Mumbai University > Electronics and telecommunication engineering > Sem 3 > Analog electronics 1

Marks: 5M

Years: Dec 15

AC load line:

When no signal is applied, the transistor voltage and current conditions are as indicated at the quiescent point (Q point) on the dc load line. When an ac signal is applied, the transistor voltage and current vary above and below point Q.

Therefore, point Q is common to both the ac and the dc load lines. Starting from the Q point, the ac load line is drawn by taking a convenient collector current change ∆$I_C$= $\frac{(∆V_CE)}{R'_L}$ .

The current and voltage changes are then measured from point Q to obtain another point on the ac load line. The ac load line is drawn through this point and point Q as shown in the Fig 3.6.

For DC load line

For the fixed bias circuit, we have

$I_C=V_CC-\frac{V_CE}{R_C}=\frac{V_CC}{R_C}-[\frac{1}{R_C}]V_CE$

=$-[\frac{1}{R_C}]V_CE+\frac{V_CC}{R_C}$

By comparing this equation with equation of straight line y = mx + c, where m is the slope of the line and c is the intercept on Y-axis, then we can draw a straight line on the graph of $I_C$ versus $V_CE$ which is having slope - $1/R_C$ and Y-intercept $V_CC/R_C$. To determine the two points on the line we assume $V_CE = V_CC$ and $V_CE$ = 0.

a) When $V_CE = V_CC$ ;$ I_C$ = 0 and we get a point A and

b) When $ V_CE$ = 0 ; $I_C$ = $V_CC/R_C$ and we get a point B

The Fig 3.5shows the output characteristics of a common emitter configuration with points A and B, and line drawn between them.

The line drawn between points A and B is called d.c. load line. The 'd.c.' word indicates that only d.c. conditions are considered, i.e. input signal is assumed to be zero.

The d.c. load line is a plot of $I_C$ versus $V_CE$. For a given value of $R_C$ and a given level of $V_CC$.

Significance of Q point:

The intersection of curves of different values of $I_B$ with d.c. load line gives different operating points. For different values of $I_B$, we have different intersection points (quiescent point or Q point) such as P, Q and R. The Q-point tends to shift its position due to any or all of the following three main factors:

Reverse saturation current, $I_CO$, which doubles for every 10 °C increase in temperature.

Base-emitter voltage, $V_BE$, which decreases by 2.5 mV per °C.

Transistor current gain, β i.e., h_fewhich increases with temperature.