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Is this even possible? Filter/impedance question

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

Reply to
billcalley
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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

Reply to
John Larkin

Did you really understand rightly? It is not that difficult to make a 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
Reply to
Ban

Hi All,

Thanks for the replies! I know about asymmetrical filters, but they are used when the filter's load and source impedances are not equal. But in this special case both the source and load that the filter sees *are* 75 ohms -- it's just that the filter's picky 75 ohm source (the amplifier) wants to see 1300 ohms. How can I get a HPF response out of that, considering that there is now a huge (purposeful) mismatch created at the filter's input? How can any passive filter act as a high impedance "buffer" like that? The filter is a 600kHz (to 12MHz) HPF, with an insertion loss of

15dB RL, and
Reply to
billcalley

Hi,

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

Reply to
Joe McElvenney

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.

Reply to
jgreimer

They're probably clueless... 1300 ohms load comes out to 0.5dB insertion loss.

Maybe they're French ?:-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
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Reply to
Jim Thompson

You can build passive LC filters with asymmetric input/output impedance. Silly example:

GENERATED BY QUICKIE LC FILTER MAKER Vin 1 0 AC 18.333V Rin 1 2 1.3000k C1 2 3 247.04p L1 3 0 263.43u C2 3 4 149.28p Rout 4 0 75.000 .AC DEC 60 59.528kHz 6.0000MegHz .PROBE .END

However, the actual input and output impedances will change significantly with frequency. Probably not what your looking for. You could pad the input which would calm the input impedance variations that the source sees. Of course, the pad will attenuate the signal further. I think you need to chat with these guys and see what they are really after.

--
Mark
Reply to
qrk

That is what I thought guys -- thanks!

-Bill

Reply to
billcalley

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