Phototransistor Symbol and Circuit Configurations

- details of the phototransistor circuit symbol and the phototransistor characteristics for use in a circuit.

The phototransistor can be used in a variety of circuits and in a number of ways dependent upon the application. Being a low cost device the phototransistor is widely used in electronic circuits and it is also easy to incorporate.

Phototransistor symbol

The phototransistor symbol for use in electronic circuit diagrams is very straightforward. It is formed from the basic transistor symbol with arrows point in to it to indicate that it is light sensitive.

The phototransistor symbol often has two arrows pointing towards it, but other phototransistor symbols show a jagged arrow. Both versions of the phototransistor symbol are acceptable and understood.

Phototransistor Symbol

Phototransistor symbol

The circuit symbol also has the convention arrow and directions on the emitter connection. It points inwards on a PNP phototransistor circuit symbol and outwards on an NPN phototransistor symbol.

It can be seen that the phototransistor symbol shown does not give a base connection. Often the base is left disconnected as the light is used to enable the current flow through the phototransistor. In some instances the base may be biased to set the required operating point. In this case the base will be shown in the normal way on the phototransistor symbol.

Phototransistor circuit configurations

The phototransistor can be used in a variety of different circuit configurations. Like more conventional transistors, the phototransistor can be used in common emitter and common collector circuits. Common base circuits are not normally used because the base connection is often left floating.

The choice of common emitter or common collector phototransistor circuit configuration depends upon the requirements for the circuit. The two phototransistor circuit configurations have slightly different operating characteristics and these may determine the circuit used.

Common emitter phototransistor circuit

The common emitter phototransistor circuit configuration is possibly the most widely used, like its more conventional straight transistor circuit. The collector is taken to the supply voltage via a collector load resistor, and the output is taken from the collector connection on the phototransistor. The circuit generates an output that moves from a high voltage state to a low voltage state when light is detected.

The circuit actually acts as an amplifier. The current generated by the light affects the base region. This is amplified by the current gain of the transistor in the normal way.

Common emitter phototransistor circuit

Common emitter phototransistor circuit

Common collector phototransistor circuit

The common collector, or emitter follower phototransistor circuit configuration has effectively the same topology as the normal common emitter transistor circuit - the emitter is taken to ground via a load resistor, and the output for the circuit being taken from the emitter connection of the device.

The circuit generates an output that moves from the low state to a high state when light is detected.

Common collector / emitter follower phototransistor circuit

Common collector / emitter follower phototransistor circuit

Phototransistor circuit operation

The phototransistor circuits can be used on one of two basic modes of operation. They are called active or linear mode and a switch mode.

Operation in the "linear" or active mode provides a response that is very broadly proportional to the light stimulus. In reality the phototransistor does not give a particularly linear output to the input stimulus and it is for this reason that this mode of operation is more correctly termed the active mode.

The operation of the phototransistor circuit in the switch mode is more widely used in view of the non-linear response of the phototransistor to light. When there is little or no light, virtually no current will flow in the transistor, and it can be said to be in the "off" state. However as the level of light increases, current starts to flow. Eventually a point is reached where the phototransistor becomes saturated and the level of current cannot increase. In this situation the phototransistor is said to be saturated. The switch mode, therefore has two levels: - "on" and "off" as in a digital or logic system. This type of phototransistor mode is useful for detecting objects, sending data or reading encoders, etc.

With most circuits not using the base connection (even if it is available), the only way to change the mode of operation of the circuit is to change the value of the load resistor. This is set by estimating the maximum current anticipated from the light levels encountered.

Using this assumption, the following equations can be used:

Active mode:   VCC   >   RL   x   Ic

Switch mode:   VCC   <   RL   x   Ic

    RL = load resistor (i.e. Rc or Re in the diagrams above).
    IC = maximum anticipated current.
    VCC = supply voltage.

Use of base connection in phototransistor circuits

On some phototransistors, the base connection is available. Access to the base connection allows the phototransistor circuit conditions to be set more appropriately for some applications.

Phototransistor circuit with base resistor

Phototransistor circuit with base resistor

High values of base resistor Rb prevent low levels of light from raising the current levels in the collector emitter circuit and in this way ensuring a more reliable digital output. All other aspects of the circuit function remain the same.

By Ian Poole

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