Posted on Oct 2, 2012

Ql and Q2 constitute a simple, high-speed FET input buffer.·Ql functions as a source follower, with the Q2 current-source load setting the drain-source channel current. The LT1010 buffer provides output drive capability for cables or whatever load is required. The LTC1052 stabilizes the circuit by comparing the filtered circuit output to a similarly filtered version of the input signal. The amplified difference between these signals is used to set Q2`s bias, and hence Ql`s channel current.

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This forces Ql"s V0 s to whatever voltage is required to match the circuit"s input and output potentials. The diode in Ql"s source line ensures that the gate never forward biases and the 2000-pF capacitor at Al provides stable loop compensation. The rc network in Al "s output prevents it from seeing high-speed edges coupled through Q2"s collector-base ]unction. A2"s output is also fed back to the shield around Ql "s gate lead, bootstrapping the circuit"s effective input capacitance to less than 1 pF.

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