Hey guys,

My electrical theory has failed me and I have been left scratching my head looking at the following astable multivibrator circuit. Just to summarize its current state:

- C1 is charging
- C2 is dis-charging
- Q1 is off
- Q2 is on

Whats got me confused (and I'm sure it's a simple explanation) is the voltages..

Is the base of Q1 drawing voltage or is it giving up voltage? See figure 1 below:


Then we have the voltage across the 1K resistor (3rd leg from the left) sitting at 6.59V. How can that be if the source voltage is only +5V? See figure 2:


And finally we have the voltage across the capacitor:


I guess to summarize my question; why do we have -1.59V on the base of Q1 and 6.59V across the 1K resistor and then 1.6V across the cap. Obviously its to do with KVL, although its been quite a few years since I have analysed a circuit. Any clarification will be greatly appreciated!

Thank you in advance

 

hi Max,
Assume that Q2 is OFF, so the right side of C2 on the collector will be at +5V.
As Q2 is OFF, means that Q1 must be ON, so Q1 Base will be at +0.7v.

When Q2 turns ON, the right side of cap C2 will effectively be at close to 0V, this means the left side of C2 is now at close to -5V, so Q1's Base will be reversed biassed, making Q1 OFF.

E

 

Hi Eric, thanks for the reply

I just looked at the simulation again and watched the exact moment of a switch, the reverse biased base will drop down to -3.7V and then it charges up again until +0.7V and it switches over once more. Why does it drop down to -3.7 exactly, whats the math behind it?

So what you're saying is.. to satisfy KVL the left side of the cap (-5V) and the reversed bias of the base (-3.7V) will require +8.7V to be drawn across the resistor?

 

hi Max,
I will compare your Sim with an LTSpice, I'll be back.
E

 

Hi Eric,

If its any easier, the sim can be found on this link:

http://www.falstad.com/circuit/e-multivib-a.html

Appreciate the effort!

 

hi Max
Q1 Base swings between ~ +0.8V and -3.6V ie: total of 4.4V

Note: the peak voltage level on Q1 collector, it is 4.4V

E

 

Thanks Eric, so where does that additional 3.7V (from the 8.7V across the resistor) come from if the source voltage is only 5V?

 

Try read this if you have time.
https://forum.allaboutcircuits.com/threads/astable-multivibrator-how-does-it-work.32164/#post-200183

 

Thanks Eric, so where does that additional 3.7V (from the 8.7V across the resistor) come from if the source voltage is only 5V?

hi,
Consider a simpler set up.
Connect a 1k from +5v to a 18uF cap, with the lower end of the cap connected to 0V.
The top end of the cap would charge to +5v and the voltage across the 1k would be 0v, now say you reversed the charged cap, what would the voltage measure across the resistor [ it would decay with time of course.]

E

 

hi Max.
This simplified circuit may help.

The cap is allowed to charge to +5v, then the transistor is switched ON, connecting the cap to 0V, note the voltage across the 1K.
The diode represents the Base/Emitter diode of a transistor.

E