Evaluating Transistors for Bipolar Logic (RTL)

Experiments on optimizing discrete logic gates based on bipolar transistors

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 I highlighted the importance of the choice of the transistor for bipolar discrete saturation logic before. The relevant parameters were already well understood in the 1960ies and devices like the 2N709 were devised, based on the technology that was available back then.

What about 2020, sixty years later? Clearly, discrete logic is not within the focus of any transistor manufacturer anymore, so we have to pick what is available. What are important criteria (to me)?

  1. Speed - low propagation delay.
  2. Availability - I don't want to work with NOS bought from some obscure vendor. Availability at PCB assemblers would be the best case.
  3. Package size - SMD is a must. Packages smaller than SOT23 would be perfect.

The last two criteria can be easily evaluated from datsheets and vendors listings. It turns out that the first criterion, speed, is not so easy to asses and merits actually testing things with real hardware.

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RTL Ring oscillator

LTspice circuit models for this project. Ringo5_inverter_XXX are ring oscillator test benches Transer_inverter simulates the transfer function

x-zip-compressed - 6.48 kB - 04/25/2020 at 16:14


  • Using a LED as base Resistor / Chaotic Ring Oscillator

    Tim06/12/2020 at 12:33 9 comments


    Out of curiosity I tried replacing the base resistor in my 5-stage ring oscillator with a red LED. Why would I do that? Well, first of all the footprint of the LED and resistor exactly matched on the PCB. So why not?

    There was a bit more rationale behind this, though: The LED will increase switching voltage and should improve noise margin, similar to what I did with "LTL". Since LEDs have a non-negligible internal capacitance, I was also hoping that the LED would act in a similar way as the reach-through capacitors in parallel to the base resistor, as I tried earlier.

    What started as a quick experiment turned into quite a curious feat, as you will notice later.

    The schematics are shown above. Please ignore the resistor values, I used 470 Ohm collector resistors in all cases. I built two versions: One with PMBT2369 and one with PMBT3904.

    PMBT2369 based RINGO5

    Results for a voltage sweep of the PMBT2369 based ring-oscillator are shown above in green, in comparison with previous results. Two things are apparent:

    1. Oscillations are only observed at a much higher operating voltage of ~2.2V instead of 1V for the others devices.
    2. The frequency is continuously rising with the voltage, no maximum is visible within the voltage range. 

    An increased operating voltage is expected since the additional voltage drop over the LED increases the turn on voltage of each inverter. The larger swing of the logic levels also leads to a reduction of propagation delay and hence oscillator frequency.

    The absence of a frequency maximum and the higer maximum frequency compared to the normal ring oscillator without reach-through capacitor (orange) suggest that the LED capacitance does indeed act in a similar way as the parallel capacitor.

    It seems that both initial assumptions have been confirmed. Does it mean that RTL with LED-instead-of base-resistor is the way to go? Probably not, as there will be serious issues with device matching when multiple gates are connected in parallel due to variation of LED forward voltage and transistor VBE. This is addressed in the LTL gate, however.

    PMBT3904 based RINGO5 with LED instead of base resistor

    Ok, let's repeat the same experiment with the "standard" switching transistor PMBT3904.

    The plot above shows the ring oscillator frequency when sweeping the voltage up to 7V and then down again. There were quite a few surprises. First, the maximum frequency is above 50MHz, far above anything I had measured with the PMBT3904 before. Then, and this is even more suprising, above 5 V  the frequency suddely drops to a very low value below 0.5 MHz - 100 times lower. When the voltage is lowered to below ~3.5 V, the frequency increases again.

    The low frequency region is shown in more detail in the figure above.

    It is clear that the frequency shows a hysteretic behavior in respect to the voltage sweep. This means that the circuit has two stable operating points between 3.5 V and 5 V - one at low oscillating frequency and one at high frequency. Some waveforms details are shown below.

    Waveform of the high frequency oscalliating mode.

    Waveform of the low frequency oscillating mode. You can clearly see fully saturated switching.

    Waveform in the high frequency operating point close to the switching point. One can see that several frequencies are superimposed and the oscilation gets instable.

    This result is quite weird.  In one mode, the oscillator is much faster than any PMBT3904 based oscillator i built before, in the other mode it is incredibly slow. What is the exact mechanism behind the metastability? Can it be used for something?

    Spice Simulations

    I set up a Spice model in LTSpice to understand better what is going on. See below for the schematics. One big source of inaccuracy is the LED model itself. I took a model for a similar LED from the OSRAM OS homepage and modified it to fit the LED I used (some noname 0603 SMD LED). However, I am certain...

