Use of Phototransistors kennethkuhn（使用光电晶体管kennethkuhn）.pdf

Use of Phototransistors by Kenneth A. Kuhn Jan. 8, 2001, rev. Feb. 3, 2008 Introduction This note is a brief overview of phototransistor characteristics and usage taken from a variety of manufacturer data sheets and application notes and the author’s experience with these devices. Complete information is available at various manufacturer web sites. The phototransistor A phototransistor is an ordinary transistor that has been modified in two ways: (1) there is a transparent window so that light can shine on the junctions and (2) the structure has been modified to maximize the light capture area. Some phototransistors have an external base lead; others do not. If there is an external base lead, it is often left floating or connected to a high impedance bias source to bias the collector current to a specific value for the no light condition. Base current is formed by light photons striking the junction. The phototransistor converts received power to a collector current. The units of this transfer function are Amperes per Watt. A more common unit that does not require that the user know the effective surface area is milliamperes per watt per square centimeter. This unit of sensitivity is much easier to use since all one has to know is the power density. Typical values for sensitivity (measured at the wavelength of peak response) range from about 0.4 2 2 to 2.0 mA /(mW / cm ) for ordinary phototransistors up to about 20 mA /(mW / cm ) for Darlington connected phototransistors. Common phototransistors can operate with a collector current up to about 20 mA. This means that the typical maximum light level that can be handled is about 20 mW/cm2 (this seemingly small number is the same as 200 Watts/meter2 which is very intense) ass