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First Test

A project log for CBJT Logic

Complementary bipolar junction transistor logic - like CMOS, but with NPNs and PNPs

Ted YapoTed Yapo 01/08/2017 at 17:2615 Comments

I decided to test the idea with a ring oscillator. After seeing that five inverters would oscillate in a SPICE simulation, I heated up the soldering iron and made this:

there are five inverters based on the original circuit:

each one has a 0.47uf supply bypass capacitor. The circuit starts to oscillate at about 0.4V, but is wildly unstable. At 0.8V, it draws 30mA with stable oscillation:

The oscillation frequency is 2.4 MHz, equal to a period of 420 ns. This implies a propagation delay of 42ns per inverter.

There are a lot of issues that would have to be solved before this becomes a usable logic family, but it's a start.

If we can figure out how to make either an AND or OR gate, we'll at least be logic-complete, then it's a matter of details.

Discussions

esot.eric wrote 01/09/2017 at 09:08 point

Nice layout

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Ted Yapo wrote 01/08/2017 at 18:03 point

Oh, this needs a base resistor, of course!  That will reduce the supply current at higher voltages.  I saw the "antique" schematic you posted - base resistor with speedup capacitor.  Makes sense.

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esot.eric wrote 01/09/2017 at 03:38 point

heh, "speedup capacitor" I guess that's exactly what I was talking about... saw it in-circuit, and didn't quite grasp its purpose.

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Yann Guidon / YGDES wrote 01/08/2017 at 17:34 point

30mA/5= 6mA per inverter at room temperature. Not the best but could still be worse, and it's hightly dependent on a very stable power supply voltage.

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Ted Yapo wrote 01/08/2017 at 17:38 point

I think the voltage supply should vary with the ambient temperature to keep everything stable.  Just add enough forced air to keep at the transistors at the same temperature and use a 2N3904 BE junction as a temperature sensor to regulate the power supply.

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Yann Guidon / YGDES wrote 01/08/2017 at 19:43 point

yup.

In comparison, TTL is extremely reliable ;-)

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K.C. Lee wrote 01/08/2017 at 21:05 point

Or run a constant current source.  BJTs are current mode devices.

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Yann Guidon / YGDES wrote 01/09/2017 at 00:32 point

@K.C. Lee : we're entering the ECL domain here :-D

Part of the allure of this circuit is the similarity with CMOS... it stands to reason that we wish to keep this aspect :-)

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Ted Yapo wrote 01/08/2017 at 17:40 point

But, my assumption is that this works like CMOS - draws very little current when idle, this 6mA/inverter is running near full speed.

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Yann Guidon / YGDES wrote 01/08/2017 at 19:42 point

no because BJT are current-driven and require current to keep conducting.

The Baker circuit with RC in parallel on the base addresses this concern, though the supply voltages need adaptations.

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Ted Yapo wrote 01/08/2017 at 21:00 point

yes, I need software that makes me think before typing.

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esot.eric wrote 01/09/2017 at 03:37 point

Interesting point about BJTs' being current-driven... As I understand, speed-wise they certainly switch faster when driven with a larger current... but there's no need for that level of current once they've switched. I wonder if an RC circuit could allow for fast switching, then lower drive-current to "maintain" a value. Might change the topology a bit (a lot)

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Yann Guidon / YGDES wrote 01/08/2017 at 17:32 point

For OR and AND, it's simply like in CMOS : high-side has inverse topology as low-side,

AND: low-side is parallel, high side series

OR : low-side series, high side parallel

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Ted Yapo wrote 01/08/2017 at 17:38 point

I think the Vbe drop is a problem here.  I can't see how to do it :-(

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Yann Guidon / YGDES wrote 01/08/2017 at 19:42 point

experiment.

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