Wednesday, April 10, 2013

We have contact!

Tim McNerney and I have established contact! If that name doesn't ring a bell, it should: he's the guy who envisioned the recreation of the Intel 4004 in the first place, got Intel to make available the schematics, and inspired others to help bring his vision to life. He's also the author of the Intel 4004 — 35th Anniversary Project web page -- if you're interested in the 4004 and haven't read his page, go there NOW.

During our conversations another option for MOSFET choices came to my attention. Rather than searching for one of the scarce few 4-terminal discrete FETs, it's possible to use two devices back-to-back to work around the problems created by the body diode created when the FET's substrate is connected to its source:

In this configuration, the two drain terminals can be used interchangeably, while the gate connection is common. In operation, if the left side drain is positive with respect to the right side the body diode on the right will be forward-biased and will conduct, but the one on the left is reverse-biased and the circuit works as intended when the gate is low. Since the FETs themselves will conduct in either direction, there shouldn't even be a voltage drop across the conducting body diode when the gate is high.

There is a drawback to this approach: the gate capacitance is doubled, since the two gates are in parallel. This will result in higher power consumption and possibly slower operation, especially since the 4004 uses resistive pull-ups rather than push-pull circuits in most cases, though there are push-pull drivers in cases where there is high capacitive loading (like in the DRAM arrays) or where timing is critical (like in the ALU).

Another drawback would appear to be increased parts counts. However, dual-FET parts are pretty common in the SMD world. Consider these two examples:

BSS84W -- Single P-Channel MOSFET

BSS84DW -- Dual P-Channel MOSFET
The package dimensions of these two devices are identical, differing only in the number and placement of the leads.

I think I'm going to add a few of these to my next parts order (along with prototyping adapters) so I can experiment with them. I want to see how they behave in a DRAM cell. That's the best part of hobbies -- there's always something to learn!

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