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

Marks: 5M

Years: May 15

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 - $\frac{1}{R_C}$ and Y-intercept $\frac{V_CC}{R_C}$. 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 ; $I_C$ = $\frac{V_CC}{R_C}$ 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. T ransistor current gain, β i.e., h_fewhich increases with temperature