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Created July 09, 2014
Last modified July 13, 2014
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This circuit amplifies a small differential signal such as that from a sensor coil or microphone, provides offset adjustment and filtering.


Small Signal Differential Amplifier & Filter

This circuit amplifies a very small signal such as that typically supplied from a sensor coil. The coil could be a magnetic field detector or a microphone.

The resistor Coil1 simulates the resistance of the coil while V4 provides a simulated voltage input from the coil. For the actual circuit, resistor Coil1 and V4 are removed -- replaced by the sensor coil.

Broken Coil Detection

R3 ties one side of the coil high. If the coil is intact, then both the plus and minus inputs will be this same high voltage and will cancel out in the differential op-amp circuit -- only the signal differential generated by the coil will be amplified. If the coil or the connecting wiring breaks, R3 will drive one input high while the other input is held low since the high voltage can no longer reach the other input through the low resistance of the coil. This will drive the output of the first stage differential op-amp to the positive rail. Zener diodes D1 and D2 pass this high condition around the capacitor so that the output of the entire circuit is driven high. If the output is being graphed, the trace will be forced to the top of the graph to indicate a broken sensor coil.

The Coil1 resistor simulating the low resistance of the coil is necessary to prevent the output from latching high during testing.

R3, D1, and D2 may be eliminated if broken coil detection is not needed.

Low Pass Filtering

The final stage is a 3 pole filter with a roll-off frequency of 2.9 kHz. The roll-off frequency is adjusted by altering C2, C3, C4 or R14, R16, R17. These three capacitors should always have the same value; the same is true for the three resistors.

For a 1kHz roll-off, the resistors should be set to 10K and the capacitors to .016uF.

To increase the frequency by two, halve the value of the capacitors or halve the value of the resistors. To halve the frequency, double either of the values. For any frequency, change the values in the same ratio as the change from 1kHz to the desired frequency, but inversely.


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