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Multistage Amplifier question...

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  • sirdroid14213
    Hi all, The learning process never ends, I am still very much a beginner, so chuckle if you like, however, I am having a tad bit of trouble remembering how to
    Message 1 of 4 , Apr 3, 2006
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      Hi all,

      The learning process never ends, I am still very much a beginner, so
      chuckle if you like, however, I am having a tad bit of trouble
      remembering how to find input resistance on a multistage amplifier.

      My issue is, actualy punching the formula into a calculator.

      IE: 100k in paralell with 50k...

      Am i correct with '100(exp 3) X-1 + 50(exp 3) X-1' ?

      If this is right, my numbers certainly are not coming up correct. If
      you can help, I would truly appreciate it.

      Thanx a million gang....

      Andy
    • rtstofer
      ... I can t seem to visualize what you are doing with the calculator. But you want (100,000 * 50,000) / (100,000 + 50,000). A more intuitive way to do it if
      Message 2 of 4 , Apr 3, 2006
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        --- In Electronics_101@yahoogroups.com, "sirdroid14213" <sirdroid@...>
        wrote:
        >
        > Hi all,
        >
        > The learning process never ends, I am still very much a beginner, so
        > chuckle if you like, however, I am having a tad bit of trouble
        > remembering how to find input resistance on a multistage amplifier.
        >
        > My issue is, actualy punching the formula into a calculator.
        >
        > IE: 100k in paralell with 50k...
        >
        > Am i correct with '100(exp 3) X-1 + 50(exp 3) X-1' ?
        >
        > If this is right, my numbers certainly are not coming up correct. If
        > you can help, I would truly appreciate it.
        >
        > Thanx a million gang....
        >
        > Andy

        I can't seem to visualize what you are doing with the calculator. But
        you want (100,000 * 50,000) / (100,000 + 50,000).

        A more intuitive way to do it if the numbers are neat: A 50k resistor
        is just 2 100k resistors in parallel. So, we have a total of 3 100k
        resistors in parallel and that is just 100k / 3 or 33.3k ohms.

        Richard

        >
      • Stefan Trethan
        Almost ;-) you need to do (100(exp 3) X-1 + 50(exp 3) X-1) X-1. What you got is the conductance in simens, if you add the last reciprocal it will be resistance
        Message 3 of 4 , Apr 3, 2006
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          Almost ;-)

          you need to do (100(exp 3) X-1 + 50(exp 3) X-1) X-1.

          What you got is the conductance in simens, if you add the last reciprocal
          it will be resistance in ohms.

          For series add resistances, for parallel add conductances.

          The formula richard gave will work as well of course, but it's only for
          two resistors and you need to type each value twice, so i never use it
          myself.

          ST


          On Mon, 03 Apr 2006 15:36:44 +0200, sirdroid14213 <sirdroid@...>
          wrote:

          > Hi all,
          >
          >
          > The learning process never ends, I am still very much a beginner, so
          >
          > chuckle if you like, however, I am having a tad bit of trouble
          >
          > remembering how to find input resistance on a multistage amplifier.
          >
          >
          > My issue is, actualy punching the formula into a calculator.
          >
          >
          > IE: 100k in paralell with 50k...
          >
          >
          > Am i correct with '100(exp 3) X-1 + 50(exp 3) X-1' ?
          >
          >
          > If this is right, my numbers certainly are not coming up correct. If
          >
          > you can help, I would truly appreciate it.
          >
          >
          > Thanx a million gang....
          >
          >
          > Andy
          >
        • 2alan.metcalf@tiscali.co.uk
          Hi Andy The mistake doing it that way is you have to multiply the result by the function x- 1 to turn it up the right way. When the are only two resistors in
          Message 4 of 4 , Apr 3, 2006
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            Hi Andy
            The mistake doing it that way is you have to multiply the result by the function "x-
            1" to turn it up the right way.

            When the are only two resistors in parallel use this formula R1 xR2 /(R1+R2)= ANS
            Also in both methods if both resistors are kilohms then ignor the (exp3) bit
            altogether and just work direckly in kilohms.

            Alan G8ETU

            From: "sirdroid14213" <sirdroid@...>
            Date sent: Mon, 03 Apr 2006 13:36:44 -0000
            Subject: [Electronics_101] Multistage Amplifier question...
            Send reply to: Electronics_101@yahoogroups.com

            > Hi all,
            >
            > The learning process never ends, I am still very much a beginner, so
            > chuckle if you like, however, I am having a tad bit of trouble
            > remembering how to find input resistance on a multistage amplifier.
            >
            > My issue is, actualy punching the formula into a calculator.
            >
            > IE: 100k in paralell with 50k...
            >
            > Am i correct with '100(exp 3) X-1 + 50(exp 3) X-1' ?
            >
            > If this is right, my numbers certainly are not coming up correct. If
            > you can help, I would truly appreciate it.
            >
            > Thanx a million gang....
            >
            > Andy
            >
            >
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