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• Even faster !

  Yann Guidon / YGDES • 01/17/2019 at 21:33 • 16 comments

  I found even faster transistors than the BFS480...
  The BFP420 is 3x faster, at 25GHz gain-bandwidth product ! This means a gain of 25 at 1GHz, for example.

  I received a few hundreds for tests. They are of course more expensive and are packaged alone in a small package (4 pins ??) so the density is lower but it could be helpful for the critical datapath, like the ALU.

  I doubt I could measure the frequency of a ring oscillator made with it so I'll have to implement at least 9 stages ! :-P
  

  One important limitation is the operating voltage : 4.5V so this limits the possible topologies.

  OTOH it could also be used to build a very good picosecond pulse generator at low voltage :-P
  

• Protoboards

  Yann Guidon / YGDES • 08/01/2018 at 03:05 • 0 comments

  By using the leftover surface of the latest PCB batch, I made enough small protoboards to solder almost 500 SOT363 parts, or the equivalent of 1000 transistors (BFS480).

  Using the #Precision current generator, I'll soon be able to prototype my first high-speed gates... When I find the time ! the relay version of #YGREC8  has priority so far...
  

• Bibliography

  Yann Guidon / YGDES • 07/12/2018 at 12:28 • 1 comment

  A very generous donator has sent me these books :-)

  I can't believe how much I'm discovering in them !
  

• moving forward

  Yann Guidon / YGDES • 07/05/2018 at 17:41 • 9 comments

  Edit: Forked to #Precision current generator
  

  ---

  I am soon getting SOT363-DIL adapters so I might soon evaluate the BFS480.

  Meanwhile, I must also evaluate the various peripheral circuits, such as current sources and current mirrors. And if I have a digitally controlled PSU, it's not  working in a suitable range : I want a better precision, from about some µA to maybe 20mA. A good PSU has a resolution of several mA...

  ECL circuits use a bias current of about 4mA but I want to to vary it. And a using a fixed resistor on a 9V battery will not provide enough stability if the output voltage varies...

  There are many example circuits for current generators, or current limiters, on the 'net. Here is a little sample of what I've found (and links to the pages) :

  
  

  
  

  My design priorities are simplicity, use of basic parts (I don't want to use an IC or Opamp) and "some stability". It will be a high-side circuit (to fit with most NPN current mirrors) so it must use a PNP transistor (as in the last picture).
  

  At first the bias-diode-based circuit (as shown just above, or in the 2nd box of the first image) looks interesting: 2 standard diodes set the base voltage, and more if a range switch is added. Very fine tuning can be added with a series adjustable resistor to change the current. However the bias might not be very stable, with a mismatch of thermal coefficients, and because there would be 2 or more PN junctions on one side, but only one on the transistor side.

  A 2-transistors circuit is chosen, as shown on the pictures. If both transistors are from the same lot (and stuck together), the thermal behaviour should be almost identical and the circuit is a bit more stable (I hope, because there is only one junction). I won't use high currents so heat won't be a significant issue. I can still add a finetuning adjustable resistor, in addition to the main current setting resistor. Its value is approximatively set as follow :

  I=U/R=Vbe/Rs= 0.7/R

  I have found a nice potentiometer, 220K log, so the minimum current is about 0.7/220000=3µA.
  

  Max current is R=U/I=0.7/0.02=35 Ohms so a fixed 33 ohms resistor is added in series.

  Another 2K potentiometer in series (R2) adds more tuning flexibility though (unlike with the diodes voltage reference) I doubt it's actually useful (I'll have to test).

  I also want to add a current probe (ammeter) in series...

  A single alkaline 9V battery should be enough, but an external PSU can also be used. The voltage difference for the circuit can reach about 8V (9V minus the Vbe and some Vce_sat) but can be increased with a higher supply voltage. The ammeter's drop should also be considered... It varies from device to device so I can't be sure yet.

  But for 5V-powered ECL, it should be good enough :-)

  ---

  OK, the first prototype doesn't work as well as expected but I built it in a hurry... at least I have a fancy box with nice knobs :-)

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  2nd prototype accepts to work. The 220K log pot is great for the range under 1mA, down to 4,7µA.
  The range above 2mA is tricky though, the low-value potentiometer might not be adapted. Maybe a log type would work better. And I still don't know why it was so difficult to make it run correctly, why I measured odd voltages, but I was initially in a hurry so I might have overlooked something.

  I want to generate not-too-unreliable currents, up to maybe 10mA. The ammeter measures 17mA when it short-circuits  the output. The potentiometer can't set the current reliably above 3mA...

  ---

  20180711:

  I replaced the pot with a 4K7 log model and it works better, though I should really replace it with a 2K2 log model. I can get a good usable range but the higher current values are still tricky,...

  Read more »

• A sidequest, already

  Yann Guidon / YGDES • 04/10/2017 at 03:13 • 0 comments

  The project has received all the 5400 dual-transistors (with the hope that 10800 transistors will be enough, but you never know, right ?) and already a question arises : how to test them ?

  I have the fast DDS but the BFS480 is one notch higher. So the idea is to see if it can be used for an avalanche pulse generator to generate sub-nanosecond signals ?

  I have seen the venerable 2N2369 used for this purpose but it's a 50-years old part. The BFS480 is not only faster, but also has lower breakdown voltage, which is a nice thing, no ? A dumb oscillator and simple charge pump should be enough to generate the low-current higher voltage that such a circuit needs.

  ---

  Some background and references

  • @Jenny List has implemented a #Avalanche Pulse Generator with a 2N3904, which is meant for higher powered applications. "The 2N3904 requires a high voltage to operate in avalanche mode, well over 100V."
  • alexwhittemore tried to make one too: #Avalanche
    
  • Kerry Wong's avalanche pulse generator
  • Jim William's AN72, AN47 and AN94, see also in EDN Simple nanosecond-width pulse generator provides high performance
  • Andrew Holme's own circuit (even more scopeporn)
  • An avalancher used to drive nanosecond lasers
  • EEVBLOG#306 (2012) citing http://people.osmocom.org/tnt/hw/pulse_gen/
  • A fancy version at http://www.siliconvalleygarage.com/projects/picosecond-pulser.html
  • (your link here)

  but I have never seen a low power UHF transistor.

  AN72 however covers the "balancing" technique, using a pulse transformer to create a symmetrical pulse. This will be needed to test the transmission lines of the clock and control pulses...
  

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