![]() Which is the transfer function of a single-pole LPF with cutoff wo with unity gain at DC. Now imagine we take the output of this integrator and feed it back into the input with a minus sign. The NPN pair driving the differential integrator forms a variable gain integrator with transfer function A/RCs, where A is the voltage gain of the diff pair and R and C are the resistor values used in the integrator. My thought was that we could take the output of such an integrator, and feed it into the opposite input terminal of the diff pair, and that feedback should form a 1-pole LPF, as follows: The EML-101 is a state variable filter, and said diff amp/diff integrator configurations form the variable gain integrators. ![]() I got the idea from the EML-101 filter schematic, as expertly redrawn by Marjan Urekar: "One project idea that I tossed out to my students, which one student is trying to get work, is to use the idea of an NPN differential amp driving a "differential integrator" to build a 1-pole LPF, which you could then do four times to get a usual 4-pole LPF. ![]()
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