a pair of bipolar transistors. The first transistor is sensitive to light, while the second transistor acts as an amplifier.

  • Advantages: More sensitive to light compared to a regular phototransistor.
  • Disadvantages: Slower response and less linear.

• Photo-FET: Not used as a standalone component but rather in optocouplers. It’s symmetrical for either polarity and hence used for AC signals. It works as a controllable resistance when applied voltage is low, in contrast to bipolar transistors.