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    voltage series feedback amplifier common collector

    I hav to design a negative feedback ampilifier using common collector configuration...negative feedback is obtained only when there is phase shift of 180 degree...how is it possible with a common collector which does not produce phase shift at output???

    •   Alt12th January 2011, 19:09

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    Re: voltage series feedback amplifier common collector

    A common emitter is an emitter follower. It has no voltage gain so it cannot have additional negative feedback. Its emitter resistor provides it with plenty of negative feedback.

    Maybe it can buffer the output of a common emitter transistor then negative feedbak can be added to the output of the emitter follower to the base of the common emitter transistor.



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    Re: voltage series feedback amplifier common collector

    Quote Originally Posted by pink beauty View Post
    I hav to design a negative feedback ampilifier using common collector configuration...negative feedback is obtained only when there is phase shift of 180 degree...how is it possible with a common collector which does not produce phase shift at output???
    Well, it sounds more complicated as it is.
    A common collector stage is also called "emitter follower" (I assume audioguru has produced a typing error).
    This circuit is a transistor amplifier with negative feedback as much as possible - equivalent to a an opamp in positive unity gain configuration (which has 100% feedback). Thus, also the common collector stage has a gain of app. unity (in fact: slightly below). There is 180 deg phase shift between the emitter current and the base-emitter voltage.
    Or do you speak about a phase shift between input and output voltages?



    •   Alt13th January 2011, 11:13

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    Re: voltage series feedback amplifier common collector

    But my circuit consists of two stages and the output from the collector of second stage is fed back to the emitter of first stage.As you said if it is a common emitter , there will be a phase shift of 360 degree across collector terminal since there are two stages.How it becomes a negative feedback?



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    Re: voltage series feedback amplifier common collector

    Quote Originally Posted by pink beauty View Post
    But my circuit consists of two stages and the output from the collector of second stage is fed back to the emitter of first stage.As you said if it is a common emitter , there will be a phase shift of 360 degree across collector terminal since there are two stages.How it becomes a negative feedback?
    This is a new information (two stages). In your own interest, you should describe your problem as detailed as possible!
    Why don't you post a circuit diagram? Your desription is rather vague ("phase shift of 360 deg across the collector...").



    •   Alt13th January 2011, 19:06

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    Re: voltage series feedback amplifier common collector

    Here is my circuit diagram...



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    Re: voltage series feedback amplifier common collector

    Thus, you have two emitter stages in series with a capacitice feedback loop from the output (second stage) to the emitter of the first stage.
    I cannot see any "common collector stage" (as mentioned in your first posting).
    And what is the problem now? Since the first emitter node is non-inverting with respect to the collector of the first transistor you have a classical negative feedback loop (because of the phase inversion of the second stage).



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    Re: voltage series feedback amplifier common collector

    thanks a lot ..one more..can voltage series feedback be applied to all types of configurations namely common base,emitter or collector ?
    Because in the book which i had studied only common collector is quoted as an example.The link for the book is given below..

    Electronic Circuits - Ii - Google Books



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    Re: voltage series feedback amplifier common collector

    Hi pink-beauty,
    at first, I prefer another classification: voltage controlled voltage feedback (instead of voltage series or - as some other authors define - series-series). That means, the feedback signal is applied as a voltage and is derived from the output voltage. As a typical illustrative example can the opamp serve when used as a noninverting amplifier.
    Regarding your question, an emitter resistance always creates a feedback voltage - however, only in common collector (common drain) configuration this voltage is derived from the output voltage. For common emitter (source) and common base (gate) this feedback signal (voltage) is derived from the output current (since the output node is the collector/drain node).

    ---------- Post added at 10:13 ---------- Previous post was at 10:09 ----------

    But, of course, other feedback modes are possible.
    For example, in the common emitter configuration you can apply voltage controlled current feedback (feedback path between collector and base). At the base node the input current is superimposed with the feedback current.



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    Re: voltage series feedback amplifier common collector

    thank you ...
    can u please suggest me any website where the concept of feedback is discussed in detail..



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    Re: voltage series feedback amplifier common collector

    Quote Originally Posted by pink beauty View Post
    thank you ...
    can u please suggest me any website where the concept of feedback is discussed in detail..
    Sorry, no I cannot. Try yourself - but I recommend not to rely on websites only. Don't forget good old textbooks.

    ---------- Post added at 19:20 ---------- Previous post was at 18:42 ----------

    I've sent you a private message - together with a pdf-paper.



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