# BJT Astable Multivibrator - What is the max. oscillation frequency?

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.

• If you're asking what the maximum frequency is, chances are you're going about something the wrong way. What are you trying to do? Commented Jan 28, 2019 at 20:26
• You should add a few-hundred ohm resistor in series with each pot, otherwise those transistors may be destroyed. Commented Jan 28, 2019 at 20:28
• Do you understand how the circuit works and why it oscillates at all? If so, it should be pretty clear which components are determining the frequency. Commented Jan 28, 2019 at 20:53
• Frequency transition is unity gain, or a gain of 1. You should have a gain of 2 for stable oscillation. Still, it may work up to 10MHZ with a gain of 2 or more.
– user105652
Commented Jan 28, 2019 at 20:54
• As @Hearth indicates, it's usually very wise to explain why you're asking a specific question; it narrows down the answers to ones that are useful to you. People are more likely to write an answer when they are confident that they're actually helping you! Commented Jan 28, 2019 at 21:11

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).