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Understanding Silicon Circuits: inside the ubiquitous 741 op amp

Understanding Silicon Circuits: inside the ubiquitous 741 op amp

How I photographed the 741 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 the 741, are available in metal cans which can be easily opened with a hacksaw.[16] I used this safer approach.
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 microscope, which 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.[17]

Conclusion

Despite being almost 50 years old, the 741 op amp illustrates a lot of interesting features of analog integrated circuits.
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 741 that’s behind it all.

We've got a winner! 741 op amp marketing letter from 1968. Courtesy of Dave Fullagar.

We’ve got a winner! 741 op amp marketing letter from 1968. Courtesy of Dave Fullagar.

Thanks to Dave Fullagar for providing information on the 741, including the letter above, which shows that the 741 was an instant success.

Notes and references

[1]
The 741 op amp is one of 25 Microchips That Shook the World

[2]
To see the variety of circuits that can be built from an op amp, see this op amp circuit collection.

[3]
You might have wondered why there is a distinction between the collector and emitter of a transistor, when the simple picture of a transistor is totally symmetrical. 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. The result is a transistor will have poor gain if the collector and emitter are swapped.

[4]
In many of the ICs that I’ve examined, it’s easy to distinguish NPN and PNP transistors by their shape: NPN transistors are rectangular, while PNP transistors have circular emitters and bases with a circular metal layer on top. For some reason, this 741 chip uses rectangular and circular transistors for both NPN and PNP transistors. Thus, a closer examination is necessary to separate the NPN and PNP transistors.

[5]
The capacitor in the 741 is located at a special point in the circuit where the effect of the capacitance is amplified due to something called the Miller effect. This allows the capacitor in the 741 to be much smaller than it would be otherwise. Given how much of the 741 die is used for the capacitor already, taking advantage of the Miller effect is very important.

[6]
An alternative way to put capacitors on a chip is the junction capacitor, which is basically a large reverse-biased diode junction. The 741 doesn’t use this technique; for more information on junction capacitors see my article on the TL431.

[7]
For more information about current mirrors, you can check wikipedia, any analog IC book, or chapter 3 of
Designing Analog Chips. If you’re interested in how analog chips work, I strongly recommend you take a look at Designing Analog Chips.

[8]
The current mirror doesn’t provide exactly the same current for a variety of reasons. For instance, the base current is small but not zero. Transistor matching is very important: if the transistors are not identical, the currents will be different. (Using a single transistor with two collectors helps with matching.) If the collector voltages are different, the Early effect will cause the currents to be different. More complex current mirror circuits can reduce these problems.

[9]



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The 741 uses are several common extensions of the current source. First, by adding additional output transistors, you can create multiple copies of the current. Second, if you use a transistor with twice the collector size, you will get an output with twice the current (for instance). Third, instead of multiple output transistors, you can use one transistor with multiple collectors; this seems bizarre if you are used to discrete 3-pin transistors, but is a normal thing to do in IC designs. Finally, by flipping the circuit and using NPN transistors in place of PNP transistors, you can create a current sink, which is the same except current flows into the circuit instead of out of the circuit.

[10]
Differential pairs are also called long-tailed pairs.
According to
Analysis and Design of Analog Integrated Circuits
the differential pair is “perhaps the most widely used two-transistor subcircuits in monolithic
analog circuits.” (p214)
For more information about differential pairs, see wikipedia, any analog IC book, or chapter 4 of
Designing Analog Chips.

[11]
You might expect 741 chips to all be pretty much the same, but the “741” name is really a category, not a single design. Manufacturers use diverse circuits for their 741 chips.
Studying data sheet schematics, I found that 741 chips can be be divided into two categories based on the circuits for the second stage and output stage. The more common variant has 24 transistors, while the less common variant has 20 transistors. As far as I can tell, nobody has pointed this out before.

Wikipedia explains the 20-transistor variant while
the 24-transistor variants are discussed in
Operational Amplifiers
IC Op-Amps Through the Ages,
UNCC class notes
and the book
Microelectronic Circuits chapter 12.
The 741 die I discuss in this article is the 24-transistor variant.

[12]
For details on the 741’s history, see this interesting discussion:
Computer history museum: Fairchild Oral History Panel.

[13]

If the output is too low, the feedback circuit pushes it higher. But if it goes too high, the feedback circuit pulls it lower. This could repeat, causing larger and larger oscillations. The capacitor blocks these oscillations.
I’ve vastly oversimplified op amp stability and frequency compensation.
Some more detailed discussions are here
and
here.

[14]
IC Op-Amps Through the Ages says: “Despite a consequent near guarantee of suboptimal performance for most applications [because of the fixed capacitor], the ease of using the 741 has made it tremendously popular, proving Fullager’s assumption that engineers are basically lazy (I mean, very time-efficient).”

[15]
The schematic is from the Fairchild LM741 datasheet. I added the missing collector-base connection on Q12 and removed R12 (which is unused in this die). The component I photographed is the Analog Devices AD741, but that datasheet doesn’t have a schematic.

[16]
A plain hacksaw works to cut open an IC can. For later ICs, I used a jeweler’s saw which gives a cleaner cut than a hacksaw – the IC doesn’t look like it was ripped open by a bear. I got a saw on eBay for $14, and used the #2 blade. Make sure you cut near the top of the IC so you don’t hit the internal pins or the die.

[17]
To form the large image of the 741 die, I used Microsoft ICE to composite four images into a larger image. The Hugin photo stitcher can also be used for this, but I had trouble with it.

 

Original article by Ken Shirriff at his blog.

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