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# Multivibrator of varying frequency

#### BobK

Jan 5, 2010
7,682
That is explained perfectly well in post #8.

Bob

#### Cannonball

May 6, 2017
193
R5 is the bios resistor for q1 along with r6, c1, and the pulses from v2, they set the voltage on the base of q1 which raises the emitter of q1 to set the frequency of the multivibrator.

#### Akshatha Venkatesh

Jan 14, 2017
145
R5 and R6 are two resistors in series between two voltage sources. If the two voltages are not equal, the voltage at the center node formed by the two resistors can be calculated using Ohm's Law.

The corner frequency is calculated using the equation for the corner frequency of a single-pole R-C circuit. For the R, use the Thevenin equivalent or R5 and R6, which is the calculated value of R5 and R6 in parallel.

ak
I am not able to understand why they have used R5 , because when I simulate the circuit taking out R5 , it still works the same , with little disturbances. What is the real need for using R5. Please help

#### Cannonball

May 6, 2017
193
R5, along with R6, c1, and pulses from v2 make up the bias voltage on the base of Q1. R5 and R6 act as a voltage divider.

R5 puts about 5 volts on the base of Q1 minus the voltage drop across it. R6 couples pulses from V2 to C1 which also biases Q1. The two sources add to produce final bias. If you remove R5, it should change the frequency but it will not turn Q1 off. You would need to remove R5 and R6 to turn off Q1 and stop the multivibrator from oscillating.

I hope this helps.

#### Akshatha Venkatesh

Jan 14, 2017
145
R5 is the bios resistor for q1 along with r6, c1, and the pulses from v2, they set the voltage on the base of q1 which raises the emitter of q1 to set the frequency of the multivibrator.
Hi, in the above circuit , to calculate Rc , what should I use as IC(Max) ? Since there's no load , what should Ic(max) be ?

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