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Originally Posted by G6Tanuki
When I've built these sorts of filters I've usually driven them from an emitter-follower, whose emitter-resistor has a value to present the 'right' load to the filter input.
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This doesn't seem to make sense because in the case of the emitter follower, it is not the emitter resistor that primarily sets the impedance (looking into the emitter) It is instead the resistance in the base circuit, which includes the Thevenin resistance of the often two resistors there and the source impedance driving the base, all divided by the transistor's hfe. You can add the emitter resistor in parallel with that, for an accurate value, but it is usually an order of magnitude higher in value. The point being that the emitter resistor value you see is usually much higher in value than the small signal output impedance of the emitter follower
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At the output-end, I use another emitter-follower - the trick here to present the 'right' impedance is to provide base-bias to the emitter-follower by using two identical resistors in series, each of 2x the required impedance - the centre-point then sits at 1/2 ths supply voltage and also presents a good match to the filter's output.
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This is about right if you ignore the emitter follower's base impedance, which is about roughly its emitter resistance multiplied by the hfe, so usually in this case the impedance is set by the parallel values of the two resistors.