cascode amplifier and miller effect

Discussion in 'Feedback and Suggestions' started by Unregistered, Mar 1, 2010.

Thread Status:
Not open for further replies.
  1. Unregistered

    Thread Starter Guest

    The Miller effect for grounded emitter is not beta times Cce, it is (Av+1) one plus voltage gain base to collector. This gain is approx RL(h+Re) where h=Ic/26mV. h=1/gm of the device. Therefore the miller capactance seen at the input of the grounded emitter circuit is Cm=(Cce)(RL/h+1), with no emitter resistance in circuit. The cacode circuit bandwidth is much improved due to the abcense of this effect. The Miller effect on input impedance dramatically reduces the operating BW with input sources that have signifiacnt output impedance.
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,535
    Where in the book are you referring to? Without context and locations this kind of feedback doesn't help much. Also, what specific verbage are you disagreeing with?
     
  3. Unregistered

    Thread Starter Guest

    Sorry again, its in Chapter three, cascode amplifier ..second paragraph.
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,535
    The only reference to Miller effect I could find is

    Volume III, Bipolar Junction transistors, BJT quirks, second paragraph from the end.

    http://www.allaboutcircuits.com/vol_3/chpt_4/16.html

    Is this it?

    The book is large, links are an absolute must when referring to a section.
     
  5. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,535
    BTW, is cascode a real word? I think this is meant to be cascade.
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
  7. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,535
    So is the OP feedback valid?
     
  8. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Last edited by a moderator: Mar 3, 2010
  9. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
  10. Unregistered

    Thread Starter Guest

    volume III chapter four..cascode amplifier

    It is in the second paragraph that beta is linked to Miller effect..when it should be Av..voltage gain related to Miller effect.
     
  11. Unregistered

    Thread Starter Guest

    While the C-B (common-base) amplifier is known for wider bandwidth than the C-E (common-emitter) configuration, the low input impedance (10s of Ω) of C-B is a limitation for many applications. The solution is to precede the C-B stage by a low gain C-E stage which has moderately high input impedance (kΩs). See Figure below. The stages are in a cascode configuration, stacked in series, as opposed to cascaded for a standard amplifier chain. See “Capacitor coupled three stage common-emitter amplifier” Capacitor coupled for a cascade example. The cascode amplifier configuration has both wide bandwidth and a moderately high input impedance.



    The cascode amplifier is combined common-emitter and common-base. This is an AC circuit equivalent with batteries and capacitors replaced by short circuits.

    The key to understanding the wide bandwidth of the cascode configuration is the Miller effect Miller effect. It is the multiplication of the bandwidth robbing collector-base capacitance by beta. This C-B capacitance is smaller than the E-B capacitance. Thus, one would think that the C-B capacitance would have little effect. However, in the C-E configuration, the collector output signal is out of phase with the input at the base. The collector signal capacitively coupled back opposes the base signal. Moreover, the collector feedback is beta times larger than the base signal. Thus, the small C-B capacitance appears beta times larger than its actual value. This capacitive gain reducing feedback increases with frequency, reducing the high frequency response of a C-E amplifier.
     
  12. Unregistered

    Thread Starter Guest

    http://web.mit.edu/klund/www/papers/jmiller.pdf

    This pdf shows simple Miller effect general case feedback example, and then the original paper by Mr. Miller. As one can see here the Miller effect is a function of the feedback impedance and the voltage gain of the amplifier.
     
  13. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    Have you thought of registering? You have been back enough and seem to like the subjects. You may like the place.
     
  14. Unregistered

    Thread Starter Guest

    Yes thanks I plan on doing it this weekend from my home pc. I'm a long time analog designer.
     
  15. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    Good. Ill have a few questions for you when you arrive. Let your presence be known in the forum. You can link back to this thread so folks know who they were talking to.
     
  16. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,535
    Since I don't really understand the base concept, I can't be of much help here. It might be simple, but I'm missing something.

    As a suggestion, maybe the OP could suggest changes to correct the text? Generally put a before and after two paragraphs.

    Ultimately it is Dennis's (our editor's) decision.
     
  17. Unregistered

    Thread Starter Guest

    Hi Bill, to simplify this one can look at it this way. If you have any impedance between the input and output of an amplifier, the current flowing through this impedance will be a function of the voltage across it and its own impedance. This voltage will be approx 1 + the gain of the amplifier. Since this impedance is connected from input to output it will have a larger signal across it than is represented by the input signal alone. This causes the current in this element to be roughly the voltage gain times the input signal, therefore the effective impedance of this feedback impedance appears to be lower than the impedance of the element by a factor of the voltage gain of the amplifier. So, if you have an inverting amplifier with a gain of 100 and a feedback capacitance, input to output, of 10pF, the input impedance to this amplifier with have a term..the Miller feedback term... which is 10pFxGain or 1,000pF of effective capacitance. This is a simple example. Now, depending on the amplifier phase shift (180 degrees +/-90) and gain and with any possible feedback impedance and phase phase (0 to +/-90) then on can have an effective Miller feedback impedance of any vaue and phase ( 0 to 360 degrees). There is nothing special about Miller effect. Its just the one term of the input impedance to an amplifier due to a feedback impedance, which is a strong function of the gain (voltage) of the amplifier.
     
  18. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    We really must bring this to a halt. The OP is strongly encouraged to take a moment to register so this illuminating thread may be continued.

    Policy constrains us to limit unregistered user inputs to pointing out simple corrections - typos, typically - to the Ebook.
     
  19. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    Here's further proof that the Ebook needs proper indexing.

    A couple of notes on the cascode.

    The two transistors can be the same or opposite polarity or two FETS may be used.

    The common base/gate transitor is used in its maximum voltage rating configuration so this configuration can run from higher supplies to develop larger output swings than CE amps.

    If the configuration is used to take advantage of the high frequency response characteristic, it is vital to ensure good layout and even screening between input and ouput otherwise all the good anti Miller work will be defeated by stray coupling.
     
Loading...
Thread Status:
Not open for further replies.