Check for Samples: LMN, LMN, LMN Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet. Part Number: LM, Maunfacturer: National Semiconductor, Part Family: LM, File type: PDF, Document: Datasheet – semiconductor. December LM LM LM Operational Amplifiers LM from b25 C to a85 C and the LM from 0 C to a70 C . LM Mil-Aero Datasheet.
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I’ve written about the famous op ampwhich came out in The LM op amp in an 8-pin metal can. The photo above shows the LM op amp in a metal can. The LM is the commercial-grade version of the LM The chip’s metal layer is clearly visible, with thin metal traces connecting the different parts of the chip.
The square bonding pads around the edge of the chip are connected by thin wires to the chip’s external pins. Under the metal layer, you can see the silicon that forms the basis of the chip.
To form transistors and resistors, a process called doping treats regions of the silicon with elements such as phosphorus or boron. In the die photo, these regions have a slightly different color, which makes the structure of the chip visible under the metal.
Die photo of the LM op amp. The LM is the commercial version of the LM While the chip seems incomprehensible at first, close examination reveals the different components and their connections. By carefully studying the die photo, I reverse engineered the circuit for the op amp.
Surprisingly, this chip has an unusual circuit design, more modern than National Semiconductor’s “classic” LM design. Although the package has the National Semiconductor logo, the internal circuitry matches the Motorola LM datasheet.
Different manufacturers have widely differing implementations of the chip, so you can’t expect two chips to behave the same just because they have the same name. Why op amps are important The function of an op amp is to take two input voltages, subtract them, multiply the difference by a huge valueor moreand output the result as a voltage. If you’ve studied 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 would you want to amplify by such a huge factor? Would amplifying a 1 volt input result in lightning shooting from the op amp? It turns out that op amps are extremely datawheet and versatile, making them a key component in analog circuits. With simple feedback circuits, you can use an op amp as an amplifier, a filter, integrator, differentiator, or a variety of other circuits.
The point of the high amplification is it improves accuracy, even if the amplification of the overall circuit is small. Transistors inside the IC Transistors are the key components in a chip. If you’ve studied electronics, you’ve probably seen a diagram of an NPN transistor like the one below, showing the collector Cbase Band emitter E of the transistor. Datasheeg it turns datasheeh 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 an NPN transistor on a op amp die. The different brown and purple colors are regions of silicon that has been doped differently, forming N and P regions.
The whitish-yellow areas lm3308 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. There’s a lot more than just the N-P-N sandwich, but if you look carefully at the vertical cross section below the lm38, you can find the N-P-N that forms the transistor. Below that is datasheef P layer connected to the base contact B. Structure of an NPN transistor in the op amp The innovative feature of the LM is the superbeta transistor, seen below.
It has a much thinner base region below the emitter. This gives the superbeta transistor a much higher beta i.
This image shows one of the superbeta transistors in the LM op amp. Note the large, round emitter.
The green rectangle below the transistor is a resistor. How the op amp works In this section, I’ll give a simplified overview of how the op amp works. The next section explains the different parts of the LM op amp. The final section describes the current mirror that provides precise currents to the op amp’s circuits.
The differential pair The key component of an op amp is the differential pair, which is 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 sink sends a fixed current I through the differential pair. If the two inputs are equal, the current is split equally between the two branches.
Otherwise, the branch with the higher input voltage gets most of the current. The schematic above shows a simple differential pair. The current sink at the bottom 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.
A small input difference is enough to direct most of the current into the “winning” branch, providing the amplification. The LM op amp circuit In this section, I’ll give a brief explanation of the LM circuit, based on a detailed discussion by Bob Widlar, the chip’s designer.
The superbeta transistors Q1 and Q2 are the heart of the chip. These form the input stage, and are connected as a differential pair. Resistors R1 and R2 provide the load for the two branches of the differential pair. By using superbeta transistors for the input, the LM achieves high performance with datasheeh low input currents. The problem with superbeta transistors is they lk308 break down and be destroyed by a small voltage difference, just 4 volts.
The LM uses a couple interesting circuits to protect the datashdet transistors.
LM Datasheet(PDF) – National Semiconductor (TI)
The first protection mechanism is the two diodes across the inputs, ensuring the voltage difference is small. On the chip, these diodes are implemented with transistors.
The second protection mechanism is transistors Q5 and Q6, which ensure that the collector-emitter voltage across the superbeta transistors is essentially zero, preventing an overload. They form a second differential amplifier that amplifies the output of the first stage. Instead of resistors, Q15 and Q16 form the load for the second stage differential amplifier.
Transistors Q7 and Q8 bias the inputs of Q9 and Q10 to the right level. The output of the op amp is driven by a high-current class AB amplifier with power transistors Q13 and Q That is, Q13 will pull the output high and Q14 will pull the output low.
To ensure that the right output transistor turns on at the right time, Q11 and Q12 bias the output transistors by two diode drops. The current mirror The schematic above uses a symbol that you may not be familiar with: A current source may seem like a strange concept, but it is very common in analog integrated circuits.
The idea is that instead of controlling currents with resistors which are inconveniently large and inaccurate on ICscurrents are generated from a current mirror. Simple modifications can scale the current or even invert it. Detail of the LM op amp schematic, showing the current source symbol. The diagram below 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, so the current on the right matches the reference current on the left.
Thus, the current mirror provides a mirror image on the right of the fixed current on the left. The current on the right copies the current on the left. In the LM, the initial current is generated not by a resistor, but by a patented four-transistor circuit that depends on one transistor having 10 times the emitter area of the others.
The photo below shows the transistor that combines 10 square emitters into one large emitter, as well as an unusual transmitter with two separate emitters. The transistor on the left has 10 emitters wired together, creating a transistor with an effective emitter size of 10 times normal. The transistor on the right has two separate emitters, providing two current outputs. Interactive chip viewer The image and schematic  below are an interactive exploration of the LM Click a component to see its location on the die and in the schematic highlighted.
The box below will give an explanation of the component. The schematic below is the full schematic for the LM; the component numbers don’t match the earlier simplified schematic. Click components in the image below for more information. How I photographed the op amp die Usually getting the die out of an IC requires concentrated acid to dissolve the epoxy package. But some ICs, such as op amps, are available in metal cans for shielding which can be easily opened with a hacksaw or even better a jeweler’s saw.
I used a metallurgical microscope for my die photos, but you can use even a basic middle-school microscope to see many of the chip features. The photo below shows the LM op amp after removing the top.
The tiny die is visible in the center, with thin wires connecting the die to the pins that surround it. The metal tab on the right indicates pin 8. To create the high-resolution die photo, I composited multiple photographs into one image details. The LM op amp has been cut open revealing the tiny die inside. Pads on the die are connected to the pins with thin bond wires. If you compare the op amp below with the LM, you’ll notice that the is much simpler.