Draw a basic low pass filter and characterize it. Draw a non-inverting amplifier and characterize it. How do you combine the two to create an active low pass filter. Write out the gain at low frequency and high frequency.

Question

Sigiloso · Accepted Answer

Basic RC filter. f-3db of 1/(2 pi RC). Unity gain at DC, rolls off at -20 dB / decade. Non-inverting amplifier. Resistor divider between output and inverting terminal. Derive transfer function from ideal op amp assumption. Active low pass filter. Put a capacitor in parallel with the resistor between the output and inverting input. Low frequency gain is same as the non inverting amplifier gain. High frequency gain is 1 because the capacitor shorts out the resistor and forms a unity gain buffer.

Sigiloso · Answer

Above answer is overthinking it. Just connect the low-pass filter output to the input (+ terminal) of the non-inverting amplifier. At low frequency it's the same as the standalone amplifier, at high frequencies it's attenuated by the same factor as the standalone low-pass filter, i.e. -20 dB/decade from the break frequency.

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