The Dual Input, Balanced Output Differential Amplifier:

The circuit is shown in the figure 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 the same DC potentials.

D.C. Analysis:

To obtain operating point (I_CC  and V_CEQ) for the differential amplifier DC equivalent circuit is drawn by reducing input voltages v₁ and v₂ to zero as shown in the figure 1.

Fig. 3

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

By applying KVL to the base emitter loop of the transistor Q₁.

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

The voltage at emitter of Q₁  is nearly equal to -V_BE if the voltage drop across R is negligible. Knowing the value of I_C voltage at the collector V_C is given as follows

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

Example - 1

The below stated specifications can be given for dual input, balanced output differential amplifier of figure 1:

RC = 2.2 kΩ, RB = 4.7 kΩ, Rin 1 = Rin 2 = 50 Ω , +VCC = 10V, -VEE = -10 V, β_dc =100 and V_BE = 0.715V.

Determine operating points (I_CQ and V_CEQ) of the 2 transistors.

Solution:

Value of the ICQ can be obtained from equation (E-1).

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

Values of I_CQ and V_CEQ are same for both transistors.

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