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Background

I have started learning transistors few weeks ago and I hit a roadblock (again). I have been following this Transistor Tutorial on Sparkfun.

Now I have moved to building an Astable Multivibrator using transistors, but I couldn't figure out the logic of how the transistors switch themselves ON and OFF over and over again. My difficulty is that, as far as I can see, the base of both transistors are biased with 0.6v as soon as the power is turned ON via R2 and R3 resistors.

This will bring both transistors into saturation mode meaning, that they should both turn ON. I am pretty confused.

Schematic

Astable Multivibrator

Questions

  1. Can you help me to understand the logic behind this circuit?

  2. How can one transistor remain OFF when both transistors are biased to 0.6v, which is sufficient for a transistor to turn ON?

Any help will be much appreciated, thanks in advance.

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  • \$\begingroup\$ Nice diagram and statement of your problem \$\endgroup\$
    – RoyC
    Mar 13, 2018 at 9:23
  • \$\begingroup\$ ...but there are also the capcitors. The base bias of one transistor will be shorted (for a while) via the capacitor if the other transistor is turned on. \$\endgroup\$
    – Curd
    Mar 15, 2018 at 10:19

3 Answers 3

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As Tony Stewart has explained one of the transistors will turn on first. When it does the voltage on its collector drops causing the voltage at the other end of the capacitor connected there to drop well below 0.6 V. The voltage across the capacitor cannot be changed instantaneously.

This point is also the base of the other transistor so it will remain turned off until the base end of the capacitor charges to 0.6 V through one of the 47 K resistors.

Lets assume Q2 turns on first. When Q2 is off the voltage on its collector the capacitor + plate is about VCC. The voltage on the - plate is about .6 V . When it turns on the collector voltage drops to 0V and the negative plate of the capacitor drops by the same amount (VCC) so the voltage on the base of Q1 is -VCC+.6 V this will firmly turn off Q1 until the negative plate charges to 0.6 V through R3.

Base and Collector waveforms

When this happens the transistor turns on pulling down the Voltage on its collector and the base of the other transistor connected through the capacitor. Causing that transistor to turn off.

Rinse and repeat.

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  • \$\begingroup\$ Ok I think now am moving somewhere. So let's say Q2 turns ON first so voltage at the collector junction of Q2 will be of 0v at this instant. What will be the voltage with respect to ground in negative plate of capacitor at this instant? Is it 0.6v or it will drop to 0v pulling the base of transistor to Q1 to low? Similarly at this instant Q1 will be off and collector voltage will be of +Vcc and does the negative plate of capacitor exhibit -VCC ?? Sorry am a beginner with these concepts struggling to understand, kindly forgive my ignorance. \$\endgroup\$
    – Harini Chandran
    Mar 14, 2018 at 11:52
  • \$\begingroup\$ Answer edited to clarify these points. \$\endgroup\$
    – RoyC
    Mar 15, 2018 at 9:10
  • \$\begingroup\$ I absolutely loved your explanation. I've been trying to understand this for a few days and this is the clearest I've seen it! \$\endgroup\$
    – marciokoko
    Sep 12, 2018 at 21:17
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In theory if both side components were identical both sides would reach Vbe threshold at the same time and amplify the collector current and pull down each other's side. But nothing is every perfectly matched so the race is always won by one side first with the smallest RC time constant on the base and/or largest hFE.

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  • \$\begingroup\$ Hi Tony, thanks for your answer. But am also looking to understand how transistor turns ON and OFF alternatively as a multivibrator and cap voltages influence the switching of these transistors. Can you please help? \$\endgroup\$
    – Harini Chandran
    Mar 14, 2018 at 11:55
  • \$\begingroup\$ tinyurl.com/yc7r3d2n play with this \$\endgroup\$
    – Tony Stewart EE75
    Mar 14, 2018 at 14:21
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I would add a simulation to "show" what happens when both sides components are "identical" ...

NB: power supply V1 has a rise time of 10 ms.

The simulation below shows that the two transistors seem to be "ON" at starting time (capacitors voltage = 0)

enter image description here

And here, when there is a "low" value for resistor R5 (little emitter feedback -> lower effective "beta") or change Q1 with a BJT with lower beta.

enter image description here

Adding the resistor on side Q2 ... which shows what side is "on" or "off".

enter image description here

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  • \$\begingroup\$ Aw, Microsim! How esoteric! :) \$\endgroup\$
    – pfabri
    Feb 2, 2022 at 13:53
  • \$\begingroup\$ No ... microcap v12, spectrum-soft.com/download/download.shtm \$\endgroup\$
    – Antonio51
    Feb 2, 2022 at 15:05
  • \$\begingroup\$ Oh, bugger. Yes, Microcap. That’s what I meant to write. Really nice piece of software, though, I have the impression that very few people use it. \$\endgroup\$
    – pfabri
    Feb 2, 2022 at 18:16
  • \$\begingroup\$ One interesting fact is that, by clicking on a "component" or a ".define ..." statement, you change the current value by some percent. So, one can see immediately what happens. \$\endgroup\$
    – Antonio51
    Feb 2, 2022 at 22:07
  • \$\begingroup\$ Yes, I too use it. Mainly for IBIS simulations. For other types of simulations I use LTSpice, because it has a much larger community of users, which helps when one gets stuck. But Microcap is a mind-blowingly versatile piece of software. I'd like to use it more, but the best way in seems to go through all the newsletters on the website and the (well-written) documentation itself. \$\endgroup\$
    – pfabri
    Feb 3, 2022 at 9:40

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