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  • Pushing RTL to <2 ns Propagation Delay

    Tim05/04/2020 at 21:46 6 comments

    So, we established that the PMBT2369 is the fastest saturation-switching transistor available today and that it achieves a minimum propagation delay of 3.5 ns at a bias current level of 10 mA. How can we go further from there? Obviously, more brute forcing by changing resistor values and increasing currents is not the way to go. Alternatively we can add components to the basic RTL gate.

    There are two approaches: Adding a reach-through capacitor in parallel to the base resistor and adding a baker clamp.


    The panel above shows circuits of both options individually and in combination including the resulting waveformt of the ring oscillator.

    The reach through cap basically shorts the base resistor for high frequency components of the input signal; the rising and falling edge. For the critical case of a falling edge of the input signal it will lead to negative biasing of the base, which will help to remove the saturation charge. You can see above, that it even leads to a negative spike of the output signal due to coupling to the collector.

    The baker clamp prevents saturation of the transistor by shorting the base with the collector if the base potential is too high. The choice of the diode is quite critical here: A schottky diode is needed to minimize forward voltage and the capacitance needs to be lower than CBC of the transistor to avoid excessive miller capacitance that could limit the turn-on time. Since the baker clamp shorts the base to the collector, also the low voltage level is increased to about 0.4V. Therefore, noise margin is drastically reduced for this solution, something that is not preferrable considering the already low noise margin of RTL.

    Spice simulation of the ring oscillator at a supply of 5V shows that both options improve switching speed and the combination of both measures is cumulative. All options use a 180 Ohm base resistor.

    The reach through cap works best, when the voltage drop across the base resistor is high, because this leads to a stronger negative overdrive of the base. This condition is optimizated when Rb is large compared to Rc, as is evident  for the strong increase in frequency for lower collector resistance in the plot above.

    Hardware testing

    Due to lack of suitable schottky diodes I was only able to test the reach-through cap in real hardware.

    Comparing an RTL oscillator with and without reach-through cap reveals quite dramatic differences. The yellow and blue graphs above are from an oscillator with the same Rb and Rc. The blue one has a 68 pF reach-through cap added in parallel. Due to negative base overdrive, no local maximum of switching speed is observed anymore and the oscillator frequency continues to rise for higher supply voltage/current.
    A scope trace shows that, despite the high frequency, there is still fully saturated switching. The signals looks a bit smoothed, which may be caused by bandwidth limitations of my probe and the scope (100 MHz):

    Finally, looking at the bias current / tpd trade off, we can see that the seemingly unsurmountable trend of pure RTL has truely been beaten and less than 2 ns tpd is achieved at  higher current. Quite an astonishing result for discrete saturation logic!

    There are some caveats: Keep in mind that the full oscillator is supplied with more than 1 W of power at this operating point, almost 200 mW per gate. Collector bias of a single gate is ~30 mA and the PCB is getting notably hot. Furthermore, since the input to the gate is basically a high-pass, the propagation delay of the gate will be dependent of the slew rate of the incoming signal. This adds another dimension to the complexity of logic design.

  • Optimizing Resistors in the PMBT2369 Ring Oscillator

    Tim05/04/2020 at 21:29 6 comments

    To speed up the propagation time of the inverters in the ring-oscillator further, I built up samples with smaller base and collector resistors.

    Sample 1
    360 Ohm
    2.4 kOhm
    Sample 2
    180 Ohm
    1.8 kOhm
    Sample 3
    180 Ohm
    470 Ohm
    The figure above shows the relationship between supply voltage and oscillator frequency. Reduction of both base and collector resistor increases speed at the same voltage. It is somewhat obvious since availability of more current will allows faster switching up to the point when the saturation charge storage limits the turn-off time. Since the PMBT2369 is engineered to reduce charge storage time, it increases switching speed up to a fairly high current. However at some point this is not sufficient anymore and the speed begins to reduce again. This is visible for Sample 3 (Rc=470, Rb=180), which has a maximum at around 3V. Also sample 2 starts to get slightly slower beyond 8V.
    This trend is confirmed when plotting supply current vs. voltage. There is a big gap between sample 2 and sample 3, I probably should have added settings in between...
    Things get much more interesting when plotting the ring oscillator frequency versus supply current. It shows that all samples follow the same relationship, so that supply current is the only governing factor to control inverter propogation delay.
    Calculating the propagation delay from the ring oscillator frequency shows that a minimum tpd of around 3.5 ns is achived at around 30 mA of supply. Since RTL gates only draw current when their input is "high", only half of the six gates (5 ringo + 1 buffer) are active at any time. This means that the bias current per gate is approximately 30mA/(6/2) = ~ 10mA.

