Phase shift in the output of the common-emitter

Thread Starter

i_nour

Joined Nov 9, 2015
31
Why does phase shift take place in the output of the common emitter amplifier when compared to the input signal?
 

MrChips

Joined Oct 2, 2009
30,964
Think of a transistor circuit as a simple voltage divider.

upload_2016-10-19_9-8-5.png


In a common emitter amplifier, the transistor replaces the low side resistor, R2.
As base current current increases, collector-emitter current will also increase, equivalent to R2 decreasing. Hence Vout falls with increasing base-emitter voltage.

If we put the transistor on the high side (R1), i.e. emitter follower configuration, Vout will increase with increasing base voltage.

Of course, if you can create a device with the opposite characteristics, i.e. increasing resistance with increasing control voltage, then you will obtain the opposite effect from that stated above.
 

ramancini8

Joined Jul 18, 2012
473
He is asking about phase shift not amplitude! There is very little phase shift through an emitter follower, but the emitter and load capacitance forms an RC circuit which introduces some phase shift. In R is small and C is big the phase shift will cause oscillations.
 

MrChips

Joined Oct 2, 2009
30,964
The TS is asking about a common emitter amplifier. I assume the TS is referring to the phase inversion at the output.
 

crutschow

Joined Mar 14, 2008
34,690
In a common-emitter amplifier when the input base voltage is increasing it causes the collector current to also increase.
With a resistive collector load, this causes the collector voltage to decrease, thus the collector voltage is 180° out-of-phase with the base voltage.
 
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