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If the phase shift were to increase to 180° before the gain dips below unity, the op amp will oscillate (See Appendix.) Fortunately the LF356 includes special “compensation” circuitry that prevents this from happening, but it does happen with some other, “uncompensated,” op amps.Ĭertain feedback networks contribute their own phase shifts. The frequency at which the phase shift equals 180°, however, is critical, as the inverting input essentially becomes noninverting at this point. Normally these phase shifts are corrected by feedback, and do not result in overall phase shifts.
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For example, the LF356’s phase shift is close to 0° below 10Hz, about 90° from about 100Hz to 1MHz, and crosses 180° at about 20MHz. Not only does an op amp’s open-loop gain decrease with frequency, but its phase shifts as well. Given this characteristic, the gain of a times one hundred LF356‑based amplifier would be accurate to 1% only out to about 500Hz! The open-loop gain of the LF356, for instance, is flat at about $2 \times 10^5$ out to only 10Hz, and then decreases at 6dB per octave. This criterion is easy to meet at DC, but is problematic at higher frequencies because the open-loop gain decreases with frequency. So long as the closed-loop (with feedback) gain is substantially lower than the open-loop gain, say by $10^3$, the errors due to the finite gain will be unimportant. Īlthough an ideal op amp has an open-loop (no feedback) gain of infinity, all practical op amps have finite open-loop gains, typically around $10^5$ at DC. Resistor noise, also called Johnson, Nyquist, or thermal noise, results from the thermal fluctuations of the resistor’s electrons, and is proportional to the square root of the resistance, temperature and bandwidth. The resistors used in an op amp’s external circuit are also noisy. The input offset voltage, $V_\mathrm \, V = 1 \, \mu V \, (RMS)$. Consequently, the op amp's output is not precisely zero when $V_+=V_-$. The transistors and other circuitry in an op amp's differential input stage do not match perfectly.
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The most important imperfections can be divided into four categories: However, in certain circuits, op amps have imperfections which break the golden rules. The last two week’s exercises should have convinced you that op amps behave ideally in a wide range of useful circuits. How does the push-pull circuit in 8.11 work? What do you think is the origin of the name? Why does the capacitor in 8.3 mask 60Hz noise?ģ. Explain why it is necessary to divide by 1000 to obtain the input offset vol t age V os in 8.
#OP AMP OFFSET NEASUREMENT PORTABLE#
NOTE: You can check out and keep the portable breadboards, VB-106 or VB-108, from the 111-Lab for yourself ( Only one each please) Reprints and other information can be found on the Physics 111 Library Site. The Art of Electronics, Horowitz & Hill Chapter 4, skim Chapters 2, 8.1
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Microelectronics Circuits, Sedra & Smith Chapter 2, skim Chapters 4, 9, & 10Īrt of Electronics Student Manual, Hayes & Horowitz Chapter 4 © 2015 Copyright by the Regents of the University of California.