Oscillator circuit is shown.

a) Calculate frequency of oscillations,

b) Calculate minimum value R1 for which circuit still oscillates,

c) Calculate amplitude of voltage VB, for which voltage VA start cuting off

Vz=6.3V, R1=100K, R2=120K, R3=470, R4=2R6=10K, C1=C2=100nF, Vd=0.7V, R4=R5.

I completed task a) and task b)

I got f_oscillations=232.7Hz and R1_min=250k.

I don't know how to solve task c)

Any idea?

Thanks in advance.