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Opto-electrical linear detection circuit

2017-03-19 22:53  
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Opto-electrical linear detection circuit
The figures (a), (b) are the light current amplification circuit which work in the linear region. The transistor in figure (a) must be germanium tube. Because the maximum open circuit voltage of photocell is only 0.6V, while the silicone tube turn on need 0.7V. When the output voltage produced by light is more than 0.3V, transistor 3AX4 starts to conduct, output voltage Usc begins to decrease from -10V. Due to the resistance of 3AX4 is lower, so it can obtain good linear output. The figure (b) adopts two photocells in series, transistor uses silicon tube. The circuit in figure (c) is connected with operational amplifier, the equivalent load resistance is zero, so the output voltage of operational amplifier is equal to the product of silicon photocell short-circuit current Isc and amplifier feedback resistor Rf, that is USc = IscRf, and the output has good linearity, as shown in (d).