Question

In the common emitter amplifier, what is the relationship between the phase of the input and output signals?

Answer 1

Step 1: Leading Phase

1. The phase of a decoupled emitter resistor in a circuit changes when the signal frequency is low enough so that the value of the emitter resistor can be compared to the reactance of the capacitor.
2. A leading phase shift is observed during this transition.
3. Therefore, the phase shift will be less than $180°$.
4. This is because the gain falls to a value that is dependent on the value of the emitter resistors and the collector load.

Step 2: Lagging Phase

1. There will be effects at the top of the frequency from the capacitance of the collector-base junction and the parasitic capacitance.
2. This is always voltage-dependent or Miller capacitance.
3. Therefore, a lag is observed and the phase shift is greater than $180°$.
4. This will be asymptotic to $90°$.

Hence, in the common emitter amplifier, the relationship between the phase of the input and output signals is $180°$.