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The importance of Phase in the frequency plots.

The Lowpass filter for this discussion.

The Lowpass filter for this discussion.

We have been talking about the phase shift when creating frequency plots, but it has not been given a lot of importance. Other than power-factor it really has not affected us that much. We have been mostly interested in the gain plot. That is just about to change.

The phase shift is very important once we start using feedback in our circuits.  A good example is op-amp circuits.   To highlight how important phase is I will proceed to do everything wrong… but we will get there!  Lets say we decided we wanted to create a bass boost amplifier.  The first thing we do is design a low pass filter.  We want a good one, so we design a two stage filter.   We will be flying coach with Wrong Way Peachfuzz as the pilot.

The gain of our filter.

The gain of our filter.

We are very happy. It cuts off at a rate of 40 db per decade. We are in high cotton!   We build the thing on a breadboard and then add it to an inverting amplifier circuit.  (Op amps were discussed a long time ago, but the link to the post is: The Inverting Op-Amp Amplifier Circuit. )

As we add it to the circuit we first put it in the feedback circuit.  Not smart…. it doesn’t work… at least not right.  We are cutting our negative feedback at the frequencies we want to be cut in this amplifier.   If it works at all we just created a high pass amplifier.   DUH!

Well we got this super-duper filter so what would happen if we installed it on the output of our amplifier?   It will cut the higher frequencies just like what we wanted.   But, for whatever reason we decided to tie the inverting input back to the actual output after the filter and not to the output of the amplifier.  We still have problems.

The phase shift due to the filter circuit.

The phase shift due to the filter circuit.

We are creating up to 180 degree phase shift in the output at the higher frequencies.   Since we are tying this back to the inverting input, our negative feedback has now become positive feedback.   (180 deg from the filter, and 180 deg due to the inverting input…. 180 + 180 = 360 and our feedback is now in phase.)

In actual reality, it is doubtful that this circuit would do a whole lot, because the feedback would be very weak due to the large decrease in the gain at these higher frequencies.  However, if we add one more section to our filter we have just created one form of an op-amp oscillator.   This circuit is shown in figure 6 on this pdf file:  TI – design of Op-Amp Sine Wave Oscillators:

The third section is used so each section only has to obtain a 60 degree shift and the gain of each section has not dropped off a lot so the feedback signal still has enough strength to cause the circuit to oscillate.

So…. I guess there has to be a moral to this whole story.  Actually there are two.  The first and most important one is:  Phase is very important and sometimes it is the most important feature.   The second and semi-joking moral is:  Sometimes being lead in the wrong direction can be a learning path.   Thanks, Mr. Peachfuzz!

We are now ready to talk about the AC and frequency characteristics of an Op-Amp.  I will be creating a video about that to go along with the video I created earlier called An Op-Amp Datasheet.


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1 comment to The importance of Phase in the frequency plots.

  • […] Sometimes it is very hard to not get “the horse ahead of the cart”. Very often in projects and design as well as learning multiple things must be accomplished at the same time, yet one of those laws of nature is matter can only be at one place at one time.  In this case the matter is me… and you… and our grey matter between our ears. We have a few more things to talk about with frequency, gain, phase, and capacitors and inductors. However, we also did not complete talking about op-amps, so we have lots of unfinished business.   But, since time and technology keeps marching on there will always be unfinished business.   Tonight’s subject is a little more complete description about the frequency related gain and phase plots I started talking about in the post,  “The importance of Phase in the frequency plots.” […]

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