Minimum input voltage for this Transistor circuit
It's pretty clear that the circuit is designed for a supply voltage of around 9V (the quiescent voltage at the output is about 4V), which means that the quiescent current through the first stage is about 375 &micro;A.

Also, the common-base configuration of the first stage is intended to work with low-impedance sources (such as when a speaker is used as a microphone), so the input signal level is best described in terms of current, rather than voltage.

It should be obvious that if the input signal exceeds &plusmn;375 &micro;A, the input stage will begin to distort the waveform, since the first stage will be forced into cutoff. The actual voltage swing that this represents depends on the actual impedance of the signal source. For example, if the source impedance really is 8&Omega;, then the maximum voltage swing would be about &plusmn;3 mV.