| 摘要 |
A pair of field effect transistors are series connected with one acting as a current source to bias the second which acts as a source follower. A photodiode, connected between the source follower gate and source, operates in response to light to change the current in the source follower. The photodiode is biased by the collector of a bipolar transistor that is connected as a negative feedback amplifier operating around the source follower stage. This configuration maintains close to zero bias on the photodiode, particularly at low light levels, and ensures that the collector current in the bipolar transistor equals the photocurrent. An output bipolar transistor with an uncommitted collector matches the feedback transistor and has its emitter and base directly connected thereto. Thus the output current will track the photocurrent, which is a linear function of light input over a very large range of levels. The circuit is linearly operative to extremely low light levels.
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