Absolute Maximum Ratings. The “Absolute Maximum Ratings” are those values beyond which the safety of the device cannot be guaranteed. The device should . Fairchild µA description. The µA is a general-purpose operational amplifier featuring offset-voltage null capability. The high common-mode input voltage. uA Frequency-compensated operational amplifier. Other names for this product: , A, uA,. Courtesy/Thanks to: Fairchild Semiconductors.

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The op amp, packaged in a TO metal can. I started with a op amp that was fairchilf in a metal can above. Cutting the top off with a hacksaw reveals the tiny silicon die belowconnected to the pins by fine wires. Inside a op amp, showing the die. This is a TO metal can package, with the top sawed off Under a microscope, the details of the silicon chip are visible, as shown below. At first, the chip looks like an incomprehensible maze, but this article will show how transistors, resistors and capacitors are formed on the chip, and explain how they combine to make the op amp.

Die photo of the op amp Why op amps are important Op amps are a key component in analog circuits. An op amp takes two input voltages, subtracts them, multiplies the difference by a huge valueor moreand outputs the result as a voltage. If you’ve fsirchild analog circuits, op amps will be familiar to you, but otherwise this may seem like a bizarre and pointless device. How often do you need to subtract two voltages?

And why amplify by such a huge factor: The answer is feedback: Op amps are used as amplifiers, filters, integrators, differentiators, and many other circuits. U741 cell phone uses op amps for filtering and amplifying audio fairchlld, camera signals, and the broadcast cell signal. If you’ve studied electronics, you’ve probably seen a fairchipd of a NPN transistor like the one below, showing the collector Cbase Band emitter E of the transistor, The transistor is illustrated as a sandwich of P silicon in between two symmetric layers of N silicon; the N-P-N layers make a NPN transistor.

It turns out that transistors on a chip look nothing like this, and the base often isn’t even in the middle! Symbol and oversimplified structure of an NPN transistor.

The photo below shows one of the transistors in the as it appears on the chip. The different faircild and purple colors are regions rairchild silicon that has been doped ua74, forming N and P regions. The whitish-yellow areas are the metal layer of the chip on top of the silicon – these form the wires connecting to the collector, emitter, and base. Underneath the photo is a cross-section drawing showing approximately how the transistor is constructed.

File:Fairchild uA741 opamp 6920.jpg

There’s a lot more than just the N-P-N sandwich you see in books, but if you look carefully at the vertical cross section below the ‘E’, you can fqirchild the N-P-N that forms the transistor. Below that is a P layer ua471 to the base contact B. But for a variety of reasons, PNP transistors have an entirely different construction. They consist of ua7411 circular emitter Psurrounded by a ring shaped base Nwhich is surrounded by the collector P.

The diagram below shows one of the PNP transistors in thealong with a cross-section showing the silicon structure. Structure of a PNP transistor in the op amp.

The output transistors in the are larger than the other transistors and have a different structure in order to produce the high-current output. The output transistors must support 25mA, compared to microamps for the internal transistors. Ya741 photo fairchil shows one of the output transistors.


Note the multiple interlocking “fingers” of the emitter and base, surrounded by the large collector. A high-current PNP transistor inside the op amp How resistors are implemented in silicon Resistors are a key component of analog chips. Thus, analog ICs are designed so only the ratio of resistors matters, not the absolute values, since the ratios remain nearly constant from chip to chip. The photo below shows two resistors in the op amp, formed using different techniques. In the pinch resistor, a layer of N silicon on top makes the conductive region much thinner i.

This allows a much higher resistance for a given size.

uA Frequency-compensated operational amplifier by Fairchild Semiconductors

Both resistors are at the same scale below, but the pinch resistor has ten times the resistance. The tradeoff is the pinch resistor is much less accurate. Two resistors from the op amp.

The left resistor is a simple ‘base resistor’, while the right resistor is a ‘pinch resistor’. How capacitors are implemented in silicon The ‘s capacitor is essentially a large metal plate separated from the silicon by an insulating layer. The main drawback of capacitors on ICs is they are physically very large. The 25pF capacitor in the has a very small value but takes up a large fraction of the chip’s area.

The current mirror There are some subcircuits that are very common in analog ICs, but may seem mysterious at first. Before explaining the ‘s circuit, I’ll first give a brief overview of the current mirror and differential pair circuits.

Schematic symbols for a current source. If you’ve looked at analog IC block diagrams, you may have seen the above symbols for ua41 current source and wondered what a current source is and why you’d use one. The fairchjld of a current fairchhild is you start with one known current and then you can “clone” multiple copies of the current with a simple transistor circuit.

The following circuit shows how a current mirror is implemented. In this case, the current is set by the resistor. Since both transistors have the same emitter voltage and base voltage, they source the same current, [8] so the current on the right matches the reference current on the left.

The current on the right copies the current on the left. A common use of a current mirror is to replace resistors.

As explained earlier, resistors inside ICs are both inconveniently large and inaccurate. It saves space to gairchild a current mirror instead of a resistor whenever possible. The large resistor snaking around the upper middle of the IC controls the initial current.

This current is then duplicated by faircnild current mirrors, providing controlled currents to various parts of the chip. Using one large resistor and current mirrors is more compact and more accurate than using multiple large resistors. The current mirror in the middle is slightly different; it provides an active load for the input stage, improving the performance.

Die for the op amp, showing the current mirrors, along with the resistor that controls the current. The differential pair The second important circuit to understand is the differential pair, the most common two-transistor subcircuit used in analog ICs. 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 fairchildd, the current is split equally.

Understanding silicon circuits: inside the ubiquitous op amp

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 fakrchild 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 ka741 a surprisingly simple circuit that routes current based on the difference in input voltages. The internal blocks of the The internal circuitry of the 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 faircihld the high-current output, which is fed to the output pin. Die for the op amp, showing the main functional units. A key innovation that led to the was Fairchuld development of a new process for building capacitors on ICs using silicon nitride. Doing away with the external capacitor made the extremely popular, either because engineers are lazy [14] or because the reduced part count was beneficial.

Another feature that made the popular is its short-circuit protection. Many integrated circuits will overheat and self-destruct if you accidentally short circuit an output.

Thethough, includes clever circuits to shut down the output before damage occurs. Interactive chip viewer The die photo and schematic below are interactive. Click components in the die photo or schematic [15] to explore the chip, and a description will be displayed below. How I photographed the die Integrated circuit usually come in a black epoxy package.

Dangerous concentrated acid is required to dissolve the epoxy package and see the die. But some ICs, such as theare available in metal cans which can be easily opened with a hacksaw. With even a basic middle-school microscope, you can get a good view of the die at low magnification but for the die photos, I used a metallurgical microscopewhich shines light from above through the lens.

A normal microscope shines light from below, which works well for transparent cells but not so well for opaque ICs. A metallurgical microscope is the secret to getting clear photos at higher magnification, since the die is brightly illuminated. Next time you’re listening to music, talking on your cell phone, or even just using your computer, think about the tiny op amps that make it possible and the that’s behind it all.

See more comments on Hacker NewsReddit and Hackaday. We’ve got a winner! Courtesy of Dave Fullagar. Thanks to Dave Fullagar for providing information on theincluding the letter above, which shows that the was an instant success. Notes and references [1] The op amp is one 25 Microchips That Shook the World and is popular enough to be on mugs and multiple tshirtsas well as available in a giant kit.

Both connect to an N layer, so why does it matter?

As you can see from the die photo, the collector and emitter are very different in a real transistor. In addition to the very large size difference, the silicon doping is different.