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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