Lecture - 4: Biasing of Differential Amplifiers

Example - 1

Design a zener constant current bias circuit as shown in fig. 5 according to the following specifications.
    (a). Emitter current -IE = 5 mA
    (b). Zener diode with Vz = 4.7 V and Iz = 53 mA.
    (c). βac = βdc = 100, VBE = 0.715V
    (d). Supply voltage - VEE = - 9 V.

Solution:

From fig. 6 using KVL we get

Fig. 5

Practically we use RE = 820 kΩ

Practically we use R2 = 68 Ω
The designed component values are:
                RE = 860 Ω
                R2 = 68 Ω

Fig. 6

Example - 2

Design the dual-input balanced output differential amplifier using the diode constant current bias to meet the following specifications.

  1. supply voltage = ± 12 V.
  2. Emitter current IE in each differential amplifier transistor = 1.5 mA.
  3. Voltage gain ≤ 60.

Solution:

The voltage at the base of transistor Q3 is

Assuming that the transistor Q3 has the same characteristics as diode D1 and D2 that is VD = VBE3, then

Practically we take RE = 240 Ω.

Practically we take R2 = 3.6 kΩ.

To obtain the differential gain of 60, the required value of the collector resistor is

The following fig. 7 shows the dual input, balanced output differential amplifier with the designed component values as RC = 1K, RE = 240 Ω, and R2 = 3.6KΩ.

Fig. 7

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