Bootstrap Time Base Generator

A bootstrap sweep generator is a time base generator circuit whose output is fed back to the input through the feedback. This will increase or decrease the input impedance of the circuit. This process of bootstrapping is used to achieve constant charging current.

Construction of Bootstrap Time Base Generator

The boot strap time base generator circuit consists of two transistors, Q₁ which acts as a switch and Q₂ which acts as an emitter follower. The transistor Q₁ is connected using an input capacitor C_B at its base and a resistor R_B through V_CC. The collector of the transistor Q₁ is connected to the base of the transistor Q₂. The collector of Q₂ is connected to V_CC while its emitter is provided with a resistor R_E across which the output is taken.

A diode D is taken whose anode is connected to V_CC while cathode is connected to the capacitor C₂ which is connected to the output. The cathode of diode D is also connected to a resistor R which is in turn connected to a capacitor C₁. This C₁ and R are connected through the base of Q₂ and collector of Q₁. The voltage that appears across the capacitor C₁ provides the output voltage V_o.

The following figure explains the construction of the boot strap time base generator.

Operation of Bootstrap Time Base Generator

Before the application of gating waveform at t = 0, as the transistor gets enough base drive from V_CC through R_B, Q₁ is ON and Q₂ is OFF. The capacitor C₂ charges to V_CC through the diode D. Then a negative trigger pulse from the gating waveform of a Monostable Multivibrator is applied at the base of Q₁ which turns Q₁ OFF. The capacitor C₂ now discharges and the capacitor C₁ charges through the resistor R. As the capacitor C₂ has large value of capacitance, its voltage levels (charge and discharge) vary at a slower rate. Hence it discharges slowly and maintains a nearly constant value during the ramp generation at the output of Q₂.

During the ramp time, the diode D is reverse biased. The capacitor C₂ provides a small current I_C1 for the capacitor C₁ to charge. As the capacitance value is high, though it provides current, it doesn’t make much difference in its charge. When Q₁ gets ON at the end of ramp time, C₁ discharges rapidly to its initial value. This voltage appears across V_O. Consequently, the diode D gets forward biased again and the capacitor C₂ gets a pulse of current to recover its small charge lost during the charging of C₁. Now, the circuit is ready to produce another ramp output.

The capacitor C2 which helps in providing some feedback current to the capacitor C1 acts as a boot strapping capacitor that provides constant current.

Output Waveforms

The output waveforms are obtained as shown in the following figure.

The pulse given at the input and the voltage V_C1 which denotes the charging and discharging of the capacitor C₁ which contributes the output are shown in the figure above.

Advantage

The main advantage of this boot strap ramp generator is that the output voltage ramp is very linear and the ramp amplitude reaches the supply voltage level.