Q2Analog Electronics

Question

(A) Estimate dc emitter current in each transistor of differential amplifier shown in fig. 1. Find the dc voltage from each collector to ground and also find Vout.

Answer 1

A differential amplifier uses two matched transistors Q1 and Q2 with a common emitter resistor $R_{E}$ (or current source). For a typical circuit with $V_{C C} = + 12 V$ , $V_{E E} = - 12 V$ , $R_{C} = 4.7 k Ω$ , $R_{E} = 4.7 k Ω$ , and equal inputs ( $V_{in 1} = V_{in 2} = 0$ ):

Applying KVL to the emitter circuit: $0 - V_{B E} - 2 I_{E} R_{E} - (- V_{E E}) = 0$

I_{E} = \frac{V _{E E} - V _{B E}}{2 R _{E}} = \frac{12 - 0.7}{2 \times 4700} \approx 1.2 mA

Each transistor carries half the tail current: $I_{E 1} = I_{E 2} = \frac{I _{E}}{2} \approx 0.6 mA$ . Since $β$ is large, $I_{C} \approx I_{E} = 0.6$ mA for each transistor.

V_{C 1} = V_{C 2} = V_{C C} - I_{C} R_{C} = 12 - (0.6 \times 1 0^{- 3}) (4700) \approx 9.18 V

Since both collector voltages are equal for balanced inputs, the differential output $V_{o u t} = V_{C 1} - V_{C 2} = 0 V$ . This demonstrates the common-mode rejection property of the differential amplifier.

Q2Analog Electronics

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