# Determine resistor values when designing astable multivibrator with BJT

I want to design an astable multivibrator with a frequency of 1kHz and duty cycle of 40%. My circuit is like this: Now,all the references I have tell me how to chose the values of R3,R4,C1 and C2 according to the formula of duty cycle and frequency. But how do I chose R1,R2,R5,R6? I have a variable power supply of 0V-25V.I have two models of BJT available, 2N3904 and BC108.

• I want to use the above mentioned circuit,so that I can obtain a better square wave. If I leave out the diodes,the waveform is not a perfect square wave. – Nafis Sep 5 '15 at 13:29

In this astable multivibrator frequency is given by the following formula:

$$f = \frac 1 {ln(2) \times (R_2C_1 + R_3C_2)} \ \approx \frac {1.443} {(R_2C_1 + R_3C_2)}$$

If C1 = C2 = C and R2 = R3 = R:

$$f \approx \frac {0.72} {RC}$$

It just takes a ratio between R2 and R3 to change the cyclic ratio, keeping $$\frac {R_2 + R_3} 2 = R$$

R1 and R4 must be low resistances. Their value won't affect frequency if they're noticeably smaller than the base resistors. Just beware of the maximum collector current.

Sources from Wikipedia.

• How does taking the diodes out affect the waveforms at the collectors? – Spehro Pefhany Sep 5 '15 at 11:40
• Just making asymmetric is not my only concern. I need a perfect square wave. Without the diodes it is not possible. – Nafis Sep 5 '15 at 13:31
• Why not add a buffering stage transistor then? Do you also need two diodes that you need the outputs in phase opposition? – user59864 Sep 5 '15 at 13:45
• I do not know about buffering stage transistor from the books I have. But,yes,I need them in opposite phases. I need two complementary waveforms. – Nafis Sep 5 '15 at 13:51
• Indeed using diodes looks like a smart tip to sharpen waveforms. From what I understand the computation I exposed should not differ and give you a sufficiently close approximation of the frequency and ratio. Just that collector resistors (R1/R2 and R5/R6 in your case) should be kept smaller than base resistors and you're good to go. – user59864 Sep 5 '15 at 13:57