# 2 transistor oscillator

Discussion in 'General Electronics Discussion' started by fender__, Aug 8, 2016.

1. ### fender__

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Aug 8, 2016
Hello

So I am doing some studying with the Tronix-1 book i recently bought. I am stuck with aa lab "2-transistor oscillator".

1. Could someone explain the current flow more in detail and how the current behaves in the capacitor, ex when it's loaded and when its unloaded.

2. In the text says I should hear a changing in the oscillation (The tone should change) but I only hear a constant tone all the time its never changing. Howcome it is like that?

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2. ### hevans1944Hop - AC8NS

4,581
2,142
Jun 21, 2012
What part of the text are you reading that "says I should hear a changing in the oscillation"? The frequency of the oscillator is varied by changing the value of R1 or C1. Smaller values (within certain limits) will result in higher frequency of oscillation and larger values (again, within limits) will result in lower frequency of oscillation.

3. ### Sunnysky

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123
Jul 15, 2016
I would only expect a single tone. each transistor inverts the signal with gain so the capacitor feedback

4. ### fender__

2
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Aug 8, 2016
well, when the speaker goes to its "rest" (the end of the text) position there should be some differences in the tone? Or is this happening so fast that you cannot hear it?

1. How many times per second is the capacitor getting charged and discharged?
2. And is it true that the voltagedrop over the capacitor is always the same as the voltage source, in this case 9V? Or is it not true in this case since the current have other paths as well?
3. How is current flowing when the capacitor is discharging, is it to the right of the capacitor?

Thank you so much for helping me out. I just need to undarstand this 100% before I move to the next lab.

/Emir

5. ### duke37

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769
Jan 9, 2011
This is a multivibrator, collector to base, collector to base. With two similar transistors, one will be on when the other is off. In this case, Q1 is npn and Q2 is pnp and both transistors go on and off together.

Assuming the oscillator is running, when Q2 is on, the collector is pulled high, when Q2 is off the collector is pulled low by the load. Thus the capacitor drives Q1 with a 9V square wave. The base of Q1 cannot rise much above 0V so the base will run from about +1V down to -8V. R1 pulls the base back up to start the progress over again. The frequency will depend on the time constant of R1 and C1.

Note that Q1 base can go down to -8V or so which is more than should be applied to the base. A diode in series with the Q1 base will protect it or you could run from a lower voltage, say 6V.