# BJT : AC Load Line Analysis

Discussion in 'Homework Help' started by Tarquin, Dec 15, 2006.

1. ### Tarquin Thread Starter Member

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

2. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
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.

3. ### Tarquin Thread Starter Member

Dec 14, 2006
11
0
Does the AC signal not affect biasing of the transistor (via the load impedance) when we consider AC load line analysis?

4. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
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?

5. ### Tarquin Thread Starter Member

Dec 14, 2006
11
0
Could u direct me to some website for details on AC load line analysis??

6. ### hgmjr Moderator

Jan 28, 2005
9,030
214
Tarquin,

Take a look at the information located at this link.