The Norton amplifier looks like an op-amp on a schematic, but it's a fundamentally different beast. Instead of amplifying the voltage difference between its inputs, it amplifies the current difference. Both inputs sit at one V_BE above ground (~0.5 V), and internal current mirrors do the differencing. The classic part is the LM3900 — four Norton amps in a 14-pin DIP, dirt cheap, and still used in industrial and automotive designs where you have a single 5–32 V supply and no negative rail.

Why bother with them when rail-to-rail op-amps exist? Two reasons:

• Native single-supply operation — inputs are current-driven through resistors, so you never worry about the input common-mode range crushing near ground.
• You bias with resistors, not voltage dividers. A single resistor from V+ to the (+) input sets the operating point. No dual op-amp buffer, no rail-splitter.

Real-world example: an AC-coupled audio preamp on a 12 V car rail. To bias the output to mid-supply (6 V), you inject a bias current into the (+) input equal to the current that flows out of the (–) input to ground through the feedback resistor. If R_f = 1 MΩ and you want V_out = 6 V, the current through R_f is (6 V − 0.5 V) / 1 MΩ = 5.5 µA. So the bias resistor from V+ (12 V) to (+) must supply 5.5 µA: R_bias = (12 − 0.5) / 5.5 µA = 2.09 MΩ. Rule of thumb: R_bias ≈ 2 × R_f × (V+ / V_out). Add an input coupling cap and an input resistor to set gain (gain ≈ R_f/R_in) and you're done.

Watch out for:

• Distortion is worse than a modern op-amp — think 1% THD, not 0.001%. Fine for squelch detectors, tone generators, ignition timing conditioners. Not for hi-fi.
• Slew rate is only ~0.5 V/µs and GBW ~2.5 MHz. Don't use it above audio.
• The (+) input is a diode, so you can't drive it directly with a voltage — always use a series resistor to convert voltage to current.
• Supply currents mirror through the substrate, so heavy loading on one amp can crosstalk into the others in the same package.

Norton amps live on because they solve the "one battery, ground reference, cheap resistors" problem elegantly. If you've ever wondered why some old-school automotive schematics have op-amp-shaped symbols with an odd diode marking on the input, you've found one.