    It appears that this is smack dab on the operating point settings that are used in the CDC6600: It sits at Ic=10mA, Ib=1mA with a maximum tpd target of 5ns. The CDC6600 uses a supply of 6V and apparantly Rb=150 Ohm, Rc=680 Ohm for fan-out of 1. This is in between the resistor settings in Sample 2 and Sample 3.

    So, in conclusion: It seems we can recreate the timing properties of the ancient CDC6600 using components that are still available today. However, this also requires bias current settings as high as those used back in the 60ies...

  • LTSpice Simulations

    Tim04/25/2020 at 16:32 0 comments

    In parallel to building the ring oscillator models I also implemented the same in LTspice. You can find the files files on the project page. Please find a very brief summary below.

    I implemented a 5-stage ring oscillator with an output buffer. Same circuit as on the PCB.. Using the transient function, it is possible to investigate frequency and waveform.

    To test the transfer function, I used the DC sweep function in the circuit below. Since the loading of the output has a significant influence on the output voltages of an RTL gate I looked at both loaded and unloaded output.

    Sweep output shown below.

    An inverter circuit is shown below. As you note, I also did some experiments with a reach through capacitor.

    Resistor variation on the PMBT2369 based inverter

    I used the ring oscillator model to investigate the impact of base and collector resistor on ring-oscillator frequency and hence propagation delay. the graph below shows the collector resistance on the x-axis and various plots as a function of base resistance.

    Impact on logic levels.

  • Standard switching transistors: 2N3904 and copies

    Tim04/01/2020 at 13:22 0 comments

    The 2N3904 is one of the most widely spread switching transistors. There are countless SMT variants available from at least 15+ different vendors in all kinds of packages. Mouser lists more than 50 variants, LCSC more than 80.
    I picked four devices in SOT23 package at random, trying to balance between more established manufacturers and lesser known chinese sources. Ironically the Nexperia part was the cheapest.
    The table above lists the four devices and their respective datasheet values. Siko admits to having a slightly slower device while the other datasheets look like they were copied from each other.

    Ring oscillator results are shown above. Minimum tpd is 20ns for the Nexperia device. The others achieve around 40ns, while Toshiba is dead last with >200ns at typical operating conditions.

    The extreme spread of these results is quite irritating - it shows how devices can easily differ by factors despite being specified the same. The only consolation is that Nexperias device is again the fastest, confirming that it may be a good idea to stick to better known vendors.

    On the other hand, the performance of the Toshiba candidate is utterly irritating, as it will hardly be able to meet datasheet specs. I double checked the package marking. The only sane explanation could be that this is a counterfeit device.

    In general, the PMBT3904 also seems to be a good candidate for bipolar logic. It is about 2-3x slower than the PMBT2369, but also 4x cheaper and available in smaller packages. More attention needs to be spent on correct biasing though, since the device is far less forgiving than the PMBT2369 when driven into saturation.

  • Digital Transistors

    Tim04/01/2020 at 13:09 0 comments

    Digital transistors look extremely appealing to build discrete logic: They come with integrated resistors so the bias emitter in an RTL gate can be omitted. Furthermore, they are available from many manufacturers in basically any package type, down to chip scale packages, at very lost cost.

    There is a nice introduction to digital transistors by Rohm here.

    The internal circuit of such a device is shown above. A wide variety of value for R1 and R2 are available. Unfortunately, the minum value for R1 is 1 kOhm, which will already limits switching currents significantly. 

    Furthermore, although many different variants are listed, not all of those are available. I picked four devices at random from different suppliers.

    The candidates are listed above. What is alarming about these devices is that basically no information about transient behavior and switching applications is listed. A few devices liste transistion frequencies, which are rather unimpressive.

    Ring oscillator results are shown above. Basically all devices enter an operational range where they are limited by saturation charge already at very low voltage. Practical gate propagation delays are in the 500-1500 ns range, a hundred times slower than the PMBT2369!  There is a very small voltage range where the saturation charge does not dominate and the devices are slightly faster, but it seem impractical to bias devices into that regime.

    These results are honestley a bit surprising to me, since I always though the main application of digital transistors would be switching? Well, maybe cost is more important than speed here.

    Verdict: Digital transistors are not suitable for discrete logic, unless you are fine with clockspeeds in the kHz range.

  • Fast switching transistors: PMBT2369 vs. MMBT2369

    Tim04/01/2020 at 06:15 0 comments

    As outlined earlier there are still two fast switching transistor types on the market: The PMBT/MMBT2369 and the BVS52. Both are offered by Nexperia and On Semi.

    The PMBT2369 is Nexperias offer, my guess is that the "P" stands for Philips. Nexperia was carved out of NXP, which used to be Philips semiconductor branch. Similarily the "M" in MMBT2369 is most likely related to Motorola, which On Semi belonged to a long time ago. The BVS52 seems to be basically the same device at both vendors, so I did not bother investigating it.

