D.C. Analysis of Differential Amplifier

D.C. Analysis of Differential Amplifier

The d.c. analysis means to obtain the operating point values i.e. I _Cq and V _CEQ for the transistors used. The supply voltages are d.c. while the input signals are a.c., so d.c. equivalent circuit can be obtained simply by reducing the input a.c. signals to zero. The d.c. equivalent circuit thus obtained is shown in the Fig.. Assuming Rs ₁ = R _S2, the source resistance is simply denoted by Rs ,

The transistors Q₁ and Q ₂ are matched transistors and hence for such a matched pair we can assume :

i)                     Both the transistors have the same characteristics.

ii)                    R _E1 = R _E2  hence R _E= R _E1 ll R _E2.

iii)                    R _C1 = R c ₂ hence denoted as R _C.

iv)                    lV _CCI = lV _EE I and both are measured with respect to ground.

As the two transistors are matched and circuit is symmetrical, it is enough to find out

operating point I _CQ and V _CEQ, for any one of the two transistors. The same is applicable for the other transistor.

Apply-g KVL to base-emitter loop of the transistor Q₁,