Multivibrator maximum frequency

oscillator

This is a basic multivibrator circuit that I am testing. The problem is that I can only get about 160kHz with the left circuit; using smaller capacitor or larger resistor will give me 0Hz. Second problem is the lack of power output under higher frequency, ie. powering a simple boost converter circuit with a MOSFET with VCC=5V, when the frequency is 20kHz, output is 8.5V and 160kHz => 6V; I have tested with different inductor values with similar result. Last problem would be the duty cycle, both transistors have 40% duty cycle instead of 50%;

My questions are:

Is there a maximum frequency for this simple multivibrator circuit? If not, why I can only got 160kHz maximum?
Does a higher frequency affect the power output of a MOSFET?
Why is the duty cycle only 40%/40% instead of 50%/50%? The output is measured at the collector of the NPN transistor.

Best Answer

Try adding a schottky anti-saturation diode between the base and collector of your transistors. With Rb1=Rb2=10k, C1=C2=100pF, Rc1=Rc2=270R and T1=T2=BC547B and BAT42 anti-saturation clamps, I get just under 1MHz on a breadboard. Without the diodes, I can't get much more than about 100-200kHz (using different component values)

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Have you considered bipolar electrolytics? Normal electrolytics only have one plate doped (if that's the right word), so they can only withstand a unipolar signal (in reverse they act as a short, if I recall correctly, which is what causes them to bulge and eventually vent/explode.) Bipolar electrolytics usually internally have two capacitors connected in inverse series, or, in some cases, both plates equally doped. You can create the same effect in a pinch by wiring two electrolytics in inverse series, like this.

  |  / +  + \  |
--| |--------| |--
  |  \      /  |

You can also get axial ceramics in values up to around 1µF, and bigger in surface mount sizes (I use 22µF 6.3V 0805 capacitors.) For example here is a 1µF cap rated for 50V.

Bjt astable multivibrator questions

Any type of cap is fine for this circuit - it shows electrolytics in the schematic due to them being cheaply available in large values (e.g. >10uF) Previously you could not get ceramics of this size, and they are more expensive for 10uF.

It's more usual to place the LED on the collector, for NPN and PNP as you can see with both circuits. Placing on the emitter side is fine, but you have to take into account the raised base voltage to turn the transistor on (Vled + Vbe = Vled + ~0.7V) and also raised saturation voltage (e.g. when transistor is fully on, which will be around Vled + ~0.2V)

The duty cycle will change if you only use a pot in place of one resistor - to keep the duty cycle the same you would need a dual pot to change both 39K resistors at once to keep the values the same.

Best Answer

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