What is the max. oscillation frequency of this Astable Multivibrator circuit? Is it directly related to transition frequency of the transistor? I plan on using it with BC547B transistor. The hardwired capacitors are 100pF or 1nF, potentiometer is 100kΩ.
with Vcc=5V you can easily get 7MHz with RcMIN=220, Rb (MIN)=1k, C=100pF or a 140ns cycle.
It takes about 70~80ns for the negative sawtooth of Vbe of =-4V (= 1V-Vcc) to reach 0.7V for Vbe to pull down the collector Rc.
You can easily go faster since Ccb <4.5pF for this transistor with a smaller C.
Frequency increases slightly with voltage but then you can violate Vebo reverse voltage rating of 6V. But then you can prevent this with reverse diodes across each Vbe and that will increase the frequency much more (2~4x) since you have clamped the negative ramp now to about +/- 0.8V
Then reduce Vcc to 1.5V and you can get back to 7MHz with the diodes not having much effect.
Probably not very high- the transistors will sit a spell before coming out of saturation. I'd guess a MHz or maybe three, could be wrong.
If you turn the pot all the way down, the maximum frequency will become zero forever (transistor will be destroyed).