with Vcc=5V you can easily get 7MHz with Rc=220, Rb=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 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

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.