| Summary: | 碩士 === 國立暨南國際大學 === 電機工程學系 === 91 === In this thesis, four self-compensation operational amplifiers with phototransistor and photodiode respectively were designed as pre-amplifiers for optical power measurement. The so-called self-compensation operational amplifier means an operational amplifier without using frequency compensation capacitor but is still stable. Since no frequency compensation capacitor is used, the layout area on the chip can be reduced.
The phototransistor optical power detecting circuit was implemented by trans-impedance amplifier with emphasis placed on the driving capability of the design of voltage amplifier. In order to make a comparison, two different voltage amplifiers, a current mirror amplifier and a cascode amplifier, were designed for this purpose. Both the pre-amplifiers have a trans-impedance gain of 40dBΩ and a 1MHz —3dB bandwidth. The minimum detectable optical power is 115.3nW.
For the photodiode optical power detecting circuit, however, the implementation was accomplished by using a two-stage trans-impedance amplifier. The first stage of the amplifier is a current amplifier, through which we can amplify the very small optical current. Due to the low noise nature of current amplifier, the signal-to-noise ratio of the pre-amplifier can then be improved. We used a trans-conductance amplifier as the second stage amplifier to transfer the optical current into voltage. The advantage of using two-stage operational amplifier is that, under certain conditions, the —3dB bandwidth is independent of the closed-loop gain of the amplifier. The final pre-amplifier we designed can have a trans-impedance gain of 92.4dBΩ, a 1MHz —3dB bandwidth, and a minimum detectable optical power of 526.6nw.
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