What’s All This Varactor-Input Amplifier Stuff, Anyway? (.PDF Download)

    Sept. 9, 2016

    In 2008, Marcello Salvatierra took the picture above of a circuit Bob Pease had just built. Bob told him it was based on a circuit that Jim Williams did in 1967. Despite working for competitors, Bob and Jim were good friends.

    I did not recognize it, so I asked some analog aficionados what it might be. Alan Martin redrew the circuit in Altium, which made its signal paths clearer. Bob Dobkin, CTO of Linear Technology, recognized it immediately. He said, “At a glance, it looks like a varactor input op amp, like the Philbrick P-2”. This makes sense; Pease wrote a chapter about the P2 in Jim Williams’ analog design book...

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    In a generic electronic system there are some inputs that are controlled by the end user. These inputs are read by electronics and acted upon by using outputs.