180 degrees phase shift in CE amplifier

[purnapragna]

hi i have adoubt like how is there is a phase shift of \[180^{\0}\] in CE amplifier output?

can anybody explain it using base-width modulation process?

thnx

purna!

 

[Kral]

purnapragna,
Are you referring to a true phase shift, as opposed the the normal inversion that is inherent in the CE topology?
Regards,
Kral

 

[v_c]

I am assuming that you are referring to small-signal ac gain ...

Note that the collector voltage is vc = Vcc - ic Rc. Since Vcc is constant it is considered "ground" for ac signals therefore the voltage at the collector is vc= -Rc ic. Now consider that fact that the collector current is beta (also denoted as hfe) times the base current (ic = hfe ib) and the fact that the base current is proportional to the base (input) voltage. All this means is that the collector current will be directly proportional to the base voltage and will be in phase with the collector current. The collector voltage on the other hand will be 180 degrees out of phase since vc = -Rc ic (because of the negative sign). Is this confusing enough?

If you look at the equivalent circuits for the CE amplifier you will see why this is happening.

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Best regards,
v_c

 

[purnapragna]

Actually i know this but question asked was how do u explain this phenomenon on the basis of base-width modulation.

thnx

purna!

 

[v_c]

OK take a look here **broken link removed**
and read the discussion in Section 5.4.2 and it may help you with your stuff.
I think base width modulation is also referred to as the Early effect?

check

From http://gabevee.tripod.com/sstubmic.html

The undesirable qualities of bipolars are base current, low input impedance, and thermal runaway due to both inherent heat properties of silicon and the phenomenon called the Early effect (AKA base width modulation) and base current recombination. The input capacitances (Miller and shunt capacitances) are also there and variable due to the aforementioned Early effect.

The bipolar transistor's input is now more controlled by the signal than would be previously realized. This is because the output of the source of the FET is now in phase. That is, the voltage signal and the current signal are in phase. This is significant when one notices that if the voltage and current were not in phase, say by 180 degrees, then as the signal voltage went positive, the base current would also go positive, because the bipolar is now conducting more, but the base current will be out of phase with the signal current, so cancellation will be the result.

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