Phototransistor Working:

A Phototransistor Working is similar to an ordinary BJT except that its collector base junction is constructed like a photodiode. Instead of a base current, the input to the transistor is in the form of illumination at the junction.

Construction and Working of Phototransistor:

Consider an ordinary BJT with its base terminal open-circuited, [Fig. 20-27(a)]. The collector base leakage current (I_CBO) acts as a base current, giving a collector current; I_C = (h_FE + 1) I_CBO. In the case of the photodiode, it was shown that the reverse saturation current is increased by the light energy at the junction. Similarly, in the phototransistor I_CBO is proportional to the collector-base illumination, [Fig. 20-27(b)]. This results in I_C also being proportional to the illumination level.

For a given amount of illumination on a very small area, the Phototransistor Working provides a much larger output current than that available from a photodiode. Thus, the phototransistor is the most sensitive of the two devices. The phototransistor circuit symbol shows a base terminal, and this is often left unconnected, but is sometime used to provide stable bias conditions.

The cross-section in Fig. 20-28 illustrates the Construction and Working of Phototransistor. The emitter area is seen to be quite small, to allow incident illumination to pass to the collector-base junction. Phototransistor packages are similar to the photodiode packages, except that three terminals are provided.

Phototransistor Characteristics and Phototransistor Specifications:

Typical output phototransistor characteristics are shown in Fig. 20-29. These are seen to be similar to BJT characteristics except that the base current levels are replaced with illumination levels. A do load line can be drawn on the characteristics in the usual way.

The partial Phototransistor Specifications is shown in F g. 20-30 shows a 3 mA minimum current at 5 mW/cm², and a sensitivity of 500 μA/mW/cm². Comparing this to the photodiode specification (Fig. 20-29) shows (as stated above) that a phototransistor is very much more sensitive than a photodiode. However, the rise and fall times (typically 5 μs and 8 μs) for a phototransistor are very much slower than the 2 ns response time for a photodiode.

Phototransistor Applications:

Two Phototransistor Applications are shown in Fig. 20-31 and 20-­32. The relay in Fig. 20-31 is energized when the incident light on the phototransistor is raised to a particular level. This occurs when the Q₁ emitter current produces sufficient voltage drop across R₂ to forward bias the BE junction of Q_2. The relay current falls again when Q₂ turns off as the light level decreases.

In Fig. 20-32, SCR₁ remains untriggered while the illumination keeps Q₁ in saturation. If the light fails, Q₁ turns off and V_R3 triggers the SCR on. This kind of circuit can be used to switch on an emergency lighting system when the normal lighting fails.