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computer science crazy
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17-04-2009, 06:25 PM


Like diodes, all transistors are light-sensitive. Phototransistors are designed specifically to take advantage of this fact .the most-common variant is an NPN bipolar transistor with an exposed base region. Here, light striking the base Replaces what would ordinarily be voltage applied to the base “ so, a photo- transistor amplifies variations in the light striking it. Phototransistors may or may not have a base lead . (if they do, the base lead allows us to bias the phototransistor's light response). The Phototransistor is similar in operation to the amplifying Transistor, but it is controlled by light rather than by the electric current of the emitter.

During the past quarter century, electric eyes have found widespread use in electronics because of their ability to control electric currents by the action of light. To the layman, one type is perhaps best known for automatically opening and closing doors, but such devices have many other important uses in television, sound motion pictures, wire photos, and still many more in industry. One of the major advantages of the Phototransistor is that it delivers very high power, which is enough in some cases to operate a switch directly without the preliminary amplification usually required.
Phototransistors are solid-state light detectors with internal gain that are used to provide analog or digital signals. They detect visible, ultraviolet and near-infrared light from a variety of sources and are more sensitive than photodiodes, semiconductor devices that require a pre-amplifier. Phototransistors feed a photocurrent output into the base of a small signal transistor. For each illumination level, the output is defined by the area of the exposed collector-base junction and the DC current gain of the transistor. The base current from the incident photons is amplified by the gain of the transistor, resulting in current gains that range from hundreds to several thousands.

The actual operation of a phototransistor depends on the biasing arrangement and light frequency. For instance , if a PN junction is forward biased, the increased current through the junctions due to incident light will be relatively insignificant. On the other hand, if the same junction is reverse biased, the increase in current flow will be considerable and is a function of the light intensity. Therefore, reverse bias is the normal mode of operation. Now, if the PN junction is the collector-base diode of a Bipolar transistor, the light-induced current effectively replaces the base current. The physical base lead of the transistor can be left as an open terminal, or it can be used to bias up to a steady state level. It is the nature of transistors that a change in base current can cause a significant change (increase) in collector current. Thus, light stimulation causes a change in base current, which in turn causes a bigger increase in collector current and, considering the current gain ,a rather large increase at that.

Characteristics :

The Phototransistor has a high power output for a photo-electric device and gives good response to a rapidly fluctuating light source .It is particularly sensitive to the wave lengths of light given off by ordinary incandescent light bulbs, and is well suited to operate with these easily available sources with good fidelity. Another virtue is the device's low impedance.
Dark current :

When the phototransistor is placed in the dark and a voltage is applied from collector to emitter a certain amount of current will flow. This current is called dark current. This current consists of leakage current of the collector base Jn multiplied by the dc current gain of the transistor. due to the presence of this current the phototransistor is not considered as completely off .The dark current is temperature dependent. It is usually specified at 25°C.

Speed of response:

The speed of response of a phototransistor is totally dominated by the capacitance of the collector - base Jn and the value of load resistance. The phototransistor takes a certain amount of time to respond to sudden Changes in light intensity. This response time is usually expressed by the rise time and fall time of the detector . As long as the light source driving the phototransistor Is not intense enough to cause optical saturation, the rise time equals to fall time. But if optical saturation occurs fall time will be much larger than the rise time. Phototransistors display rise time and fall time in a range of

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