The phototransistors look like the normal transistor, except for the fact that the base terminal is not present in the case of the phototransistor. Photo-transistors convert incident light into photocurrent. Instead of providing the base current to activate the transistor, the light rays are used to illuminate the base region. The base terminal consists of the material that shows sensitivity to light. The circuit symbol of the phototransistor is similar to that of the conventional transistor, but the base terminal can be omitted. The two arrows pointing towards the phototransistor indicate that the phototransistor is triggered by the light that falls on it.
Advantages of the phototransistor
1. Higher efficiency compared to the photodiode: the efficiency of the phototransistor is greater than that of the photodiode. This is because the current gain in the case of the phototransistor is greater than that of the photodiode, therefore, even if the amount of incident light in both is the same, the phototransistor will generate more photocurrent than the photodiode.
2. Faster response: the response time of the phototransistor is greater than that of the photodiode, this provides the advantage of using the phototransistor in our circuit.
3. Lower noise interference: the main disadvantage of photodiodes, especially that of avalanche photodiodes, is that it is not immune to noise interference. On the contrary, phototransistors are immune to noise interference.
4. Economical: the phototransistor is less expensive than other light sensitive devices, therefore, it is economical to use phototransistors in light sensitive applications.
5. Less complex: the phototransistor design is simple and less complex compared to LDR and photodiodes.
Disadvantages of phototransistors
1. Effect of electromagnetic energy: the efficiency of phototransistors decreases when the electromagnetic field interferes in the region of operation. This results in poor conversion efficiency of the phototransistors.
2. Low efficiency at high frequency: due to the large area of the base region of the collector, the capacitance increases. Because of this, it can not effectively convert light into photocurrent in higher frequency ranges.
3. Slower: phototransistors are slow compared to photodiodes. Photodiodes can convert light into photocurrent much faster than phototransistors.
4. Electric peaks: arises in phototransistors more frequently compared to photodiodes.