Is this even possible? Filter/impedance question
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Posted by billcalley on August 9, 2005, 12:12 am
Please log in for more thread options Hi All, A company I'm contracting for told me to design a circuit I did not even think was possible. But is it? They have an amplifier that needs to see 1300 ohms at its output, even though the amplifier itself only has an output impedance of 75 ohms. So far, so good. But they want a *passive* highpass filter to be at the output of this 75 ohm amplifier! In other words, they want the input to the highpass filter to present the 75 ohm amplifier with 1300 ohms! (And then the HPF must also have an output impedance of 75 ohms after all this). I would have thought I needed an active buffer to accomplish this. How could I even begin to design such a structure, much less maintain the HP filter shape? Has anyone ever heard of this before? Thank You, -Bill | ||||||||||||||||
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Posted by John Larkin on August 9, 2005, 12:43 am
Please log in for more thread options wrote: You'd need a transformer to do this, and it would result in a best-case voltage gain through the filter (ignoring losses) of 0.24. Your customer sounds a little confused. John | ||||||||||||||||
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Posted by Ban on August 9, 2005, 8:23 am
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billcalley wrote: > Hi All,
Did you really understand rightly? It is not that difficult to make a
> > A company I'm contracting for told me to design a circuit I did not > even think was possible. But is it? They have an amplifier that > needs to see 1300 ohms at its output, even though the amplifier > itself only has an output impedance of 75 ohms. So far, so good. > But they want a *passive* highpass filter to be at the output of this > 75 ohm amplifier! In other words, they want the input to the highpass > filter to present the 75 ohm amplifier with 1300 ohms! (And then the > HPF must also have an output impedance of 75 ohms after all this). I > would have thought I needed an active buffer to accomplish this. How > could I even begin to design such a structure, much less maintain the > HP filter shape? Has anyone ever heard of this before? > > highpass with 1300R input impedance, in fact the needed transformer itself will act as a passive highpass if you design it right. The turns relation must be 4.17:1and the attenuation will be 12.5dB depending on the losses in the transformer. But since the amp is loaded with only 1.3k, its level is almost +6dB higher. You will probably need a Zobel network on the output of the amp to prevent gain peaking. I wonder why they want the high impedance on the amp output, if a simple 1:1 transformer does the same thing without any level loss. -- ciao Ban Bordighera, Italy | ||||||||||||||||
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Posted by Joe McElvenney on August 9, 2005, 2:33 pm
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Hi, > A company I'm contracting for told me to design a circuit I did not
>even think was possible. But is it? They have an amplifier that needs >to see 1300 ohms at its output, even though the amplifier itself only >has an output impedance of 75 ohms. So far, so good. But they want a >*passive* highpass filter to be at the output of this 75 ohm amplifier! > In other words, they want the input to the highpass filter to present >the 75 ohm amplifier with 1300 ohms! (And then the HPF must also have >an output impedance of 75 ohms after all this). I would have thought I >needed an active buffer to accomplish this. How could I even begin to >design such a structure, much less maintain the HP filter shape? Has >anyone ever heard of this before? > Passive filters with asymmetrical in/out impedances are quite common. Design an odd order Butterworth (or Tchebycheff) filter using one of the impedances, figuratively cut it in half through the middle reactance, scale one half to the second impedance and then glue back together. Look up p.18 of - "Simplified Modern Filter Design" by Philip R. Geffe. Cheers - Joe | ||||||||||||||||
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Posted by jgreimer on August 9, 2005, 3:20 pm
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> Hi All,
> > A company I'm contracting for told me to design a circuit I did not > even think was possible. But is it? They have an amplifier that needs > to see 1300 ohms at its output, even though the amplifier itself only > has an output impedance of 75 ohms. So far, so good. But they want a > *passive* highpass filter to be at the output of this 75 ohm amplifier! > In other words, they want the input to the highpass filter to present > the 75 ohm amplifier with 1300 ohms! (And then the HPF must also have > an output impedance of 75 ohms after all this). I would have thought I > needed an active buffer to accomplish this. How could I even begin to > design such a structure, much less maintain the HP filter shape? Has > anyone ever heard of this before? > > Thank You, > > -Bill > You left out most of the requirements of the filter. What is - the corner frequency? - the slope (db/decade)? - the insertion loss? My first thought is a pi or T filter but I need the above details first. | ||||||||||||||||
>
> A company I'm contracting for told me to design a circuit I did not
>even think was possible. But is it? They have an amplifier that needs
>to see 1300 ohms at its output, even though the amplifier itself only
>has an output impedance of 75 ohms. So far, so good. But they want a
>*passive* highpass filter to be at the output of this 75 ohm amplifier!
> In other words, they want the input to the highpass filter to present
>the 75 ohm amplifier with 1300 ohms! (And then the HPF must also have
>an output impedance of 75 ohms after all this). I would have thought I
>needed an active buffer to accomplish this. How could I even begin to
>design such a structure, much less maintain the HP filter shape? Has
>anyone ever heard of this before?
>
>Thank You,
>
>-Bill