Inside a TTL Logic IC

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Since 2014 is generally considered the 50th anniversary of TTL logic, I thought I’d take a TTL logic chip apart and do a little analysis.

So I started with a DM7438N, lot code M:P9006Y. Looking at National Semiconductor’s device marking convention document, I take this to mean that it was manufactured in week 6 of 1990 at a subcontractor’s fab in the United States and assembled in Malaysia.

The 7438 is a quad 2-input NAND buffer with open-collector outputs. That means the die should look symmetrical to a degree.

To take it apart, I used a rotary tool to carve out the encapsulation material on the top and the bottom, and then picked at it with side cutters until the chip fell out. Sadly I cracked off a corner of the die including one bond pad, but it’s still possible to figure out how it works.

What does all this do? See the image below. I’ve cropped all but one gate and highlighted the various semiconducting regions in different colors. I’ve also given designators to all the components.

Red represents the N-type collector epitaxial diffusion. Cyan represents the P-type base diffusion, and purple represents the N+ emitter region.

The schematic looks like this:

That dual-emitter transistor (Q1) sure looks strange!

How does it work? Well, if both A and B inputs are a logic high, then Q1 is off, but some current flows from R1 (4K ohm) through the base collector junction (since it is, after all, a PN junction) and feeds the base of Q2. Q2 turns on, and its emitter current feeds R3 (1K ohm) and Q3. Q3 turns on as well, and the output Y gets driven low. The non-inverted version of the output signal is available at the collector of Q3 (biased through R2, a 1.6K ohm resistor), but this particular chip doesn’t use it.

If either A or B goes low, then Q1 gets turned on. Current flows through the base emitter junction and the base gets pulled to about 0.6V above ground. No current flows through the base of Q2 because the voltage on the collector of Q1 is just too low for any current to flow. Q3 therefore stays off, and the output Y goes high impedance. By the way, this is what open collector means–the collector of the output stage transistor is left “open” with no corresponding transistor above it to pull it high.

Diodes D1 and D2 are just for input protection.

There are a couple of unused components. There is a resistor right below R1, and another resistor below R2. There are two extra transistors with a shared collector to the left of Q3. A different top metal mask could connect these extra components into the circuit and change the function of the device.

Can you think of some other gates that could be built by changing the top metal mask? Remember that there is only one metal layer which limits where you can route the traces.

XL741 – Discrete Op-Amp

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We’ve done it again! My friends at Evil Mad Scientist Laboratories have a new kit for sale. Following on the success of our Three Fives discrete 555 timer kit, we’ve had a lot of requests for a discrete 741 op-amp.

The XL741 is based on the datasheet schematic of the original uA741 op-amp IC from 1968. You can wire it up in a classic op-amp circuit and probe nodes inside the IC so you can see how the chip works. Play with differential pairs, modify the compensation, and change bias currents to your heart’s content!

Flea Market Find–Dual Gun CRT

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At the electronics flea market on Saturday, I found a 3ABP7 dual gun CRT. This one was built by DuMont, most likely intended for the 3″ version of their 5″ Type 279 Dual Beam Oscilloscope.

So of course I had to fire it up. There are two sets of deflection coils, so I drove them with one deflection board and cross-wired the deflection coils to flip the image around on the second gun.

The guns themselves have a common cathode connection and separate grids, which forces me to drive them both in parallel since my deflection board video amplifier keeps the grid at a constant potential and drives the video onto the cathode.
Dual gun 3ABP2 CRT

Here’s a closer look at the guns. The filaments are connected in parallel so this tube uses twice the normal current.
Dual gun 3ABP2 CRT

Compensating CRT Deflection Coils

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Things look really weird when you use an uncompensated deflection coil with a vector graphics display. This is because the coil looks like an inductive load to the driver amplifier, and the parasitic capacitance makes it ring. Ringing and overshoot create the strange-looking display, basically extending every line past its destination.

One way to compensate for that is to add a series RC snubber in parallel with each deflection coil. You can perform some calculations to figure out the values of the resistor and capacitor but they won’t get you very close to the answer. There are just too many parasitics to model. It’s much faster just to build a RC substitution box and tweak the values until you get the result you want.

CRT Board BOM Updates

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I have corrected some mistakes in the bills of materials for the CRT driver boards (the files in GitHub have already been updated):

  • ScopePower U2 part number from TS271CN (DIP package) to TS271IDT (SOIC).
  • ScopePower R3 part number from VR37000003305JR500 (33 meg) to┬áVR37000001005JR500 (10 meg).

If you already ordered parts, I apologize. The TS271CN is a great little op amp and the DIP package version will work fine in a breadboard. That’s what I will do with the ones I accidentally ordered. The 33 meg resistor is useful as a bleeder resistor if you build the post deflection acceleration module (to be posted soon).

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