Identifying Various Feedback Networks

Discussion in 'Homework Help' started by jegues, Dec 10, 2011.

  1. jegues

    Thread Starter Well-Known Member

    Sep 13, 2010
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    43
    When given an arbitrary amplifier I sometimes have trouble identifying whether or not the amplifier is,
    • Series-series
    • Shunt-series
    • Series-shunt
    • Shunt-shunt
    Does anyone have any techniques in order to correctly identify the type of feedback amplifier?

    For examples, I have attached a figure with 3 random amplifiers.

    What I've been doing so far is checking to see if the input or output only gets a portion of the input/output current, and if this is the case it must be shunt connection. Otherwise series connected.

    Does anyone have any tips on correctly identifying the what type of feedback amplifier we're looking at in various circuits?

    Thanks again!
     
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    There is a clear way to differentiate between the two type of feedback. If you disconnect your load, and the feedback signal goes to zero (output current goes zero), that's current feedback (load current > zero, feedback > zero ). If, on the other hand, you short the load, and the feedback goes to zero (output voltage goes zero), that's voltage feedback (load voltage > zero, feedback > zero).
    The input summing is shunt when feedback connects to the same node that the input source is connects. Otherwise we have series feedback.

    Maybe this will helps you
    http://forum.allaboutcircuits.com/showthread.php?p=249786#post249786 ( post 10 )

    From "Analysis and Design of Integrated Electronic Circuits" by Paul M. Chirlian 1987 page 811:

    "We have classified feedback as positive or negative. Let us consider some additional classifications. There is 'voltage feedback' and 'current feedback'. In voltage feedback, the signal fed back (which can be either a voltage or a current) is proportional to the output voltage, whereas in current feedback, the signal fed back is proportional to the output current. When voltage negative feedback is used, the output voltage tends to become independant of the value of Av, whereas current negative feedback tends to stabilize the output current. ...Block diagrams for voltage and current feedback are shown in Fig. 16-2. Figures 16-2a and 16-2b represent voltage feedback since the signal fed back is proportional to the output voltage. Now, look at the inputs to the amplifiers. In Fig. 16-2a the feedback signal is placed in series with the input signal, so V3=V1+bV2

    This is called 'series feedback'. In Fig. 16-2b, the feedback signal is in parallel (shunt) with the input. Now we consider that the feedback signal is a current that is proportional to the output voltage. This is called 'shunt feedback' I3=I2+bV2

    In this case b is not dimensionless. Its units are mhos (Siemens). Note that V3=V1 in this case. This implies that, if the input generator has no series impedance, then there is no feedback. Note that the amplifier of Fig. 13-4 used voltage shunt feedback. In that case, the resistor R1 could be considered to be the generator resistance.

    In Figs. 16-2c and 16-2d we have examples of current feedback. Now the feedback signal is proportional to the load current. Figure 16-2c is an example of 'current series feedback'. Here the feedback signal is a voltage that is proportional to the output current. Figure 16-2d represents 'current shunt feedback'. Note the in Fig. 16-2c the units of b are ohms while in Fig. 16-2d, b is dimensionless. When voltage feedback is used the input to the b network is placed in parallel with the output. Thus the word shunt can be used to replace voltage. For instance voltage-series feedback can be called shunt-series feedback and voltage-shunt feedback can be called shunt-shunt feedback. In a similar way current-series feedback can be called series-series feedback and current-shunt feedback can be called series-shunt feedback.
     
    Last edited: Dec 11, 2011
    BaharuddinAziz and screen1988 like this.
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