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EE 230 Lecture 20 Spring 2010

EE 230 Lecture 20 Spring 2010 - EE 230 Lecture 20 Nonlinear...

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EE 230 Lecture 20 Nonlinear Op Amp Applications The Comparator Nonlinear Analysis Methods
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Quiz 14 What is the major purpose of compensation when designing an operatinal amplifier?
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And the number is ? 1 3 8 4 6 7 5 2 9 ?
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Quiz 14 What is the major purpose of compensation when designing an operatinal amplifier? Compensation is used to make the frequency response of an operational amplifier approximately first-order
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Offset Voltage Consider a basic noninverting voltage amplifier OUT F V IN V R A = =1+ V R If offset voltages are present F OUT I F OS N R V = 1+ V + R R 1+ V R If the desired voltage gain is large, the effects of V OS are a major problem By superposition, it readily follows that F OUT,OFFSET OS R V = 1+ V R Review from Last Lecture
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Management of V OS with Capacitor Coupling R V IN R F1 V OS1 R V OUT R F2 V OS2 Effects can be reduced even further with a second blocking capacitor F1 F2 F2 OUT, OFFSET OS1 OS2 R R R V = 1+ 1+ V 1+ V R R R ⎞ ⎛ + ⎟ ⎜ ⎠ ⎝ F2 OUT, OFFSET OS2 R V = 1+ V R OUT, OFFSET OS2 V = V Review from Last Lecture
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Bias and Offset Currents I BIAS is small for bipolar input op amps, extremely small for FET input op amps Can be neglected in most designs regardless of whether FET or Bipolar input is significantly smaller (/5 to /20) Have been a question about I BIAS on many interviews I BIAS around 50 nA for 741, I OFFSET around 3nA OFSET BIAS1 BIAS2 I =I -I I OFFSET is a random variable with zero mean for most designs I BIAS around 20 fA for LMP2231, I OFFSET around 5fA I BIAS is the current that must flow for the internal transistors to operate correctly Review from Last Lecture
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Bias and Offset Currents OUT B1 2 V I R ± For 741, if R2=10K, VOUT 50nA*10K=.5mV if R2=1M, VOUT 50nA*1M=50mV For LMP2231, if R2=10K, VOUT 20fA*10K=.0.2nV if R2=1M, VOUT 20fA*1M=20nV Can use superposition Will consider only the contributions by I B1 and I B2 Effects of bias currents on most other useful circuits is very small too In those rare applications where it is of concern, using a better Op Amp is a good solution Review from Last Lecture
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Compensation 0 20dB/ decade 0dB BW A Compensation refers to adjusting the frequency dependent gain characteristics of the op amp so that the time and frequency domain performance of the feedback amplifier is acceptable Usually involves making the amplifier look like a first-order lowpass circuit If compensation is not done on cascaded-type op amps, feedback circuits using the op amp are usually unstable Natural type of response
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