Amplifier impedances

Discussion in 'Homework Help' started by rahuldandekar, Sep 27, 2006.

  1. rahuldandekar

    Thread Starter New Member

    Sep 27, 2006
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    The texbook has been helpful, very helpful so far... our prof did nothing much in the way of explaining transistor circuits, but the Semiconductors volume really helped. However, I cannot get past this concept of input and output impedance... And the chapter on Amplifier impedances in the textbook is pending.

    So, please help. :) Thanks in advance.
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    Amplifiers provide gain as a function of frequency. They do this by making a copy of the input signal on the output. If the gain is a function of frequency, then impedance which is the ratio of voltage to current can be expected to change as a function of frequency. The ratio of voltage to current can change either because of changes in magnitude, or changes in phase.

    The same is true of any vector quantity. You can change either the magnitude or the direction of a vector. Frequency dependent quantities behave just like vectors in the complex plane.

    It is the frequency dependence that gives AC circuits all of their magic, and complexity.
     
  3. Dave

    Retired Moderator

    Nov 17, 2003
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    Can you provide a link to the section where you are having problems? And explain specifically what you do not understand in this section, this way we can guide you better.

    Thanks.

    Dave
     
  4. madmax1984ro

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    Dec 10, 2006
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    I am having the same difficulty. I'm very interested in learning how amplifiers work and I read and learned a huge amount from this book but one important thing i don't understand is amplifier impedances. It's a pending section in Volume III Chapter 4 (Bipolar Junction Transistor). It's probably supposed to appear as a section in other chapters as well but if it was at least in this one i (and others too) would get a basic understanding of what it's about.
     
  5. Papabravo

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    Feb 24, 2006
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    Except for resistors in a DC circuit, impedance is a function of frequency. That means that you can expect the impedance to change as the frequency of the input changes. The following article may help clarify things.

    http://en.wikipedia.org/wiki/Electrical_impedance

    If you think about an AC voltage source with an internal impedance connected to the input of the amplifier. It should be clear that the AC voltage source, and the internal impeadance of the AC source, and the input impedance of the amplifier form a series circuit. The total impedance determines the current and the voltages at each point can be derived from the voltage divider theorem.

    The output of an amplifier can also be considered as an AC source with an internal resistance connected to a load impedance. The anaylysis is just the same as for the input.
     
  6. madmax1984ro

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    Dec 10, 2006
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    I've just found some good sources on impedance matching and learned a lot. For valve amps they used impedance matching for maximum power transfer. Nowadays it's actually better to have greater input impedance. It's actually very simple (in my head at least): if you have a perfect AC source with a series (resistive) impedance coupled to an outside (smaller) impedance then most of the voltage will be dropped across the internal impedance. That's bad. For maximum voltage transfer you must have (much) greater outside (load or input) impedance.

    There is also impedance from the way the circuit responds to certain frequencies. I found some formulas that calculate this but I don't really understand them :)

    Please feel free to correct me if I'm wrong.
     
  7. Dave

    Retired Moderator

    Nov 17, 2003
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    What exactly are you trying to understand about amplifier impedances? Is it an appreciation of the small-signal operation of the transistor where you look at analysing the impedance-components of the amplifier configuration (i.e. common emitter/collector etc) or are you looking at this from a purely amplifier level (i.e. impedances from within the amplifer circuit itself)?

    Dave
     
  8. madmax1984ro

    New Member

    Dec 10, 2006
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    Well, that's the point... a lot of people talk about output (input) impedance and impedance matching and I don't always know for sure what they're talking about.
     
  9. Papabravo

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    Feb 24, 2006
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    There is no confusion. They are talking about one and only one thing. To facilitate the transfer of power from one subsystem to another the optimal situation occurs when the output impedance of one subsystem is equal to the input impedance of the next subsystem. The match is done using complex conjugate pairs. A Smith Chart can be very handy in such situations.

    Since impedance is a complex valued, frequency dependent quantity, achieving a perfect match over a wide band of frequencies is nearly impossible. What you get are compromises.
     
  10. Dave

    Retired Moderator

    Nov 17, 2003
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    Impedance matching generally refers to the need to match the impedance if the load with that of the source to maximise power transfer and reduce the effects of reflections from the load back to the source. This is a concept that has significance in TM lines where standing wave patterns develop on the line, and also dielectric heating. It is the case that in dielectric heating that in order to ensure the maximum power transfer from the magnetron to the load, that the load impedance is matched to that of the magnetron - this is achieved through tuning circuits in the magnetron which will adjust the source impedance to ensure that if 1kW of power is distributed then 1kW is absorbed in the load. Power reflections back to the magnetron (as a result of imbalanced source and load impedances) are potential damaging for the equipment and very inefficient (particular if you consider that industrial dielectric heating applications can be using powers upto 15kW).

    There is good article over at HyperPhysics on Impedance Matching.

    Dave
     
  11. beenthere

    Retired Moderator

    Apr 20, 2004
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    Hi,

    Just to ring in on the practical aspect of impedance and amplifiers - an amplifier should have a high impedance to avoid loading effects on the signal. Those effects include loss of amplitude and pulse broadening. A perfect amplifier will have no effect on the signal it amplifies.

    On the other hand, an amplifier needs to have a low output impedance. That lets it drive a signal into less-than-perfect loads without distortion.
     
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