Input Coupling capacitor

Thread Starter

Murdock39

Joined Apr 21, 2021
11
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.
 

KeithWalker

Joined Jul 10, 2017
1,919
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.
 

crutschow

Joined Mar 14, 2008
27,761
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.
 

Thread Starter

Murdock39

Joined Apr 21, 2021
11
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.
 
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