Other uses of these emitter resistors...

Discussion in 'General Electronics Chat' started by Himanshoo, Apr 28, 2015.

  1. Himanshoo

    Thread Starter Member

    Apr 3, 2015
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    For what purpose exactly 180 ohm resistor are placed here...yes it provide some sort of negative feed back to ensure stability..but here in this circuit that work is already accomplished by the feedback base resistor of Q1..and rest is done by compensating action of the circuit ....Then what exact benefit we can have by inserting these emitter resistance if overall gain of the circuit is not an issue....how much effect it have on collector current of the transistors..
     
  2. ian field

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    Oct 27, 2012
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    Its NFB for Q2, Q1 is just a compensated automatic bias generator - presumably the circuit configuration has to match Q2 for the compensated bias to be valid.

    Its basically a "bootstrap" bias that allows increased input impedance on Q2.
     
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  3. OBW0549

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    Mar 2, 2015
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    The emitter resistors are put there to reduce the effects of Vbe mismatch between the two transistors, to make the collector currents more nearly equal.
     
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  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I have always used a decoupling cap across the 180ohm.
    Max.
     
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  5. Himanshoo

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    Apr 3, 2015
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    just to prevent ac degenration or for some other purpose...here in this circuit i mean only dc conditons....
     
  6. Himanshoo

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    Apr 3, 2015
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    please elaborate your point..
     
  7. Himanshoo

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    Apr 3, 2015
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    but whats the need of increasing the input impedance for Q2 ..does loading effect anyhow try to vary constant collector current of Q1...
     
  8. dl324

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    Mar 30, 2015
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    They're used to set the DC bias of the Q2. The emitter resistor of Q2 gives headroom for the negative part of the coupled signal.

    I don't see any negative feedback in the circuit. Simple DC biasing with an AC coupled input.
     
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  9. dl324

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    It's more typically called a compensation cap and is used to modify frequency response. It's not uncommon to split the emitter resistance into multiple resistors so you can tweak stage gain at multiple frequencies.
     
  10. Himanshoo

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    Apr 3, 2015
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    no as far as i know emitter resistor gives a king of negative feedback in order to stablize the transistor from thermal effects....
    take a look..
    http://www.allaboutcircuits.com/vol_3/chpt_4/12.html
     
  11. Himanshoo

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    Apr 3, 2015
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    are you talking about shunt capacitors that are required to give -3db gain roll of at higher frequencies....
     
  12. dl324

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    Strange definition of feedback. I think of it as feeding back part of the output signal.
     
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  13. Himanshoo

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    Apr 3, 2015
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    :)
     
  14. ian field

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    If the 180R on Q2 was shunted by a capacitor, there would be no AC NFB - only DC NFB.

    The stage won't have much gain - but not much distortion either.

    It has one trait of a direct coupled amplifier, DC NFB is pretty much required.
     
  15. OBW0549

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    The circuit you showed can be looked at as a traditional current mirror circuit, with modifications: the 10K resistors in series with the transistor bases allow an AC signal to be injected into the base of Q2, thereby allowing Q2 to serve as an amplifier; and the 180 ohm emitter resistors provide emitter degeneration, improving bias stability.

    If you Google on "current mirror with emitter degeneration" you will find a wealth of material on this; one good explanation is in

    http://www.ami.ac.uk/courses/ami4409_amsicd/u01/

    in the text for Figure 7.
     
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  16. Himanshoo

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    Apr 3, 2015
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    I am getting your point regarding Q2 acting as an amplifier...But what about emitter resistance of Q1 ...since bias stabilty is improved (though no to much extent..)by using collector feedback base resistor ....What I think of Re_Q1 is that it would be present just to match the collector current flowing in Q2 transistor equal i.e to counteract the effect of addition of Re_Q2 which is meant for bias stability...
     
  17. OBW0549

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    You are over-thinking this.

    Go back to my first comment: the emitter resistors are put there to reduce the effects of Vbe mismatch between the two transistors, so as to equalize the collector currents. It really is as simple as that. Emitter degeneration is often used for exactly that purpose when making current mirrors from discrete devices, because of the difficulty in getting precisely matched transistors.
     
  18. WBahn

    Moderator

    Mar 31, 2012
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    I described this in detail in another one of your threads and points out that using ballast resistors (another name for the degeneration resistors) you could mirror a small signal transistor with a power transistor and get good performance.
     
  19. Himanshoo

    Thread Starter Member

    Apr 3, 2015
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    As you said in my previous thread—->“But the more you drop across the ballast resistor the less overhead you have to work with. Everything is a compromise.”



    my inference of above :The addition of emitter resistor can be advantageous as if we increase voltage drop at emitter resistor the lesser Vbe is(due to negative feedback)..and the LESSER we have to worry about Vbe since the MAGNITUDE of Vbe is now quite manageable and also the effects of Vbe mismatch are also less since the value of Vbe have decreased..

    Is this what you meant by term “overhead”..
     
  20. Himanshoo

    Thread Starter Member

    Apr 3, 2015
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    Do you agree with post #19
     
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