EE 435 Lect 18 Spring 2010

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

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Unformatted text preview: EE 435 Lecture 18 Breaking the Loop for Loop Gain Analysis Other methods of gain enhancement 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 .• • • • • 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 .• • • • • 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 W V .• • • • • Review from last lecture .• • • • • 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 I N 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 .• • • • • 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...
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This note was uploaded on 01/31/2012 for the course EE 345 taught by Professor Geiger during the Fall '11 term at Iowa State.

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

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