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Re: Why are transistor input and output impedances important?

  1. Oct 19, 2011 #1
    I'm currently studying transistor amplifiers and It is not entirely clear how impedance relates to amplification. For the emitter follower configuration, the book I'm reading implies that low output impedance means high voltage gain and, for any amplifier in general, high input impedance is good for voltage gain though I'm not sure why. Can anyone please explain why this is the case?
     
    Last edited: Oct 19, 2011
  2. jcsd
  3. Oct 19, 2011 #2
    I don't think there is any particular important relation between impedance and voltage gain. It is not that emitter follower has high voltage gain. In fact the voltage gain of a follower is very close to one only. Straightly speaking, it is slightly less than one. It just transform the output signal to a low impedance output so it is better to drive a load. It is usually used to buffer a high impedance source.

    In a common emitter transistor circuit with a resistor Re on the emitter and Rc on the collector, generally the gain is close to [itex] A=-\frac {R_C}{R_E}\;[/itex]. As you can see, to get higher gain, Rc has to be higher, so the impedance at the collector become higher and it is not very good in driving the load. So you put an emitter follower following the circuit to buffer the stage so you get better drive.

    On the side note, look into the three basic transistor configurations: Common emitter( normal amplifier), common base( in differential pair and more for high frequency application) and common collector ( emitter follower that use for buffering). Look at their different characteristics.
     
  4. Oct 19, 2011 #3

    AlephZero

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    I think the book is trying to make a general point about connecting "building blocks" of circuits together.

    You can think of the output impedance of a circuit, and the load impedance that it is driving, as a potential divider. To get most of the output voltage across the load, you want the output impedance to be low compared with the load impedance.

    Similarly you want the input impedance to be high compared with the impedance of whatever is providing the input signal, so you get most of the input voltage across the input impedance of the amplifier, not across the output impedance of the signal source.

    But there are other practical issues involved, so don't treat this as more than an "arm-waving" explanation.

    And note that the above is only talking about input and output voltages, not currents or power levels.
     
  5. Oct 19, 2011 #4
    It is not to do with gain it is to do with impedance matching and signal loss.

    An amplifier processes a signal from a source and delivers it to load.

    Each of the source, amplifier and load have their own input and/ or output impedances.

    So suppose we have an amplifier with input and output impedances of 1k, a signal from a source generating 10 volts with an output impedance of 10k. Further suppose the amplifier feeds a load of 100.

    Firstly the ouput impedance of the source and the input impedance of the amp form a potential divider for the 10 volt signal.

    Approximately 9 volts is dropped across the source impedance and only 1 volt is input to the amplifier ie a reduction of 9:1 in the signal before any amplification takes place.

    At the output we have a similar situation. The output impedance of the amp forms a potential divider with the load.

    Say the amp gain is 20 then it generates 20 x 1 volts = 20 volts but only 1.8 volts is delivered to the load, the rest appears across the amp output impedance.

    Can you see what amp conditions are required to improve this?
     
  6. Nov 4, 2012 #5
    Thanks and...

    thanks!
     
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