Why absence of input coupling capacitor in BJT amplifier causes the gain to be low? I have run simulation on Multisim and I am getting this.

 

How about posting the circuit?

Bob

 

It is difficult to give a definitive answer without a circuit diagram but I would guess that without a coupling capacitor, your signal source is changing the DC bias on the base of the transistor.

 

Most likely pulling it near 0V.

Bob

 

How about posting the circuit?

Bob

If I remove the capacitor from here my sinusoidal peak voltage drops from 550 mV to some nV

 

My guess in post #3 was correct. Without a coupling capacitor, the signal source is shorting out the DC bias on the transistor base,

 

Your schematic shows the input signal is at 0VDC. Then without the input coupling capacitor, the base of the transistor is not biased anymore, it is at 0VDC and is a rectifier when the signal level is above about 600mV peak.

 

Remember that the ideal voltage source at the input has zero impedance so it acts as a DC short to ground.
That capacitor blocks the DC so the input bias is not shorted.

 

Your schematic shows the input signal is at 0VDC. Then without the input coupling capacitor, the base of the transistor is not biased anymore, it is at 0VDC and is a rectifier when the signal level is above about 600mV peak.

Thanks, Audioguru again! I never analyzed the circuit from that angle.