Amplifier phase difference

[amaresh92]

greetings,

i wonder if anyone could tell me that how and why there is 180 phase shift in single stage transistor amplifier circuit?

 

[Studiot]

First I recommend reading this thread.

https://www.physicsforums.com/showthread.php?t=401308&highlight=transistor

The answer to your question is that the phase difference depends upon the configuration of the transistor.

Common Emitter (output form the collector) amplifiers have a 180 deg phase difference

Common Collector (output form the emitter) amplifiers are in phase ( = 0 phase difference)

The phase difference occurs because raising the base voltage or applying a positive going voltage to the base causes the collector voltage to fall, as outlined in the referenced thread.

Conversely the emitter voltage rises, to maintain the base emittter voltage which is theoretically constant.

One type of phase splitter is to take two outputs; one (in phase) from the collector and one (reverse phase) from the collector.

 

[yungman]

greetings,

i wonder if anyone could tell me that how and why there is 180 phase shift in single stage transistor amplifier circuit?

Asumming it is a NPN transistor. Let the emitter resistor R1 be 1K to ground and bias the base to 1.7V. The R1 will have 1V drop and set up the emitter current to be 1mA. Let the collector resistor R2 be 2K and connect the collector through R2 to Vcc=6V. The collector voltage then will be 6V-2V(1mA into 2K)=+4V. You follow that?

Then if I drive +0.5V extra into the base (1.7+0.5=2.2V), then the emitter voltage become 1.5V (0.5V increase). This make the emitter current 1.5mA. In turn the collector current will increase to 1.5mA.

Now let's look at the voltage at the collector. 1.5mA across 2K give 3V. Therefore the collector voltage will be 6-3=3V.

SO by increasing the base voltage by +0.5V, the collector voltage drop by 1V or the collector volt change by -1V. You see the the voltage move opposite direction or 180 degree off phase? And the gain is -2

$$G=\frac{-1}{0.5}=-2$$.