PE2-1.3 Differential Amplifiers & Op-Amps
You can analyse the basic differential pair, design one- and two-stage op-amps with frequency compensation and common mode feedback, and account for input range limitations, slew rate and PSRR.
Differential signalling is the reason analog ICs work at all in a noisy chip: common mode noise appears on both inputs and cancels, which single-ended circuits cannot do. Frequency compensation of the two-stage op-amp is the unit's hardest and most valuable content — you are deliberately slowing the amplifier so that feedback around it stays stable, and Miller compensation with a nulling resistor is the standard answer. Slew rate reappears here as a bias-current limit rather than a datasheet number, which is what makes it designable.
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Single ended versus differential amplifiers
Why analog chips use two wires rather than one: common disturbances cancel. It is the single most important structural choice in analog integrated design.
The basic differential pair and its analysis
The circuit that does it, and how gain and common mode range come out. Analyse it once properly and the operational amplifiers below are assemblies of it.
One stage operational amplifier
The simplest complete amplifier, which is the differential pair with an active load. It has good bandwidth and limited swing.
Two stage operational amplifier
Adding a second stage for gain and swing, which is what most designs actually use. It also creates the stability problem the next item solves.
Frequency compensation of the two stage op-amp
Making the two stage amplifier stable in feedback, which is where the pole splitting trick appears. It is the most examined topic in this course for good reason.
Common mode feedback
The extra loop a fully differential amplifier needs to hold its output level. It is quietly omitted from many schematics and fatal on silicon, so look for it deliberately.
Input range limitations
What input voltages the amplifier can actually accept. It becomes a serious constraint as supply voltages shrink.
Slew rate and PSRR
Two large signal limits that no small signal analysis reveals. Slew rate is set by the tail current and the compensation capacitor, which connects it to the item above.
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