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Imagine hooking the output to the inverting terminal

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Unformatted text preview: without burdening the source of V in with any current! • Imagine hooking the output to the inverting terminal: • If the output is less than Vin, it shoots positive • If the output is greater than Vin, it shoots negative – result is that output quickly forces itself to be exactly V in negative feedback loop Vin + Vin Winter 2008 5 + Important note: op-amp output terminal sources/sinks current at will: not like inputs that have no current flow Winter 2008 6 UCSD: Physics 121; 2008 UCSD: Physics 121; 2008 Positive feedback pathology Op-Amp “Golden Rules” Rules” • In the configuration below, if the + input is even a smidge higher than Vin, the output goes way positive • This makes the + terminal even more positive than more positive Vin, making the situation worse • This system will immediately “rail” at the supply rail” voltage • When an op-amp is configured in any negativeany negativefeedback arrangement, it will obey the following two rules: – The inputs to the op-amp draw or source no current (true whether negative feedback or not) – could rail either direction, depending on initial offset – The op-amp output will do whatever it can (within its limitations) to make the voltage difference between the two inputs zero Vin + Winter 2008 Lecture 9 positive feedback: BAD 7 Winter 2008 8 2 Op-Amps 02/14/2008 UCSD: Physics 121; 2008 UCSD: Physics 121; 2008 Inverting amplifier example Non-inverting Amplifier R2 R2 R1 R1 Vin Vout + • Applying the rules: Vout + Vin • Now neg. terminal held at V in neg. terminal at “virtual ground” terminal ground” – so current through R 1 is If = V in/R1 (to left, into ground) – so current through R1 is If = V in/R 1 • This current cannot come from op-amp input • Current does not flow into op-amp (one of our rules) – – – – – so the current through R1 must go through R 2 – voltage drop across R 2 is then IfR2 = V in (R2/R 1) • So Vout = 0 Vin (R2/R1) = Vin (R2/R1) • Thus we amplify Vin by factor R2/R1 by so comes through R2 ( delive...
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