Circuit Diagram
Waveform:
Here transistor Q1 acts as a switch and transistor Q2 acts as an emitter follower (i.e. a unit gain amplifier).
Circuit Operation: Initially transistor Q1 is ON and Q2 is OFF. Therefore capacitor C1 is charged to VCC through the diode forward resistance (RF). At this instance output voltage is zero. When negative pulse is applied to the base of transistor Q1, it turns OFF. Since transistor Q2 is an emitter follower, therefore the output voltage V0 is same as base voltage of transistor Q2. When Q1 turns OFF, the capacitor C1 starts charging capacitor C through resistor (R). As a result of these both the base voltage of Q2 and output voltage begins to increase from zero. As the output voltage increases diode D becomes reverse biased, because of the fact that the output voltage is coupled through the capacitor (C1) to the diode. Since the value of capacitor (C1) is much larger than that of capacitor (C), the voltage across capacitor (C1) practically remains constant. Thus voltage drop across resistor (R) and hence current (IR) remains constant, means capacitor C is charged with constant current.
This causes voltage across capacitor C (and hence the output voltage) to increase linearly with time. The circuit pulls itself up by its own bootstrap and hence it is known as bootstrap sweep circuit.