Congratulations to normandinf for solving last week’s trivia question! There were quite a few responses on our Facebook page as well, and everyone knew that the circuit was a High-pass filter. Good job everyone!
For those of you who were stumped, here is the solution. With the circuit open in Multisim, go to the toolbar and click Simulate>>Analyses>>AC Analysis. I have my analysis configured as shown in Figure 1.
Figure 1: AC Analysis Frequency parameters
In the Output tab, add V(output) into the selected variables for analysis, as in figure 2.
Figure 2: AC Analysis Output
Click Simulate and you’ll see something similar to figure 3.
Figure 3: AC Analysis simulation
So, we can clearly see that it is a high-pass filter. To find the cut-off frequency, we can use the cursors to find that the -3 dB frequency is about 1.59 kHz. But what is that little dip on the end, after the 10MHz range? Let’s take a closer look, by increasing our frequency range in our analysis. Figure 4 shows the new frequency range.
Figure 4: AC Analysis, new frequency range
When we simulate, now we see something a little more confusing, shown in figure 5.
Figure 5: AC Analysis simulation with larger frequency range
When I first saw this, I was a bit confused. I had specifically looked up the configuration of a high-pass filter and built one,and yet – here was what seemed to be a band-pass filter staring me in the face.So which is it? A high-pass filter, or a band-pass filter?
The answer lies in the fact that the circuit is an active high-pass filter, and the maximum pass band frequency is determined by the characteristics of the op-amp I used. So normandinf was absolutely right when he said there was a gain of -2 at high frequencies.
I hope you enjoyed trying to solve last week’s trivia question, and I hope this post was educational! I know I learned a lot.
Mahtab
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