AC18: High pass filters - amplitude and phase response..

This video begins by recapping the principals of a high pass filter and explores its complex voltage transfer function of a filter constructed from an inductor and resistor. Using the three key frequency points (0 rad/s, infinite rad/s and the cut-off rad/s) we construct an idealised Bode plot for the filter. Using the simple rules for plotting a realistic Bode plot that we saw in the previous video, we will compare the real and idealised frequency response of the filter. This video will give you a comprehensive understanding of how to use Bode plots to represent the amplitude and phase response of a high pass filter.

[@johnnywintery8608]

Thanks for the video! One question, I’ve tried to simulate this filter in spice, it works just as expected, but when I attach a load resistor (e.g 50 ohms) the results look quite different, what’s going on?

[@thecircuithelper]

Ahh, good question, but don't forget when you add a load resistor to the output of the CR HPF, you're essentially creating a parallel resistor combination, this effectively reduces the total resistance and alters the cut off frequency of the filter accordingly. There are two options to overcome this, first you can ensure that your load resistance is much bigger than the resistor in the filter (thus the combined resistance is dominated by the resistor in the filter, not the load), second (and more reasonable) approach is to simply use the load resistance as the R in the filter... all you need to do is to rearrange the equation for the cut off frequency to find the required C you need for a given load and cut off!

[@simonyoungglostog]

What are the pros and cons of using a Capacitor/Resistor -v- resistor/inductor for this filter?

[@thecircuithelper]

That’s a very good question, to which there are several possible answers! I’d say the main advantage of using C over L in a high pass filter is that it provides DC isolation… this is why you typically see some kind of high pass filter at the input to an audio amp, where the ‘amp’ side of the filter has a DC bias, which would be problematic for the audio source. The second reason is perhaps more operational, in that high quality capacitors are widely available at low cost with tight tolerances, inductors less so!

[@simonyoungglostog]

@@thecircuithelper There's something in inductors potentially 'spiking' a voltage that bothers me. Not sure if it's apt here.

[@thecircuithelper]

Well… it’s possible to get an inductive kick, but that requires the current through the inductor to be varied on a short time scale (V=Ldi/dt), possible but not so likely in most filtering applications. To be honest, most practical filters use both L and C, they tend not to use R, because resistors dissipate power while inductors and capacitors don’t (at least in theory). Check out Butterworth filters for an example…

[@simonyoungglostog]

@@thecircuithelper Of course, thank you, I didn't consider the 'powe-lossr' aspect.