    I compared On Semis and Nexperias flavors of the 2369. As you can see in the table above, their datasheet values are basically the same. I measured hfe and found the onsemi device to have slightly higher gain.

    The plot above shows a comparison of both devices. It appears that the PMBT2369 is quite a bit faster. The MMBT2369 shows a maximum at around 5V supply. At higher voltage (current), the saturation charge seems to dominate switching behavior. This trend can only be seen at >8V for the PMBT2369.

    Minimum tpd of the MMBT2369 based RTL inverter is 6.8 ns, that of the PMBT2369 based inverter is 4.8 ns.

    It's quite difficult to asses the actual origin of the speed difference, just speculating: The higher current gain will increase the miller capacitance of the MMBT2369. However this is mostly relevant while the device is turned on. The critical condition is the storage time of the saturation charge, which flows out of the base when VBE is already zero and the miller capactance plays no role.

    The PMBT2369 definitly appears to be the faster of the two. In fact, the PMBT2369 seems to be the fastest bipolar switching transistor that is still available in high volume today.

    Why even look at other devices? Unfortunately it seems that the PMBT2369 is only available in the relatively large SOT23 package. To build dense discrete logic (yes, it makes no sense, I know), it would be of interest to use much smaller packages such as SOT523 or SOT723.

  • How to assess switching speed

    Tim04/01/2020 at 05:51 0 comments

    Why is it so difficult to identify the fastest transistor for our application from merily looking at the datasheets?

    One issue is that many transistor types are not characterized for saturation switching operation anymore and hence little information about that use case is available in the datasheet. As it turns out, many newer transistor types are not optimized for this operational mode either...

    A second issue is that most datasheets only list maximum and minimum values. These usually represent garuanteed test limits, but they are not necessarily representative for the typical performance of the device.

    In an ideal world, all of this could be solved by simulation. Unfortunately, I had very mixed results using spice models provided by different vendors. In some cases this may be, because the spice model is not finetuned to switching operations.

    In the end, the only way is to actually test various candidate devices. The tool of the trade is the ring oscillator.

    The images above shows a single RTL inverter including design settings. I used a 2.4 kOhm collector resistor and 360 Ohm base resistor for all investigations. To form an oscillator, an odd number of inverters have to be connected in a circular fasion.

    The picture above shows the design I used for most of my investigation. The first five inverters form the ring oscillator, the sixth inverter is used as an output buffer to make sure there is a well defined load on the oscillator.

    Btw - the wire was used to fix a flaw in the first revision of the PCB.

    The image above shows a smaple measurement from the article i linked above. By changing supply voltage it is possible to investigate the behavior it difference current settings. The propagation deleay of individual gates can be easily deduced from the oscillator frequency by tpd=1/(2*5*fosc).

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Adrian Freed wrote 08/19/2022 at 06:09 point

Interesting experiments! The speed peak around 3-volts for comparable resistor choices is not far from RTL itself which is a nice confirmation. Your LED experiment reminded me of high voltage DTL which uses a zener (MHTL)

  Are you sure? yes | no

Tim wrote 08/19/2022 at 16:34 point

Interesting. I wonder if the zener diode DTL also had an instable oscillation mode. But I guess that was suppressed by their base bleed transistor.

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wmeyer48 wrote 07/28/2021 at 19:30 point

With regard to speed of the transistors, it is not always desirable. Consider the 2N3646, which is a fast high-beta device. Years ago, when we were starting to deal with emissions issues, testing showed that a 2N3646 (the only one in a fairly large design) was the sole source of excessive radiation. There was no need for such a specialized device, so the remedy was replacement with a 2N3904. I see that latter day devices like 2SC3646 reduce the 350MHz rating of the 2N part to 120MHz. 

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Keith wrote 04/27/2020 at 06:35 point

Have you tried adding a schottky diode connected between base and collector of the transistors? I used to design 74F series logic and they were essential to avoid devices saturating.

  Are you sure? yes | no

Tim wrote 05/16/2020 at 18:01 point

I looked at it in simulation now. Will try once I get suitable devices.

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Yann Guidon / YGDES wrote 05/31/2020 at 14:06 point

Hey Keith !

Would you like to join us in the #Hackaday TTLers  group and tell us your F stories ?

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Yann Guidon / YGDES wrote 04/01/2020 at 11:19 point

What is the effect of removing the base resistors ?

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Tim wrote 04/01/2020 at 12:16 point

You will move to a different place on the power/speed tradeoff curve. Will add some data later.

And you may encounter funny issues with device matching...

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