Posted on Mar 28, 2013 11454
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. Normally, this open-loop configuration would be quite drifty because there is no de feedback. The LTC1052 contributes this function to stabilize the circuit. It does this 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. 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 junction. A2"s output is also fed back to the shield around Ql"s gate lead, bootstrapping the circuit"s effective in_put capacitance down to less than 1 pF.