# Determine resistor values when designing astable multivibrator with BJT

I want to design an astable multivibrator with a frequency of 1 kHz and a duty cycle of 40%.

This is my circuit:

Now, all the references I have tell me how to choose the values of R3, R4, C1 and C2 according to the formula of duty cycle and frequency. But how do I choose R1, R2, R5 and R6?

I have a variable power supply of 0..25 V. I also 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. Sep 5, 2015 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? Sep 5, 2015 at 11:40
• Just making asymmetric is not my only concern. I need a perfect square wave. Without the diodes it is not possible. Sep 5, 2015 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, 2015 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. Sep 5, 2015 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, 2015 at 13:57

5 years later, and I find this question on Google. So, I'll add my comments for the next guy looking for info.

There's lots of documentation describing the selection of R2 and R3, and the capacitors, but little documentation on the effect of R1 and R4.

I'm playing with my scope and a multivibrator circuit (without steering diodes). I've noticed that my waveform is much more sawtooth shaped than square when I'm using 1k/10nF RC networks and 270R collector resistors (at about 70kHz). However, increasing the RC network to 51k/100pF, and keeping my 270R collector resistors really sharpens up the waveform (at about 150kHz).

Here's the 51k:270R

And with 1k/270R

About the best analysis I can come up with is this (and I might be wrong): These transistors are operating in common-emitter mode. Therefore, gain is proportional to the ratio of the resistors R1:R2 and R4:R3. My transistors (BC337-40) should be capable of a gain of 250, but using 1k:270R sets the max gain permitted to about 4. A higher gain would drive the transistors to the rail (saturation) much faster than a lower gain. So using a 51k/270R configuration sets the gain to ~200 and reduces the rise time, sharpening up the wave.

Think about it this way - as the capacitor comes up to voltage, that increase is multiplied by the gain of the circuit, until it hits your V+ rail (saturation). If the gain is low then the increase is also low, and there's a broader zone where the (capacitor voltage * gain) is still less than V+.

So if your goal is sharp square waves, you should use high beta transistors and the highest ratio of R1/4:R2/3 that you can manage. Steering diodes may or may not be necessary depending on just HOW square you need your wave to be, the frequency, and the particulars of your transistors.