domingo, 27 de junio de 2010

Schottky Transistors


When the input of a saturated transistor is changed, the output does not change immediately; it takes extra time, called storage time, to come out of saturation. In fact, storage time accounts for a significant portion of the propagation delay in the original TTL logic family.

Storage time can be eliminated and propagation delay can be reduced by ensuring that transistors do not saturate in normal operation. Contemporary TTL logic families do this by placing a Schottky diode between the base and collector of each transistor that might saturate, as shown in Figure BJT-7. The resulting transistors, which do not saturate, are called Schottky-clamped transistors or Schottky transistors for short.

When forward biased, a Schottky diode's voltage drop is much less than a standard diode's, 0.25 V vs. 0.6 V. In a standard saturated transistor, the base-tocollector voltage is 0.4 V, as shown in Figure BJT-8(a). In a Schottky transistor, the Schottky diode shunts current from the base into the collector before the transistor goes into saturation, as shown in (b). Figure BJT-9 is the circuit diagram of a simple inverter using a Schottky transistor.
C.I: 17.557.095 

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