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HO14_214W09_Practical_fb_2pp

# HO14_214W09_Practical_fb_2pp - Handout#14 EE 214 Winter...

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B. Murmann, B. Wooley EE214 Winter 2008-09 1 Practical Feedback Amplifiers Handout #14 EE 214 Winter 2009 B. Murmann and B. A. Wooley Stanford University B. Murmann, B. Wooley EE214 Winter 2008-09 2 Practical Feedback Amplifiers In the two-port approach to analyzing real feedback amplifier circuit implementations, an equivalent forward amplifier and an equivalent feedback network are defined so as to enable use of the ideal feedback equations. In the preceding introduction to feedback amplifiers, the feedback network was assumed to be ideal. However, in practice the feedback network loads the forward amplifier. This loading, as well as the presence of non-ideal source and load impedances, can be taken into account by modifying the forward amplifier, as illustrated in the following examples.

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B. Murmann, B. Wooley EE214 Winter 2008-09 3 Modeling Shunt-Shunt Feedback Amplifiers Consider the two-port model for a shunt-shunt feedback amplifier with finite input and output admittances for both the forward amplifier and the feedback network, as well as finite source and load admittances. Since the forward amplifier and the feedback network in a shunt-shunt circuit share common voltages at the input and output, a short-circuit admittance ( y-parameter ) representation is used for both networks. Y-parameter representation of a two-port network: i 1 = y 11 v 1 + y 12 v 2 i 2 = y 21 v 1 + y 22 v 2 B. Murmann, B. Wooley EE214 Winter 2008-09 4 where y 11 ! i 1 v 1 v 2 = 0 y 12 ! i 1 v 2 v 1 = 0 y 21 ! i 2 v 1 v 2 = 0 y 22 ! i 2 v 2 v 1 = 0 The shunt-shunt feedback amplifier can thus be modeled as follows: Forward amplifier i S
B. Murmann, B. Wooley EE214 Winter 2008-09 5 Summing currents at the input and output of the overall amplifier i S = (y S + y 11a + y 11f )v i + (y 12a + y 12f )v o 0 = (y 21a + y 21f )v i + (y L + y 22a + y 22f )v o Define y i ! y S + y 11a + y 11f y o ! y L + y 22a + y 22f Then v i = ! y o y 21a + y 21f v o i S = y i ! y o y 21a + y 21f " # \$ % v o + (y 12a + y 12f )v o = 1 y 21a + y 21f " # \$ % ! y i y o + (y 21a + y 21f )(y 12a + y 12f ) ( ) * + v o B. Murmann, B. Wooley EE214 Winter 2008-09 6 Comparing this result with the ideal feedback equation ! v o i S = " (y 21a + y 21f ) y i y o " (y 21a + y 21f )(y 12a + y 12f ) = " y 21a + y 21f y i y o # \$ % ( 1 " y 21a + y 21f y i y o # \$ % ( (y 12a + y 12f ) a ! ! y 21a + y 21f y i y o f ! y 12a + y 12f it is apparent that a feedback representation can be used by defining v o i s = A = a 1 + af

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B. Murmann, B. Wooley EE214 Winter 2008-09 7 For the preceding definitions of a and f, it is difficult to establish equivalent circuits for these functions. However, the situation can be simplified by realizing that it may often be possible to neglect reverse transmission in the forward amplifier and forward transmission in the feedback network. That is, it can often be assumed that
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HO14_214W09_Practical_fb_2pp - Handout#14 EE 214 Winter...

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