Created by Created February 27, 2012 Last modified February 27, 2012 Tags

### Summary

Example of a high pass filter to help clarify the role of a capacitor.

### Description

Lots of times I see questions arise when a capacitor is used in a high pass filter. Folks are pretty used seeing capacitors in low pass applications, and it is not too hard to wrap your head around how a capacitor can be a low pass filter. It's a charge storage element, so the voltage across a capacitor wont immediately change, instead the voltage will change slowly as the capacitor charges or discharges through the resistor.

However when we flip things around and use the same capacitor as a high pass filter, for example in this circuit things start to get a little confusing. This quick circuit helps you explore the capacitor, and realize that its actually not doing anything different, we are just measuring at a different place.

Things to do in CircuitLab

• Run the DC Simulation, and note the DC operating voltages. Note that V_neg at DC is 0.

• Run the transient simulation, and first ignore the voltage at V_neg, and focus only on the V(V_pos)-V(V_neg) trace, as it reacts to the square wave V1. What does this look like? Does this behavior make sense?

• Now take a look at what the voltage V(V_neg) is doing. Note that V_neg is the voltage at V_neg relative to GND.

• Change V1 from a square wave to a sine wave (remove V(V_pos)-V(V_neg) from the outputs for clarity) and run the transient simulation again. How does the output waveform at V_neg compare to the input sinewave? Does it have the same DC value? Were the amplitude and phase affected?

• For some advanced exploration, run the Frequency analysis. How is this consistent with the rest of your observations about the gain at DC and at High Frequencies?

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