Multivibrator maximum frequency

Question

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:

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

Answer 1

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)

Answer 2

Putting in a larger resistor for R8 and R9 will slow the multivibrator down, not speed it up. It could also result in insufficient base bias, which might explain the dead time in the duty cycle of your transistors. Try decreasing R8 and R9. I'd be surprised if you couldn't get above 1 Mhz with jellybean transistors like the 2n2222 or BC548. You could also buffer with another transistor stage after the multivibrator to increase output voltage.