Free Trial

Safari Books Online is a digital library providing on-demand subscription access to thousands of learning resources.


  • Create BookmarkCreate Bookmark
  • Create Note or TagCreate Note or Tag
  • PrintPrint
Share this Page URL
Help

Chapter 8: Designing Lowpass and Highpas... > 8.13 Sallen & Key Third-Order Lowpas...

8.13  Sallen & Key Third-Order Lowpass in a Single Stage

So far we have created first- and second-order responses by using a single stage containing a single op-amp. It is also possible to create third- and fourth-order responses in a single stage, though I should warn at once that the component sensitivities are worse.

Figure 8.5 shows third-order Butterworth lowpass Sallen & Key filters implemented as single stages. As for the second-order stages, this can be done in two different ways; with a unity-gain buffer or by using voltage amplification. By altering the capacitor ratios one can obtain any third-order characteristic with any gain. Figure 8.5a shows a unity-gain lowpass filter; starting with R = 1 kΩ, we find that there are no simple capacitor ratios, and C2 is inconveniently big at 562 nF, more than twice the size of C1. Figure 8.5b shows an alternative circuit with a gain of 1.5 times (there is nothing magical about this figure), which gives the same Butterworth response as Figure 8.5a. As usual, this gain may be an embarrassment rather than a help in a crossover filter.


  

You are currently reading a PREVIEW of this book.

                                                                                                                    

Get instant access to over $1 million worth of books and videos.

  

Start a Free Trial


  
  • Safari Books Online
  • Create BookmarkCreate Bookmark
  • Create Note or TagCreate Note or Tag
  • PrintPrint