Common emitter negative feedback

Discussion in 'Homework Help' started by Saviour Muscat, Sep 19, 2014.

  1. Saviour Muscat

    Thread Starter Member

    Sep 19, 2014
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    Hello
    I am 29 year old man working as a maintenance technician, currently I am studying electronics to enrich my knowledge to open new doors in the future. I am certain that here you will give me the help I need,

    My difficulty is understanding the principle of operation and how does it work the negative feedback (emitter resistor with out capacitor) of a common emitter amplifier. I tried on the web several search attempts but didn't
    fully understood the text.

    What I understood was that when the collector current increases the voltage across the emitter resistance increases and this change in voltage opposes the fixed voltage supplied by the potential divider circuit and the base emitter voltage decrease(Vbe= Vb -Ve) therefore the collector current decreases and the output remains stable under all conditions as temperature ,supply variation and ac input voltages variations .

    What I need is that someone corrects what I understood or otherwise confirm that my thoughts are correct.

    Thank you and good wishes

    SM
     
  2. crutschow

    Expert

    Mar 14, 2008
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    Yes, your understanding appears to be basically correct. In general, negative feedback feeds back part of the signal to the input to stabilize the gain and/or operating point of the circuit. The idea is the make the circuit operation dependent upon the feedback element values (which can be quite stable) instead of the transistor gain, which is quite variable.

    Note that the degree of stability that negative feedback gives is determined by the amount of feedback. The more feedback, the greater the stability (the trade-off is that the circuit gain decreases in proportion to the amount of feedback).

    Op amps circuits normally use a high amount of negative feedback to give very stable circuit operation, where the feedback basically determines the circuit input-output transfer characteristics. This works because the op amp has such a high open-loop gain (at least at low frequencies) that, for reasonable circuit gains, the feedback value can be the predominant factor determining the circuit response. That's why op amps are used for so many analog signal processing applications. They are sort of the "swiss-army-knife" of analog circuits.
     
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  3. MaxHeadRoom

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    Jul 18, 2013
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    Another view is to look at the purpose of the capacitor often called a decoupling capacitor, which essentially means that the intention is to make the emitter the same potential as the power common as far as the signal is concerned.
    Max.
     
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  4. #12

    Expert

    Nov 30, 2010
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    The focus of this concept is the base to emitter voltage. If you increase the base voltage by a tenth of a volt, more current will flow from the collector through the emitter. If the emitter is just grounded, the only limit to the increase in current is the gain of the transistor (Ic = .1 Hfe). As soon as you add some resistance between the emitter and ground, the increase in current is limited by (change in base voltage) / (collector current times emitter resistor). Very predictable!

    You can do this in rather convoluted loops that control the voltage gain of several stages at once. You can do this in AC only, DC only, or both at once. Very convenient!
     
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  5. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    An unbypassed emitter resistor (or cathode resistor in a tube amplifier) provides negative CURRENT feedback. It tends to want to reduce the base-emitter current variations, thus reducing overall gain.
     
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  6. MrChips

    Moderator

    Oct 2, 2009
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    Your understanding is correct. To get an insight on feedback, consider the following three situations:
    1. positive feedback
    2. no feedback
    3. negative feedback
    As an analogy, consider your foot pressing on the gas pedal of an automobile moving on the highway.

    1) Positive feedback means that as the car goes faster your foot presses harder on the gas pedal and the car moves even faster. The end result is the car keeps on accelerating for ever until you blow the engine or the hwy patrol comes after you.

    2) No feedback means you are deaf and blind as to how fast you are going.

    3) Negative feedback means that you hear the engine rpm, watch how fast you past every other car and observe the speedometer. You then moderate the pressure on the gas pedal and hold the car at a comfortable speed.

    In electronic amplifiers, positive feedback results in run-away and oscillations. Negative feedback keeps the output in check at a manageable voltage.

    With no bypass capacitor across the emitter resistor, negative feedback is used to stabilize the DC operating voltages (Q-point) of the transistor amplifier. Hence the AC gain is reduced. By applying a capacitor across the emitter resistor, you restore some of the AC gain (frequency dependent) without affecting the DC balance point (Q-point).
     
    Last edited: Sep 20, 2014
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  7. Saviour Muscat

    Thread Starter Member

    Sep 19, 2014
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    Thank for your replies I appreciated so much, sorry for my late reply because today I was at work.

    If the base ac voltage increases the emitter voltage also increase, quantitatively the ac input voltage increase is it smaller than the Ve ?

    Kindly can someone reply


    Thanks
    SM
     
    Last edited by a moderator: Sep 20, 2014
  8. MrChips

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    Oct 2, 2009
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    The base voltage will "always" be about 0.6V more positive than the emitter voltage.
    Hence the emitter voltage tracks the base voltage closely. The AC gain is almost unity.
     
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  9. #12

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    Nov 30, 2010
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    The change in the voltage at the emitter is always a little less than the change in voltage at the base. This is because the base to emitter voltage has to increase a tiny bit to cause an increase in collector current...and vise-versa. The base to emitter voltage will always be a little bit less than at idle when you are trying to suppress current through the collector. The end result is that delta Ve is always a little bit less than delta Vb in the AC analysis. The relationship is NOT linear because delta Ic/delta Vbe is a log function. This is a source of distortion which responds very well to negative feedback.
     
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  10. Saviour Muscat

    Thread Starter Member

    Sep 19, 2014
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    Thanks I understood
    How the feedback stabilize if there is a increased change in ac input voltage at the base if the emitter increased change is smaller?
     
    Last edited: Sep 20, 2014
  11. Saviour Muscat

    Thread Starter Member

    Sep 19, 2014
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    Sorry I confused I am very sorry many thanks
     
  12. crutschow

    Expert

    Mar 14, 2008
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    True it's the emitter current that generates the feedback, but it is the emitter resistor voltage from the current that's feed back to the base which reduces the voltage gain of the circuit, not the current gain. It has no direct effect on the small base-emitter current which is always determined by the transistor Beta and collector current in the active region.
     
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  13. b1u3sf4n09

    Member

    May 23, 2014
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    It also can be used to saturate the output of the amplifier, leading to an only on or only off configuration, hence how we get the comparator.

    99% of the time, positive feedback is a bad thing, but I didn't want Savior Muscat to think that it was always meant to be avoided.
     
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  14. MrChips

    Moderator

    Oct 2, 2009
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    Positive feedback does have its applications.
    It is use in regenerative radio receivers, oscillator circuits and to provide hysteresis in comparator circuits.
     
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  15. vk6zgo

    Active Member

    Jul 21, 2012
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    I agree with Eric,

    Greybeards like myself were taught that "CURRENT negative feedback" was when the feedback voltage was derived from a resistance in series with the load,& "VOLTAGE negative feedback" was when the feedback voltage was derived from a voltage divider across the load.

    This is the definition in "Terman" & "The Radiotron Designer's Handbook".

    In this case,it is simple a matter of terminology,as implied by Eric's use of uppercase.
     
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  16. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Dr. Wheeler:
    It's gratifying to know that someone else here knows about the Radiotron Designer's Handbook! It's been my constant companion since I was a tyke. Alas, I can't find my Terman book; no doubt it's been loaned out to a non-greybeard. :)

    Eric the Elder.
     
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  17. KL7AJ

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