Solution to last week’s trivia question

[MahtabZ]

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