How to measure DC Operating Point of a simple transistor circuit

Load lines shows the behavior linear components in the circuit. DC load line gives the I-V relationship in the DC equivalent circuit. The inverse of slope of DC line gives the DC load resistance, \$R_E+R_C\$.

AC load lines give the I-V relationship when AC equivalent model is considered. But the AC load seen by the amplifier, \$r_c=R_C||R_L\$, 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.

• DC load line is applicable only when DC equivalent circuit is considered.

• After calculating Q point (\$I_{CQ},V_{CEQ}\$) from DC load line, \$I_{max}\$ and \$V_{max}\$ can be calculated as, $$I_{max} = I_{CQ} + \frac{V_{CEQ}}{r_c}$$ $$V_{max} = V_{CEQ} + I_{CQ}r_c$$

• Putting Q-point in middle means: \$I_{max} = 2I_{CQ}\$ and \$V_{max} = 2V_{CEQ}\$. Comparing this equations with equations for \$I_{max}\$ and \$V_{max}\$, it is clear that the value of \$r_c\$ should be \$V_{CEQ}/I_{CQ}\$. So after drawing DC LL, select Q-point such that $$\frac{V_{CEQ}}{I_{CQ}} = r_c$$ This will fix the Q point in the middle of the AC LL.

But again at the same time "Vce under 0.2V" means there is saturation which means Ic must be maximum

The "theory" is just a "rule of thumb" when turning a BJT on to act like a switch. The current that flows is still dependent on base current and collector voltage: -

Once the collector voltage has lowered beyond a point the BJT leaves the "active" region and enters saturation where it is "like" a base-current-controlled variable resistor. See the different rising slopes extending from the origin - if you approximated these to straight lines each has a V/I ratio just like in a MOSFET: -

Of course it can be really confusing the way that MOSFET's saturation region IS NOT the BJT's saturation region but the reason is clear if you consider that for a BJT it is saturation of the base whereas in a MOSFET (or JFET) it is saturation of the DS channel - two totally alien effects but called the same thing.

EDITED SECTION

I've added this because some internet pictures of a BJT's operating characteristic are incorrect. See below at an incorrect diagram of the BJT in saturation: -

There is NOT one line representing all base currents - each base current graph is individual and not "merged" as per the incorrect picture above.