What exactly are the Zeners doing in this circuit?
I've been trying to improve my understanding of BJT's, by trying to 'improve' the classic astable multivibrator. In an effort to 'sharpen' the transistions, I included a Zener diode between the capacitor and the base of the transistor. My thinking was to 'protect' the base from the capacitor charge curve until it was well past the ~0.7V threshold. What ended up happening instead was an increase in oscillation frequency, and an increase in oscillation stability with very small capacitance values.
With the 'classic' multivibrator circuit, I can only get to about ~100kHz. With the Zeners (as well as R7,R8 and R9), I see stable oscillations well into the low MHz range.
Can someone help me understand how to analyze how the Zeners are working?
[The breadboard circuit performs very similarly to the simulation in LTSPice, in this case, about 3MHz with rise/fall times <10ns]