98 IN THE BEGINNING
across the transistor will be less than 200 mV during the "on-condition." R2 biases the transistor base positive during the off condition to provide greater tolerance to noise, I00, and signal variations. Capacitance C was selected to remove all of the minority carriers from the base when the transistor is being turned off. The effect of C on a test circuit driven by a fast step is shown in Figure 7. Note that the delay due to hole storage is only a few millimicroseconds.
We run the circuits under saturated conditions to achieve stability and a wide tolerance to parameters without the need for clamp diodes. Unlike vacuum tubes, which always need an appreciable voltage across them for operation, a transistor requires practically no voltage across it. In spite of the delay in turning off saturated transistors, these circuits are faster than most vacuum tube circuits. Faster circuit speed is not due to the fact that the transistors are faster than vacuum tubes, but because they operate at much lower voltage levels. A vacuum tube takes several volts to turn it from fully "on" to fully "off'; a transistor takes less than 1 V.