1
\$\begingroup\$

I want to design an astable multivibrator with a frequency of 1kHz and duty cycle of 40%. My circuit is like this: enter image description here

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.

\$\endgroup\$
  • \$\begingroup\$ 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. \$\endgroup\$ – Nafis Sep 5 '15 at 13:29
1
\$\begingroup\$

In this astable multivibrator

Astable multivibrator using BJT

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.

\$\endgroup\$
  • \$\begingroup\$ How does taking the diodes out affect the waveforms at the collectors? \$\endgroup\$ – Spehro Pefhany Sep 5 '15 at 11:40
  • \$\begingroup\$ Just making asymmetric is not my only concern. I need a perfect square wave. Without the diodes it is not possible. \$\endgroup\$ – Nafis Sep 5 '15 at 13:31
  • \$\begingroup\$ Why not add a buffering stage transistor then? Do you also need two diodes that you need the outputs in phase opposition? \$\endgroup\$ – user59864 Sep 5 '15 at 13:45
  • \$\begingroup\$ 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. \$\endgroup\$ – Nafis Sep 5 '15 at 13:51
  • \$\begingroup\$ 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. \$\endgroup\$ – user59864 Sep 5 '15 at 13:57

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.