Input and Output Impedance Help

Discussion in 'General Electronics Chat' started by setsunaseiei, Mar 26, 2009.

  1. setsunaseiei

    Thread Starter Member

    Mar 25, 2009
    12
    0
    Is there a trick to increase the input impedance of a CE amp and decrease the output impedance provided the required gain?
    If its multistage, how come when I AC couple 2 CE amps, the first stage gets clipped? How can I decrease the effect of loading on my circuit?
    I think it's about the impedances but I'm a neophyte when it comes to this.
    Thanks.
    ,,,,,,,
    V/ (~c~).
     
  2. PRS

    Well-Known Member

    Aug 24, 2008
    989
    35
    You can increase the input impedance by adding a feedback resistor to the emitter. If your RE is say, 1k, you can put a 100 ohm resistor in series with 900 ohms and put your bypass cap between them. Here you sacrifice gain for high input resistance. But design for the right gain in the first place.

    You can increase the input resistance with negative feedback from the output, but you're not ready for that, I think. And, anyway it, too, reduces gain. Gain is always being traded for other benefits with feedback.

    You can increase input resistance by using a small Ic and base biasing resistors, but too small an Ic limits your amp's ability to drive the load.

    But your clipping may be fixable by another method. Post your circuit then you'll get a lot of response from the technical gurus in this forum. ;)
     
  3. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,040
    287
    Adding a feedback resistor in the emittier will indeed increase the input impedance but it WON'T decrease the output impedance. An emitter resistor gives you CURRENT feedback. Voltage feedback WILL do both things for you, though.

    Eric
     
  4. PRS

    Well-Known Member

    Aug 24, 2008
    989
    35
    All too true, KL7AJ. I focused on the input impedence only. You're right, feedback will decrease the output impedence. You can also decrease it by reducing Rc.

    I think all these variables of gain vs this and that is what keeps electronics engineers in demand. :)
     
  5. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    A good negative feedback will make the trick.
     
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