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Understanding Silicon Circuits: inside the ubiquitous 741 op amp

Understanding Silicon Circuits: inside the ubiquitous 741 op amp

IC component: The differential pair

The second important circuit to understand is the differential pair, the most common two-transistor subcircuit used in analog ICs.[10]
You may have wondered how the op amp subtracts two voltages; it’s not obvious how to make a subtraction circuit. This is the job of the differential pair.

Schematic of a simple differential pair circuit. The current source sends a fixed current I through the differential pair. If the two inputs are equal, the current is split equally.

Schematic of a simple differential pair circuit. The current source sends a fixed current I through the differential pair. If the two inputs are equal, the current is split equally.

The schematic above shows a simple differential pair. The key is the current source at the top provides a fixed current I, which is split between the two input transistors. If the input voltages are equal, the current will be split equally into the two branches (I1 and I2). If one of the input voltages is a bit higher than the other, the corresponding transistor will conduct more current, so one branch gets more current and the other branch gets less. As one input continues to increase, more current gets pulled into that branch. Thus, the differential pair is a surprisingly simple circuit that routes current based on the difference in input voltages.



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The internal blocks of the 741

The internal circuitry of the 741 op amp has been explained in many places[11], so I’ll just give a brief description of the main blocks. The interactive chip viewer below provides more explanation.

The two input pins are connected to the differential amplifier, which is based on the differential pair described above. The output from the differential amplifier goes to the second (gain) stage, which provides additional amplification of the signal.
Finally, the output stage has large transistors to generate the high-current output, which is fed to the output pin.

Die for the 741 op amp, showing the main functional units.

Die for the 741 op amp, showing the main functional units.

A key innovation that led to the 741 was Fairchild’s development of a new process for building capacitors on ICs using silicon nitride.[12]
Op amps before the 741 required an external capacitor to prevent oscillation, which was inconvenient.[13]
Dave Fullagar had the idea to put the compensation capacitor on the 741 chip using the new manufacturing process. Doing away with the external capacitor made the 741 extremely popular, either because engineers are lazy[14] or because the reduced part count was beneficial.

Another feature that made the 741 popular is its short-circuit protection. Many integrated circuits will overheat and self-destruct if you accidentally short circuit an output. The 741, though, includes clever circuits to shut down the output before damage occurs.

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