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. 8.7.1. Assuming R_S1 = R_S2, the source resistance is simply denoted by R_S.

• The transistors Q1 and Q2 are matched transistors and hence for such a matched pair we can assume :

i) Both the transistors have the same characteristics.

ii) R_EI = R_E2 hence R_E = R_E1 || R_E2 .

iii) R_C1 = R_C2 hence denoted as R_C.

iv) |V_CC| = |V_EE| 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.

• Applying KVL to base-emitter loop of the transistor Q₁

From the equation (8.7.6), we can observe that

i) R_E determines the emitter current of Q₁ and Q₂ for the known value of V_EE

ii) The emitter current through Q₁ and Q₂ is independent of collector resistance R_C.

• Now let us determine V_CE. As I_E is known and I_E ≅ I_C, we can determine the collector voltage of Q₁ as

V_C = V_CC – I_C – I_C R_C …. (8-7.7)

• Neglecting the drop across RS, we can say that the voltage at the emitter of Qi is approximately equal to -VBE. Hence the collector to emitter voltage is

• Hence I_E = I_C = I_CQ while V_CE = V_CEQ for given values of V_CC and_VEE.

Key Point : In the equation (8.7.6), the sign of VEE is already considered to be negative, while deriving it. Hence while using this equation to solve the problem, only the magnitude of VEE should be used and negative sign of VEE should not be used again.

• Thus for both the transistors, we can determine operating point values, using equations (8.7.6) and (8.7.8). With the same biasing arrangement, the d.c. analysis remains same for all the four possible configurations of differential amplifier.

Key Point : The d.c. analysis and expressions for ICQ and VCEQ remain same for all the differential amplifier circuit configurations.

Ex. 8.7.1 Calculate the operating point values for the circuit shown in the Fig. 8.7.2.

• The sign of VEE is already considered while deriving the equation hence VEE must be replaced by 12 V. And assume V_BE = 0.7 V.