# BJT : AC Load Line Analysis

#### Tarquin

Joined Dec 14, 2006
11
What is the effect of frequency of AC signal on the location of operating point in case of AC load line analysis of BJT ?

#### Papabravo

Joined Feb 24, 2006
20,615
What is the effect of frequency of AC signal on the location of operating point in case of AC load line analysis of BJT ?
The Q-point or operating point is established by the DC analysis which establishes the DC load line. The AC load line has a different slope then the DC load line and the two load lines intersect at the Q-point. In the common emitter amplifier, as you increase the frequency you should expect the gain to go down and the phase shift to move away from -180 degrees. The Q-point remains unchanged.

#### Tarquin

Joined Dec 14, 2006
11
Does the AC signal not affect biasing of the transistor (via the load impedance) when we consider AC load line analysis?

#### Papabravo

Joined Feb 24, 2006
20,615
Does the AC signal not affect biasing of the transistor (via the load impedance) when we consider AC load line analysis?
The biasing of a transistor is on the input. Biasing is a DC thing. If the input signal is AC coupled then for small variations the transistor operates in it's linear region and there is no distortion. If the signals are large and drive the transistor into cutoff or saturation then you get nonlinear behavior and clipping but the Q point stays essentially the same. I could be wrong though. If there is any effect at all it will not be a first order one.

The whole reason it is called a Q point is that is does not move. If it did move it would be called a Q smudge, or a Q ellipse. If the Q point was not a point then the load line would wander around and be called a load band instead of a load line.

Where are you going with this strange notion anyway?

#### Tarquin

Joined Dec 14, 2006
11
Could u direct me to some website for details on AC load line analysis??

#### hgmjr

Joined Jan 28, 2005
9,027
Could u direct me to some website for details on AC load line analysis??
Tarquin,

Take a look at the information located at this link.