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Die Photos and Analysis of the LM108 Operational Amplifier

Die Photos and Analysis of the LM108 Operational Amplifier
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Transistors inside the IC

Transistors are the key components in a chip. The LM108 op amp uses NPN and PNP bipolar transistors, while many newer op amps use low-power CMOS transistors instead.
If you’ve studied electronics, you’ve probably seen a diagram of an NPN transistor like the one below, showing the collector (C), base (B), and emitter (E) of the transistor. A transistor is usually illustrated as a sandwich of P silicon in between two symmetric layers of N silicon; the N-P-N layers make an NPN transistor.
But 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.

Symbol and oversimplified structure of an NPN transistor.

The photo below shows an NPN transistor on a 741 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 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. There’s a lot more than just the N-P-N sandwich, but if you look carefully at the vertical cross section below the ‘E’, you can find the N-P-N that forms the transistor. The emitter (E) wire is connected to N+ silicon. Below that is a P layer connected to the base contact (B). And below that is an N+ layer connected (indirectly) to the collector (C).

Structure of an NPN transistor in the 741 op amp

Structure of an NPN transistor in the 741 op amp

The innovative feature of the LM108 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.e. amplification), but makes the transistor much more delicate: just 4 volts between the collector and emitter can “punch through” the thin base and destroy the transistor.

This image shows one of the superbeta transistors in the LM308 op amp. Note the large, round emitter. The green rectangle below the transistor is a resistor.

This image shows one of the superbeta transistors in the LM108 op amp. Note the large, round emitter. The green rectangle below the transistor is a resistor.
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