Astable multivibrator with transistors - Page 1
Forum section: electronics / beginners

author: jeanguypataterub
author_rank: Contributor
timestamp: December 18, 2020, 03:31:44 pm
content: I was looking for a bit of clarification. I am a newbie and was working with an astable multivibrator transistor circuit and was reading that when Q2 starts to conduct the right side of C2 gets pulled to zero volts and the left side gets pulld down to -5.4 volts. How can this be? You would think that it would only get puled down to 0volts.I can't get my head around this black magic. Can someone help please?https://www.electronics-tutorials.ws/wp-content/uploads/2018/05/waveforms-tim20.gifPV

author: jmelson
author_rank: Super Contributor
timestamp: December 18, 2020, 03:38:11 pm
content: When one transistor turns on, the capacitor pulls off bias from the other transistor. But, then the charge on the capacitor runs out, and the transistor that was cut off starts conducting again. That couples through the other cap to the other transistor to shut it off.Jon

author: jeanguypataterub
author_rank: Contributor
timestamp: December 18, 2020, 03:41:30 pm
content: So does the voltage actually go below 0V into the negative?

author: Paul Rose
author_rank: Regular Contributor
timestamp: December 18, 2020, 04:29:46 pm
content: If C2 is charged to 5.4v (so right terminal is 5.4v higher than left terminal, and then you "close" Q2 (turn it on), collector of Q2 (and therefore right terminal of C2) get pulled low.The left terminal of C2, at that instant is still 5.4 lower than the right terminal (and that right terminal is now 0), so the left terminal of C2 (and the base of Q1) are negative with respect to your 0v point.

author: mikerj
author_rank: Super Contributor
timestamp: December 18, 2020, 04:32:24 pm
content: Yes, the voltage will actually go below zero on the transistor base. This is actually not desirable, the negative voltage will scale with the supply voltage so with higher supply voltages it could cause breakdown of the base-emitter junction. A diode can be added to each transistor to prevent this.If you download LTSpice a basic Astable circuit is included in the examples, so you see how this work:

author: Paul Rose
author_rank: Regular Contributor
timestamp: December 18, 2020, 04:36:53 pm
content: Quote from: mikerj on December 18, 2020, 04:32:24 pmYes. With the 6v supply shown in the schematic, you are probably "OK" with reverse biased B-E. But with higher voltage, you will definitely want the diodes.

author: jeanguypataterub
author_rank: Contributor
timestamp: December 18, 2020, 04:41:40 pm
content: Thanks for the help guys. Much appreciated!

author: Ian.M
author_rank: Super Contributor
timestamp: December 18, 2020, 05:54:27 pm
content: Quote from: mikerj on December 18, 2020, 04:32:24 pmLTspice 'noob' hint:LTspice labels waveforms by the node name. If you want meaningful names for your waveforms (and you should!), you need to know that unnamed nodes use auto generated node numbers n001 upwards, but those numbers will change if you modify the circuit connections or add components, so I *STRONGLY* recommend explicitly naming all nodes of interest in your circuit using the 'Label Net' tool (signpost icon with a letter A on it). Mike has only labelled Q1's base (as Q1_Base). I'd personally label all the bases and collectors( b1, b2, c1, c2). You cant have two names for the same node so only one label per node will be effective. The ground symbol is a special case of a node label, and always 'wins' so don't try to label any transistor electrode directly connected to ground! If two points have identical node labels, they are the same net as-if there was an invisible wire between them.HTHIan.

author: TimFox
author_rank: Super Contributor
timestamp: December 18, 2020, 06:06:36 pm
content: The classic multivibrator circuits were invented for vacuum tubes before the War. With vacuum triodes, the grid voltage must go quite negative to shut off the tube, and zero grid-cathode voltage will give substantial cathode-plate current. Positive grid-cathode voltage will conduct cathode-grid current much as with a BJT. One important difference is that the negative grid-cathode voltage can go very negative without any problem. Also, when the triode is turned on, the plate-cathode voltage will be substantial.

author: Ian.M
author_rank: Super Contributor
timestamp: December 18, 2020, 07:24:15 pm
content: A JFET version is much closer in behavior to the original vacuum tube multivibrator as JFET gates behave much like tube grids (except they don't exibit contact potential self biassing if left floating).Here's a LTspice sim of one for your entertainment and edification. I've added C3 and R5 to make it self-biassing, avoiding the need for a separate gate bias supply, and increased the supply voltage to 9V so it runs under similar conditions to L.T's BJD educational example. If you want to build it in real life, select a pair of JFETs with reasonably matched Vgs cutoff voltages.