[MUSIC] Welcome back to Linear Circuits. This is Dr Ferri. We're starting a new module on frequency response. This module will focus a lot on the frequency characteristics of circuits. For example, if I put a low frequency signal into a circuit, I'm looking at the input and output behavior shown here for a sine wave at high frequency. So this is a much higher frequency sine wave. And I'm showing the input and output signals. Notice they're different. There's a phase delay between the input and output here. And an amplitude difference that I'm not seeing here at the low frequency. So in other words, the circuit treats signals of different frequencies differently. The input-output behavior is different. We can see it a little bit more dramatically when we see a square wave. So, for example, at a low frequency square wave, the signal's input and output are almost on top of one other. But they're very different. The input and output are very different at high frequencies. So the circuit treats frequencies differently. Now we utilize that or exploit that when we develop filters. We build circuits purposefully to have different characteristics. For example, a common application is in sensors. I get a sensor signal, and a lot of times, it has noise on it. You see the noise here as this variation there. And I want to clean it up, I want something like this. So I take this signal and put it through a filter that gets rid of the high frequency, and then I get a much nicer signal. Another application where we see filtering is in music, in particular, audio equalizers. So if I look at a signal like this, so pretend this is my audio equalizer. And I have low frequency and a high frequency about the same, then my signal will sound like this. [SOUND] Now if I take my equalizer, and I boost the low frequency and keep the high frequency the same, then what you will hear is that low frequency hum. So in the first one, there was a little bit of a low frequency hum or buzzing sound. Here, what we'll find is that the high frequency is diminished, and you'll hear a little bit more of that buzzing sound. [SOUND] Now in this particular case, I've reversed it, I boosted the high frequencies and got rid of the low frequencies. So in other words, I should have a clearer signal. I shouldn't have that buzzing sound. And instead, you'll hear a high frequency. That actually sounds almost a little bit more shrill because I'm amplifying it. [SOUND] So in the rest of this module, we will spend a lot of time looking at frequency characteristics of signals, in particular, voltage signals. And we will spend a lot of time looking at frequency response and Bode plots of circuits. Now I've introduced the term Bode plots in here, because that term is often used in real applications. For example, audio applications and audio speakers, they're always giving Bode plots to show the frequency characteristics. And then we will spend a lot of time also looking at filters and designing filters. All right, thank you. [MUSIC]
