With out reducing RA = 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 RA & diode D to approximately 2/3 Vcc & discharges through RB& Q1 until the capacitor voltage equals approximately 1/3 Vcc, 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 Vcc, upper comparator turns transistor Q1 ON & C rapidly discharges through transistor Q1.
· When voltage across C equals to 1/3 Vcc, 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 = VccC/3IC ………………………… (1)
IC = (Vcc - VBE)/R = constant current
Therefore the free – running frequency of ramp generator is
f0 = 3IC/ Vcc C ……………………….(2)