hey guys, its exam i got one small question.
What is the difference btwn the two circuits below, i know that there is the op amp in the middle, but its a voltage follower and i cant see what it does exactly. i tried googling it, but there's not much explaination on what a voltage follower does. Thanks for your time!


PS. i read this online, as well as in my text book. "provides a high input impedance, a low output impedance, and unity gain" what does that mean exactly? would the impedance before the voltage follower be ignored then if it has close to infinite impedance?

 

In circuit #1, the second RC lowpass filter loads down the first RC lowpass filter. Then the response of the filter is poor.

The opamp stops the loading down so the second circuit does what it is supposed to do.

 

Circuit 1 can be thought of as an RC voltage divider stage, followed by another CR voltage divider.

In circuit 1, the second CR stage adds some additional loading on the first stage and changes the transfer function.

The addition of the voltage follower removes the loading effect of circuit 2 on circuit 1. In this case, the overall transfer function is just the product of the transfer function of the first RC part, times the transfer function of the second CR part.

Without the follower, this would not be the case. Then you would have to analyze the entire circuit together, taking account of the loading effect of the second stage on the first stage.

 

Perfect, that's what i was looking for. Thank you guys!

 

Sorry, i made a mistake.
The second filter is a highpass filter not a lowpass filter.

 

Indeed. One of the advantages of active filters (and this circuit actually qualifies) is that you can isolate the different poles and get more ideal (as well as MUCH easier to design) responses.

Back in the olden days, filter buffers were quite common, using an emitter or cathode follower instead of an op-amp voltage follower...but with essentially the same results.

Eric

 

By the way....I'm assuming your example is merely representative. If the R values and C values are the same, the circuit will do precisely nothing! A low-pass filter followed by a perfect reciprocal hi-pass filter is a piece of wire! If the values are staggered, however, it can be either a bandpass or bandstop filter, of course.

Just being pedantic here.

Eric