3-44
Figure 3-47C.—Transistor sawtooth generator (pnp).
Since Q1 is a pnp transistor, a positive voltage must be used to drive it to cutoff. Figure 3-47, view
(B), shows a rectangular wave input 500 microseconds long on the positive alternation. At T0, the
positive gate applied to the base of Q1 cuts off Q1. This effectively removes the transistor from the circuit
(opens the switch), and C1 charges through R3 toward 20 volts. Starting with a charge of
-
2.5 volts at T0,
C1 charges (T0 to T1) for 500 microseconds to -4.25 volts at T1. Let's determine the percent of charge:
This allows nearly a linear rise of voltage across C1.
Increasing the value of R3 or C1 increases the time constant. The capacitor will not charge to as high
a voltage in the same period of time. Decreasing the width of the gate and maintaining the same time
constant also prevents the capacitor from charging as much. With less charge on the capacitor, and the
same voltage applied, linearity has been improved. Decreasing R3 or C1 or increasing gate width
decreases linearity. Changing the applied voltage will change the charge on the capacitor. The percentage
of charge remains constant; however, it does not affect linearity.
At T1, the positive alternation of the input gate ends, and Q1 returns to a forward-bias condition. A
transistor that is near saturation has very low resistance, so C1 discharges rapidly between T1 and T2, as
shown in figure 3-47, view (C). The capacitor discharges in less than 200 microseconds, the length of the
negative alternation of the gate. The negative gate is made longer than the discharge time of the capacitor
to ensure that the circuit has returned to its original condition.
From T1 to T2, the capacitor discharges and the circuit returns to its original condition, ready for
another positive gate to arrive. The next positive gate arrives at T2 and the actions repeats.
