DC load line

• Load line shows the behavior of liner components in the circuits. The DC word indicates that this line is drawn under dc operating conditions without any ac input signal. Word load used because the slope of line is given by resistor which acts as load.

• DC load line gives The V-I relationship in the DC equivalent circuit. The inverse slope of the line is given by Re + Rc.

• The Q point is bias point on the load line which gives the dc operating conditions of the transistor. The dc load line is the set of a infinite number of Q point.

For DC load line,for the fixed bias circuit, we have,

In the output circuit, the load equation can be written as

VCE = VCC- IC *RC

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 ICversus VCE which is having slope - 1/RC and Y-intercept VCC/RC and X intecep as VCC.

This equation involves two unknown VCE and IC and therefore can not be solved. To solve this equation output characteristic ( IC vs VCE) is used. The load equation is the equation of a straight line and given by two points:

IC= 0, VCE = VCC

& VCE = 0, IC= VCC / RC

The intersection of this line which is also called dc load line and the characteristic gives the operating point Q .

AC load line

AC load lines give the I-V relationship when AC equivalent model is considered.

• But the AC load seen by the amplifier, rc=RC||RL is different from the DC load and hence the slope of AC load line is different. But these two lines intersect at the point where AC signal part becomes 0. ie, at Q point.

• Thus ac load line represents the ac operating conditions of transistor.

• The parallel combination of RC||RL act as load.

The following figure shows the DC and AC load line:

Effect of transformer coupling on power conversion efficiency

Transformer coupling provide better power conversion efficiency. As transformer can increase or decrease voltage or current levels according to the turns ratio, as explained below. In addition, the impedance connected to one side of a transformer can be made to appear either larger or smaller (step up or step down) at the other side of the transformer, depending on the square of the transformer winding turns ratio.

If the transformer is ideal then no power losses are considered.Hence it provides better efficiency.