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Lowpass filters that don't: A tale of leakage current

February 07, 2011 | Kendall Castor-Perry | 222901583
Lowpass filters that don't: A tale of leakage current The Filter Wizard, Kendall Castor-Perry, gives us a tale of leakage currents and their relationship to true lowpass filters.
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Im glad that some of my readers were able to spot the problem with the lowpass filter we ended up with at the end of Gee, I see!. First to get back with a good answer was Roger Littlewood. Was it mean of me to get you all thinking about some simple engineering issues in this way? Sorry if you think it was. But the answer is pretty basic, and youd feel embarrassed if you flunked the question in a job interview.

To put you out of your misery: Bias Current. Ring a bell? Look again at Figure 1 below. The op-amps we used take a small amount of bias or, more properly said, leakage current at their input terminals. The trouble is, theres no DC return path to ground (or to either supply) for the current at the inputs of U2, U4 and U6 that connect to the resistor network that stretches along the top of the filter. Signal arrives here only through capacitors:

(Click on image to enlarge)

Figure 1: The filter from Gee, I see!

Consequently U2, U4 and U6 arent able to establish proper DC operating conditions with respect to the supplies. Whatever type of amplifier is used, the non-inverting inputs will drift out to close to one of the supply rails. Feedback will take the other amplifier in each GIC with it, so all the circuit voltages will end up near the supply rails. Typically, both the input and the output stage of the op-amps will be out of their linear region and the amplifiers wont work properly. AC simulation in SPICE may not show this effect fully. Even if the model delivers realistic leakage behaviour at the inputs, the effective signal level in such a simulation is zero, and this wont provoke any non-linear effects in the amplifiers.
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