EE 230 Lecture 18 Spring 2010

EE 230 Lecture 18 Spring 2010 - EE 230 Lecture 18 Nonideal...

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EE 230 Lecture 18 Nonideal Op Amp Characteristics Output Saturation Slew Rate Offset Voltage
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Quiz 12 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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Quiz 12 The operational amplifier has a GB of 20MHz. Determine the 3dB bandwidth of the closed-loop amplifier. Solution: ( ) 0 1+K BW = GB 2 0 1 R K R = 0 GB BW = 1+K 20MHz = 2.2MHz 9 =
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Gain, Bandwidth and GB Summary of Effects of GB on Basic Inverting and Noninverting Amplifiers 2 0 1 R K= R 0 GB BW 1+K = 0 GB BW K = 2 0 1 R K=1+ R () =− 0 FB 0 K As 1+K 1+s GB = 0 FB 0 K K 1+s GB V OUT V 2 V 1 GB As= s 1 A = s+BW Adequate model for most applications 0A GB =A 0 20dB/ decade 0dB BW 0 A FB (s) BW A 1 Review from Last Lecture
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Example: If the input to the amplifier is .01sin(2 π 10000t), determine the actual and desired output if the op amp is the LMP2231 biased with +/- 2.5V supplies. sin(2 10000 ) OUTDesired V t π =• ω |p| A 0 20dB/ decade 0dB BW ω sig 2 10000 SIG ω () FB 100 As = 100 1+ s 2 10000 FB 100 Aj = 100 1+ j 2 10000 2 130000 FB 100 = 1+j7.7 12.9 FB 2 100 Aj = 1+7.7 = .01*12.9sin(2 10000 82.6 ) o t OUT V .12sin(2 10000 82.6 ) o t OUT V 82.6 o ⎛⎞ ∠= ⎜⎟ ⎝⎠ -1 FB j7.7 = - t a n 1 Review from Last Lecture
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Measurement of GB Most direct: measure A o measure ω b A o is difficult to measure (and exact value usually not of concern) ω b is difficult to measure (and exact value seldom of concern) Direct method of determining GB is not practical If a circuit is adversely affected be a parameter, then this circuit is often useful for measuring that parameter provided relationship between performance and parameter is determined/known. GB=A o ω b Review from Last Lecture
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Strategy for Measuring GB 1. Build FB noninverting amplifier with gain K o 2. Measure BW 3. GB=(K 0 )(BW) Keep gain (K 0 ) quite large (maybe 100) and amplitude small enough so there is no SR distortion. With large K 0 , frequency where gain drops 3dB will be small enough that it can be accurately measured. Review from Last Lecture
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Determination of proper Op Amp orientation Put in frequency dependent model for op amp in the OVERALL CIRCUIT and determine which orientation of the op amp has all poles in LHP () GB As= s This can be somewhat tedious if there are several op amps because they must all be oriented correctly Experience is useful at providing guidance on how to orient the op amps An unstable circuit can be embedded in a larger circuit that is stable and
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EE 230 Lecture 18 Spring 2010 - EE 230 Lecture 18 Nonideal...

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