AA 320 Summary Notes #5

AA 320 Summary Notes #5 - AA320 Aerospace Instrumentation...

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AA320 – Aerospace Instrumentation Notes on Op-Amp Bandwidth, Inverting & Summing Op-Amps, Comparators. Op-amp bandwidth : In exercise 2 of Lab 4 you saw that the open loop gain G was extremely high. It is only when we use negative feedback (closing the loop) that the op amp is useful. The large G insures that the closed-loop voltage gain will only depend on the values of external resistors. Suppose the op amp had this same huge open loop gain at all frequencies. What happens when we put an AC signal into the op amp? Necessarily there is a time lag between output and input (the transistors in the op-amp are not infinitely fast). For a sinusoid input, the time lag translates into a phase lag. At some high frequency, f osc , the output will be 180 ° out of phase with input (when input is max, output will be min). Thus, if we connect a feedback resistor between V out and V , it will actually be positive feedback. In practice, this would mean that with no input at all to V + , the op-amp would oscillate: it would produce a sinusoidal output at frequency f osc on its own . That would make the op-amp useless. To eliminate this problem, all op-amps have a built-in low pass filter that reduces the open-loop gain more and more at higher and higher frequencies. The gain G ~ 10 7 that is typical of op amps is really the DC gain , G 0 . The op-amp's internal low pass filter causes the open loop gain at finite frequencies to fall off as: G ( f ) = G 0 / 1 + ( f / f 1 ) 2 where f 1 is only a few Hz (~10 Hz for the LF411 used in our lab). At frequencies above ~ 100 Hz, a very good approximation is G G 0 f 1 /f , which we can write as
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AA 320 Summary Notes #5 - AA320 Aerospace Instrumentation...

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