Phototransistor

Phototransistor

• A transistor providing internal current multiplication when exposed to light is called phototransistor.

• It has light sensitive collector to base junction. A lens is used so that the base is exposed to the light.

• The base collector area is kept large as it is light sensitive.

• The base terminal is generally not brought out hence phototransistor acts as a two terminal device.

• The construction of a phototransistor is shown in Fig. 6.9.1 (a) while its symbol is shown in Fig. 6.9.1 (b).

• When no light is incident small leakage current flows from collector to emitter called I_CEO due to small thermal generation. This is called dark current which is very small of the order of 10^‒9 A.

• When base is exposed to light, base current is produced due to generated electron‒hole pairs. This is photocurrent. This is denoted as I_λ.

• This produces collector current which is β times I_λ.

  I_C = βI_λ

• The base current I_λ increases as light intensity increases hence collector current also increases with the light intensity.

Characteristics

• While biasing phototransistor, the base terminal is kept open.

• The collector is connected to positive of supply through a resistance R_C.

• While the emitter is grounded as shown in Fig. 6.9.2 (a).

• As light intensity increases, the base current I_λ increases. This is shown in Fig. 6.9.2 (b).

• The output characteristics are shown in the Fig. 6.9.2 (c) which shows that as the intensity of the light increases (H) then the collector current also increases.

• The phototransistor speed is good but not as fast as photodiode.

• If the base terminal is brought out, the phototransistor can be used as a conventional transistor.

Applications

• The various applications of phototransistor are,

1. In computer logic circuits.

2. In level indicators and relays.

3. In punch card readers.

4. In alarm systems.

5. In various opto electronic systems like counting systems, smoke detector circuits intrusion detector circuits, etc.

• Let us discuss few phototransistor applications in detail :

1. Light Operated Relay Circuit

• The light operated relay circuit using phototransistor is shown in Fig. 6.9.3.

• The transistor Q₁ is a phototransistor while the transistor Q₂ is the conventional BJT.

• When the incident light is enough, the Q₁ conducts heavily which drives the Q₂ into saturation. Hence collector current through the coil, enerzises the relay.

• The diode acts as a protecting device which protects Q₂ from large voltage transient to occur at the collector of Q₂, when the transistor turns off.

2. Darkness Operated Relay Circuit

• The darkness operated relay circuit also can be constructed in the similar fashion which is used to de‒enerzise the relay coil. The circuit is shown in Fig. 6.9.4.

• When there is dark, the transistor Q₂ remains properly biased, keeping relay coil energized.

• When there is enough light, the phototransistor Q₁ turns on which reduces forward bias of base‒emitter junction of Q₂, turning it off. This de‒energizes the relay.

3. Light Interruption System

• The phototransistor is also used in the alarm circuits. One such light interruption system using phototransistor is shown in Fig. 6.9.5.

• Normally the phototransistor is ON and which holds the gate of SCR low hence keeping it OFF.

• When the light is interrupted, the phototransistor Q₁ becomes OFF. The high voltage swing at collector of Q₁ turns SCR ON. This activates the alarm system.

• The switch S can be used to reset the alarm.

Review Questions

1. Explain the construction and working principle of a phototransistor.

2. Draw and explain the characteristics of phototransistor.

3. State the applications of phototransistor and discuss any one in detail.

4. Write a note on phototransistor.