The attached circuit is a preamplifier for a frequency counter. Does it need a diode to prevent reverse bias of the base-emitter junction for larger input signals?

 

Does it need a diode to prevent reverse bias of the base-emitter junction for larger input signals?

Only if the signal Voltage peak-to-peak is more that 10V.

 

See

 

@Bordodynov I do not understand what each line in the graphs represents.

 

Graphs of sine voltage signal at the points: in, b, out
For different amplitudes of input voltage.
Light green and blue are barely visible because they are too small input voltages.
Red and darker green are are amplified "normally".
Torquoise (or whatever the last color is)... well judge yourself.. hint it is 10V amplitude...