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EE 435 Lect 18 Spring 2010

EE 435 Lect 18 Spring 2010 - EE 435 Lecture 18 Breaking the...

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EE 435 Lecture 18 Breaking the Loop for Loop Gain Analysis Other methods of gain enhancement
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Two-stage amplifier with LHP Zero Compensation od oo m C L C c m c m md g g g sC C C s g g sC g g ) s ( A + + + = 5 2 5 5 1 = 1 5 5 1 C m C m g g C g z z 1 location can be programmed by R C If g c >g m5 , z 1 in RHP and if g c <g m5 , z 1 in LHP R C has almost no effect on p 1 and p 2 .• Review from last lecture .• • •
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Two-stage amplifier with LHP Zero Compensation = 1 5 5 1 C m C m g g C g z p 2 p 1 X X z 1 Analytical formulation for compensation requirements not easy to obtain (must consider at least 3 rd –order poles and both T(s) and poles not mathematically tractable) C C often chosen to meet phase margin (or settling/overshoot) requirements after all other degrees of freedom used with computer simulation from magnitude and phase plots p 3 X .• Review from last lecture .• • •
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Basic Two-Stage Op Amp with LHP zero Realization of R C Transistors in triode region Very little current will flow through transistors (and no dc current) V DD or GND often used for V XX or V YY V BQ well-established since it determines I Q5 Using an actual resistor not a good idea (will not track gm5 over process and temp) C OX EB L R = μ C WV .• Review from last lecture .• • •
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Loop Gain - A β But what if the amplifier is not ideal? The Loop Gain is ( ) [ ] ( ) 2 O LP V O L 1 2 IN 2 1 IN G G A A G G G +G +G +G G +G = + 1 1 2 R β = R +R 2 1 2 G β = G +G ( ) ( ) [ ] ( ) O 1 2 V O L 1 2 IN 2 1 I V N L G G +G A G G G +G +G +G G + A G + = Note that A VL is affected by both its own input and output impedance and that of the β network This is a really “messy” expression Any “breaking” of the loop that does not result in this expression will result in some errors though they may be small .• Review from last lecture .• • •
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Loop Gain - A β But what if the amplifier is not ideal? Most authors talk about breaking the loop to determine the loop gain A β In many if not most applications, breaking the loop will alter the loading of either the A amplifier or the β amplifier or both Should break the loop in such a way that the loading effects of A and β are approximately included Consequently, breaking the loop will often alter the actual loop gain a little Q-point must not be altered when breaking the loop
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