# In circuit A the signal form mic AC (very low amplitude )

1. Apr 9, 2012

### kpraneethin00

consider two circuits:
In circuit A the signal form mic AC (very low amplitude ) is combined with Vbb(to drive the BJT), as the two voltage sources are series they adds up and its amplitude increases and it is passed to bjt and then to loudspeaker.

in circuit B the signal form mic AC (very low amplitude ) is combined with V(DC voltage source) so voltage source in series adds up and its amplitude increases it is directly passed to the loudspeaker without BJT.

my doubt is:
I find in the amplification circuits BJT is used. by the above second method the voltage amplitude increases by combining the low amplitude signal from the mic with an external DC source, as the voltage increases current increases(I=V/R) so in that case the a high amplitude signal reaches the loudspeaker in circuit B.
THEN WHAT ROLE THE BJT plays in CIRCUIT A, what happens if CIRCUIT B IS BROUGHT UNDER PRACTICE.

Last edited: Apr 9, 2012
2. Apr 9, 2012

### vk6kro

You should not connect DC to a speaker, but even if you did, the sound from the speaker would only be caused by the small varying signal from the microphone.
You probably would not hear anything.

A speaker has very little resistance, so if you placed this directly across a microphone, the microphone would produce very little signal because the microphone has a comparatively high internal resistance.