EE 230 Lecture 19 Spring 2010

EE 230 Lecture 19 Spring 2010 - EE 230 Lecture 19 Nonideal...

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EE 230 Lecture 19 Nonideal Op Amp Characteristics Offset Voltage Common-mode input range Compensation
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Quiz 13 The operational amplifier has a GB of 20MHz. Determine the 3dB bandwidth of the closed-loop amplifier. 2 0 1 R K R =
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And the number is ? 1 3 8 4 6 7 5 2 9 ?
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Slew Rate The slew rate of an op amp is the maximum rate of change that can occur in the output voltage of an op amp Usually the positive going slew rate and the negative going slew rate are the same Slew rate is usually specified in the units of V/ μ sec Slewing can occur in any circuit for any type of input waveform Slew is usually most problematic at higher frequencies when large output excursions are desired Review from Last Lecture
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Slew Rate If V IN is a square wave, this circuit will always exhibit slew rate limitations Desired Output Actual Output Showing Slew Assume V IN is a rather low amplitude, low frequency square wave Review from Last Lecture
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Offset Voltages V OUT V IN 1 A 0 V OS Ideal OA transfer characteristics Actual typical OA transfer characteristics A 0 is the dc gain of the Op Amp and is very large V OS is called the input offset voltage (or just offset voltage) and represents the dc shift from the ideal crossing at the origin V OS is a random variable at the design stage and varies from one device to another after fabrication Can be positive or negative Review from Last Lecture
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Offset Voltages Typical distribution of transfer characteristics after fabrication Distribution of commercial parts if premium parts have been removed Review from Last Lecture
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Offset Voltages Model: () OUT 0 IN OS V = A V - V V OS Can be modeled with a dc voltage source in series with either terminal Polarity of the source is not known on batch since can be positive or negative Polarity of offset voltage for each individual op amp can be measured Review from Last Lecture
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Offset Voltage Consider a basic noninverting voltage amplifier OUT F V IN V R A == 1 + VR 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 1 + V R
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Offset Voltage F OUT I F OS N R V= 1 + V + R R 1+ V R ⎛⎞ ⎜⎟ ⎝⎠ Example: Determine the effects of the offset voltage on the output if the gain of the feedback amplifier is 500, the offset voltage is 3mV and the input is 0.001sin1000t OUT I OS N V =500V + 500V OUT V =0.5sin1000t+ 1.5V Note the offset voltage effects on the output are larger than the signal!
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EE 230 Lecture 19 Spring 2010 - EE 230 Lecture 19 Nonideal...

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