Dual Input, Balanced Output Differential Amplifier:

The circuit is shown in fig. 1, v₁ and v₂ are the two inputs, applied to the bases of Q₁ and Q₂ transistors. The output voltage is measured between the two collectors C₁ and C₂ , which are at same dc potentials.

D.C. Analysis:

To obtain the operating point (I_CC and V_CEQ) for differential amplifier dc equivalent circuit is drawn by reducing the input voltages v₁ and v₂ to zero as shown in fig. 3.

Fig. 3

The internal resistances of the input signals are denoted by R_S because R_S1= R_S2. Since both emitter biased sections of the different amplifier are symmetrical in all respects, therefore, the operating point for only one section need to be determined. The same values of I_CQ and V_CEQ can be used for second transistor Q₂.

Applying KVL to the base emitter loop of the transistor Q₁.

The value of R_E sets up the emitter current in transistors Q₁ and Q₂ for a given value of V_EE. The emitter current in Q₁ and Q₂ are independent of collector resistance R_C.

The voltage at the emitter of Q₁ is approximately equal to -V_BE if the voltage drop across R is negligible. Knowing the value of I_C the voltage at the collector V_Cis given by

          V_C =V_CC – I_C R_C

and V_CE = V_C – V_E

               = V_CC – I_C R_C + V_BE

       V_CE = V_CC + V_BE – I_CR_C       (E-2)

From the two equations V_CEQ and I_CQ can be determined. This dc analysis applicable for all types of differential amplifier.

Example - 1

The following specifications are given for the dual input, balanced-output differential amplifier of fig.1:
R_C = 2.2 kΩ, R_B = 4.7 kΩ, R_{in 1} = R_{in 2} = 50 Ω , +V_CC = 10V, -V_EE = -10 V, β_dc =100 and V_BE = 0.715V.
Determine the operating points (I_CQ and V_CEQ) of the two transistors.

Solution:

The value of I_CQ can be obtained from equation (E-1).

      

The voltage V_CEQ can be obtained from equation (E-2).

The values of I_CQ and V_CEQ are same for both the transistors.

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