Amplifiers (BJT's)

Discussion in 'General Electronics Chat' started by Hurt_it_Circuit, Aug 1, 2013.

  1. Hurt_it_Circuit

    Thread Starter Member

    Oct 2, 2012
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    I am a little confused about the input and output resistances of CE, CB & CC configurations in the BJT. Why do we care about these resistances if we are looking to amplify the voltage or current? What is the significance if the output resistance is a high value or a low value? What is the significance if the input resistance is a low or high value?
     
  2. WBahn

    Moderator

    Mar 31, 2012
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    Because the circuits driving, or being driven, by your amplifiers see these impedances as part of the circuit they are interacting with.

    Say you want to take a signal and divide it by two and then send it into an amplifier that has a gain of 0.5 and you want to drive a 10Ω load. There are lots of reasons you might divide it by two first -- perhaps you need to scale it or perhaps the amplifier can't handle the full voltage of the signal -- and so you use two 10kΩ resistors for your divider. But what happens if the input impedance of your amplifer is 1kΩ to ground? Now you have 1kΩ in parallel with the lower 10kΩ resistor of your divider and instead of getting 50% of the signal at the amplifier input you get 8.3%. Now let's say that amplifier's output impedance is 100Ω. So it forms a voltage divider giving you only 9.1% of the no-load voltage across the load. So you've gone from an overall gain of 25% (the 50% form the input voltage divider and the 50% from the amplifier) to less than 0.4%(the 8.3% from the combination of the input voltage divider and the input impedance, the 50% from the amplifier, and the 9.1% from the output impedance interacting with the load). Hence, because of poor input and output impedance design, your final signal is barely 1% of what you expected.
     
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  3. Hurt_it_Circuit

    Thread Starter Member

    Oct 2, 2012
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    I see so is it safe to say then you typically (most circumstances) want a low input and output resistances? Or does it really depend on the application? If there is an application where you would want high input and output resistances could you give an example of such instance?
     
  4. WBahn

    Moderator

    Mar 31, 2012
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    Look at the example I gave and ask whether it would be better if the input impedance were 100Ω instead of 1kΩ, or if 10kΩ? Then ask if it would be better if the output impedance were 10Ω instead of 100Ω, or if 1kΩ would be better.

    In general (there are always exceptions) it depends one what kind of amplifier it is. If it is a voltage amplifier, you want high input impedance and low output impedance. If it is a current amplifier, you want low input impedance and high output impedance. If it is a power amplifier, you want the input and output impedance to match the impedances driving them and being driven by them, respectively.
     
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  5. LvW

    Active Member

    Jun 13, 2013
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    Let me put it very simple:

    *In most - if not in all - cases the signal voltage to be amplified cannot be provided with a source resistance Rs=0. Hat means, it is not an ideal signal source. However, of course you want that this signal voltage does not change while being connected to the amplifier.
    That means: Your amplifier must not draw any input current. But this is impossible.
    Thus - at least you require that the input resistance of your amplifier is as large as possible. As an example, the input resistance of an opamp, therefore, is in the order of 1...10 Mohms.
    *For similar reasons, the output resistance of a voltage amplifier should be as small as possible, because a load to be connected (and the corresponding current) must have only a negligible influence on the amplified signal voltage. (The foregoing assumes that the load - if it`s value is known - is not contained in gain calculation).
    This is the reason for a very low opamp output resistance, which is further reduced due to negative feedback.
     
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  6. Veracohr

    Well-Known Member

    Jan 3, 2011
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    An ideal voltage amplifier has infinite input impedance and 0 output impedance. Think of it as a simple 2-resistor voltage divider: the output resistance of a source is like the first resistor, the input resistance of the load is like the second.

    Current sources are an example of when high output resistance is good.
     
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