Editing Diode–transistor logic (section)

== Speed improvement ==
[[File:Transistor Clock.jpg|thumb|A digital clock made only with discrete transistors, diodes and resistors, no integrated circuits. This clock uses 550 switching diodes and 196 transistors to divide 60&nbsp;Hz power-line frequency down to one pulse per second and provide a display of hours, minutes and seconds.]]

The DTL [[propagation delay]] is relatively large. When the transistor goes into saturation from all inputs being high, charge is stored in the base region. When it comes out of saturation (one input goes low) this charge has to be removed and will dominate the propagation time.

One way to speed up DTL is to add a small "speed-up" capacitor across R3.  The capacitor helps to turn off the transistor by removing the stored base charge; the capacitor also helps to turn on the transistor by increasing the initial base drive.<ref>{{citation |editor-last=Roehr |editor-first=William D. |title=High-Speed Switching Transistor Handbook |year=1963 |publisher=Motorola, Inc. }}. Page 32 states: "As the input signal changes, the charge on the capacitor is forced into the base of the transistor.  This charge can effectively cancel the transistor stored charge, resulting in a reduction of storage time.  This method is very effective if the output impedance of the preceding stage is low so that the peak reverse current into the transistor is high."</ref>

Another way to speed up DTL is to avoid saturating the switching transistor.  That can be done with a [[Baker clamp]].  The Baker clamp is named for Richard H.&nbsp;Baker, who described it in his 1956 technical report "Maximum Efficiency Switching Circuits".<ref>{{citation |first=R. H. |last=Baker |title=Maximum Efficiency Switching Circuits |year=1956 |url=http://www.dtic.mil/srch/doc?collection=t3&id=AD0096497 |archive-url=https://web.archive.org/web/20150925061100/http://www.dtic.mil/srch/doc?collection=t3&id=AD0096497 |url-status=dead |archive-date=September 25, 2015 |work=MIT Lincoln Laboratory Report TR-110 }}</ref>

In 1964, [[James R. Biard]] filed a patent for the [[Schottky transistor]].<ref>{{cite patent |inventor-last=Biard |inventor-first=James R. |inventorlink=James R. Biard |title=Unitary Semiconductor High Speed Switching Device Utilizing a Barrier Diode |country-code=US |patent-number=3463975 |publication-date= December 31, 1964 |issue-date= August 26, 1969 }}</ref> In his patent the Schottky diode prevented the transistor from saturating by minimizing the forward bias on the collector–base transistor junction, thus reducing the minority carrier injection to a negligible amount. The diode could also be integrated on the same die, had a compact layout, no minority-carrier charge storage, and was faster than a conventional junction diode. His patent also showed how the Schottky transistor could be used in DTL circuits and improve the switching speed of other saturated logic designs, such as Schottky-TTL, at a low cost.