The photodiode is really a phototransistor, but I am using only the collector and the base (phototransistors are far too slow). The wire sticking up is the unused emitter.

The simple circuit is built Manhattan-style on a piece of copper board, with little pads glued down as necessary. I just breadboarded this circuit on a previously used project board, discarding what was there already.

The photodiode connects directly to the BNC connector. The DC bias voltage comes from the clips. The 50-ohm impedance of the scope high-speed vertical amplifier provides the load resistance.

The capacitor bypasses the power supply leads. Without it, I would get a 20 millivolt RF signal from a nearby FM Broadcast station, whose antenna I can see from my window. This setup solves that problem, and is otherwise superior to using a protoboard. I tried this on a protoboard, and like this. It's true what people say about protoboards. They are usually more trouble than they are worth. With a little practice, you can wire together a high-quality rf circuit in a few minutes. I recommend it.

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