S5-4.1 The Operational Amplifier
You can describe the op-amp block diagram and ideal characteristics, analyse inverting, non-inverting and follower configurations with ideal and non-ideal devices, and account for offset, bias current, slew rate, CMRR and PSRR.
The op-amp is the component that lets you design with feedback rather than with transistors, and the two ideal rules - no input current, and the inputs are driven to equal voltage - solve most circuits in one line. The parameters in this unit are all the ways a real device breaks those rules, so learn each as a specific failure of an ideal assumption. Slew rate is the one that surprises people, because it is a large-signal limit that no small-signal analysis will ever reveal.
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Op-amp block diagram
The device that lets you design with feedback rather than with individual transistors. Start here even if the internals look uninteresting, because the block structure explains the parameters later in this topic.
Ideal op-amp characteristics
The two rules that solve most circuits in one line: no current into the inputs, and the inputs driven to the same voltage. Everything below is a way in which a real device breaks one of them.
Inverting and non-inverting amplifiers with ideal op-amps
The two configurations everything else is built from, worked with the ideal rules. Gain from a ratio of two resistors is the result to be able to produce instantly.
Inverting and non-inverting amplifiers with non-ideal op-amps
The same two circuits with the ideal assumptions removed. Doing them twice is the point: the difference between the two answers is what the specification sheet is telling you.
The voltage follower
The extreme case of the non-inverting configuration, and the most useful circuit in the topic. Unity gain sounds pointless until you need to stop one stage loading the next.
Input offset voltage and output offset voltage
The first specific way a real device misbehaves: it does not output zero for zero input. Small, and it dominates when you are amplifying something small.
Input offset and bias currents
The second: the inputs do draw a little current after all. It matters most with large resistors, which is exactly when people assume it will not.
Slew rate
The one that surprises people, because it is a large signal limit that no small signal analysis will reveal. A circuit can be perfectly linear on paper and still turn a sine wave into a triangle.
CMRR and PSRR
Two rejection figures that say how well the device ignores what it should ignore. They are the numbers to look for when a circuit works on the bench and fails in a noisy cabinet.
Lab: implement voltage follower, inverting and non-inverting amplifiers with an op-amp
The bench version of the configurations. Building all three in one sitting is worth it because the differences between them are easiest to see side by side.
Lab: measure the op-amp parameters
The bench version of the non-ideal items. Measuring the parameters yourself is what turns a datasheet column into something you believe.
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