Astable Multivibrator Applications and Ramp generator

Astable Multivibrator

Applications:

With out reducing R_A = 0 ohm, the astable multivibrator can be used to produce square wave output. Simply by connecting diode D across Resistor RB. The capacitor C charges through R_A & diode D to approximately 2/3 V_cc & discharges through RB& Q1 until the capacitor voltage equals approximately 1/3 V_cc, then the cycle repeats.

To obtain a square wave output, RA must be a combination of a fixed resistor & potentiometer so that the potentiometer can be adjusted for the exact square wave.

Free – running Ramp generator:

·           The astablemultivibrator can be used as a free – running ramp generator when resistor RA& RB are replaced by a current mirror.

·           The current mirror starts charging capacitor C toward Vcc at a constant rate.

·           When voltage across C equals to 2/3 V_cc, upper comparator turns transistor Q1 ON & C rapidly discharges through transistor Q1.

·           When voltage across C equals to 1/3 V_cc, lower comparator switches transistor OFF & then capacitor C starts charging up again..

·           Thus the charge – discharge cycle keeps repeating.

·           The discharging time of the capacitor is relatively negligible compared to its charging time.

·           The time period of the ramp waveform is equal to the charging time & is approximately is given by,

T = V_ccC/3I_C ………………………… (1)

I_C = (V_cc - V_BE)/R = constant current

Therefore the free – running frequency of ramp generator is

f₀ = 3I_C/ V_cc C ……………………….(2)