Hannon S. Yourke


Oral History – Hannon Yourke


Beginning Work on Current Steering Transistor Circuits


The initial motivation for this work was to come up with non-saturating techniques for doing logic.  What was in place at the time was all based on saturated circuits, which take an awful long time to come out of saturation.  So, I worked on this idea and came up with what eventually became known as emitter coupled logic (ECL). The most important parameters for these circuits was switching speed and power dissipation.  Of course, the devices were quire different than now.  They had very high collector resistance and, to keep them out of saturation and in a region of high bandwidth, you had to work with higher voltages.  I think that bipolar transistors (as we had then) are not used at all now. One of the types we used was the Philco surface barrier transistor, which was about the best device you could get your hands on at that time.   These were very uniform – I guess they were etched to achieve very specific characteristics.  The one problem with these was that the devices would fail if the well specified collector breakdown voltage was even slightly and transiently exceeded – otherwise, these were very reliable.


Using Different Transistor Types


There was also a semiconductor group at IBM, which came up with the drift transistor.  Internally, this transistor was used for our work and also this was the type which was delivered in our machines - there was a commitment (to use these transistors) that was made for the Stretch program, and  my




Oral History – Hannon Yourke


invention (the current steering circuit) was used in the Stretch computer.  This program actually came out much later than the first machines to use those circuits.  The first machine to use the circuits was the IBM 7090.  In parallel with the Stretch program they started development of a lower performance machines, also using current steering circuits.  Stretch, I believe, came out in 1961, and the 7090 in 1959. 


Current Steering Circuit

 has Broad Applicability


It was a very general technique.  The patent, as I wrote it, really covered all types of possible alternatives.  Even though the Stretch machine was built with a combination of PNP and NPN transistors (so you didn’t have to voltage translate), my patent included a voltage translation.  The paper I wrote (“Millimicrosecond Transistor Current Switching Circuits”) in 1957 for the IRE shows an assortment of circuits, including triggers, exclusive-or circuits, line drivers, and others.  In fact, every circuit in the Stretch circuit manual was designed using this technique. So, you can see that the patent was very basic and also very important.  


As far as mainframe high speed and high performance logic, this circuit remained the basis until the time I left the company - I retired in 1985. 



Go To Yourke Oral History, Page 3


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