opamp_comments

opamp_comments - Vin Ad Vout Z2 Z1 Z1 + Z2 Ad Z1 / (Z1 + Z2...

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1 Z 2 Z 1 A d V in V out V out /V in = Z 1 + Z 2 Z 1 × A d Z 1 /(Z 1 + Z 2 ) 1 + A d Z 1 1 + Z 2 ) 1 If the amplifier gain is reasonably large, so that A d Z 1 / (Z 1 + Z 2 ) >> 1, then V out in Z 1 + Z 2 Z 1 , which is the desired mode of operation. Negative feedback can become positive if there is 180° phase shift in the amplifier. This can and will happen at sufficiently high frequencies because of the amplifiers' high- frequency rolloff (due to transistor capacitances). If the feedback is positive and of magnitude greater than 1, then the feedback circuit will oscillate. To avoid this, the op- amp high-frequency response must be tailored so that the gain of the feedback loop decreases gracefully to below unity before the phase shifts add up to more than 180°. This is called compensation.
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2 -Vee 50k 280k 10ž 10ž 10ž 50ž 50ž Above is one op-amp circuit. Several more are shown in a related hand-out. Most op-amps can be broken down into three parts: A differential input stage, a second voltage-gain stage, and an output buffer stage.
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This note was uploaded on 08/06/2008 for the course ECE 137AB taught by Professor Rodwell during the Winter '08 term at UCSB.

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opamp_comments - Vin Ad Vout Z2 Z1 Z1 + Z2 Ad Z1 / (Z1 + Z2...

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