Astable Multivibrator with two Transistors

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Relaxation oscillator with two bipolar transistors is given in figure bellow.


Another name of this circuit is astable mutivibrator. It is based upon two discrete transistors interconnected with so called regenerative feedback that is type of positive feedback. Output of the transistor Q1 is connected with input of the transistor Q2 and vice versa. Connection between collector of one transistor with corresponding base of another transistor is done with R-C elements (RB1-C2 and RB2-C1). This circuit is not stable and it start to oscillate. It is desirable situation that if one transistor is cut-off, opposite one should be saturated and vice versa. Condition of the saturation is given by:


Typical waveforms of the base and the collector voltage of given transistor in astable multivibrator is given in figure bellow.


Blue trace is collector voltage, and red trace is base voltage. While transistor is cut-off, output (collector) voltage is at VCC of 5V. During that time, other transistor is saturated and it collector voltage is low as 0.2V. Capacitor in the positive feedback is charging with time constant of Ï„=RB·C. Charging last until threshold voltage is reached. Transistor is now turned on and saturated. This transition is very fast, and output voltage drops from 5V to 0.2V in a very short time. This is one of the terminals of other capacitor, and it’s voltage fast drops for amount of 4.8V. Because capacitor voltage between it’s plates can’t be changed in time instant, the other capacitor terminal drops for the same amount and cut-off other transistor. Now, other transistor is cut-off, and it’s base is charging again with the same time constant. First transistor remains saturated until the base of the other reaches threshold voltage a then whole process is repeating. If two base resistors, RB1 and RB2 are the same, and if capacitor values are the same then duty cycle is 50%. This is most common situation for relaxation oscillator with discrete transistors. Frequency of relaxation oscillator with discrete transistors can be calculated via charging time of two bases. For each of the bases, period of charging is:




frequency of relaxation oscillator is


When transistor is cut-off, output voltage is increasing by charging capacitor of R-C integrator consisting of capacitor C and resistor RC. This is the reason why is the rising edge of the signal for relaxation oscillator is much slower the the falling edge. The smaller the RC is, the sharper is the rising edge and vice versa. As it was mentioned above, since conducting transistor has to operate in saturation while conducting, RB has to be at least five time or more larger then the RC resistor.

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External links:

Relaxation Oscillator on Wikipedia

Relaxation Oscillator on Hyperphysics

Relaxation Oscillator Learnabout-Electronics


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