There are four problems, each worth up to 25 points Figure 1 below is an NPN dif

Question

There are four problems, each worth up to 25 points Figure 1 below is an NPN differential pair with resistive loads. Assume current gain b of each of the transistors is 50 and that all three transistors have an Early voltage of 30V. Assume V for all BJTs is 0.7V. +5V +5V Q1 02 EF 03 0.25mA Fig. 1. NPN differential pair with resistive loads. Differential input voltage V and common-mode voltage Vare indicated 1. Determine the common-mode input voltage range for the differential amplifier. The lower limit of the common-mode input voltage range occurs when the collector-to-emitter voltage of Q becomes 0.3V or less. The upper limit of the common-mode range occurs when the collector-to-emitter voltage of either Q, or becomes 0.3V or less.

Explanation / Answer

Lower limit of common mode voltage:

When Vce of Q3 = 0.3V

The Vbe of BJT = 0.7V

VCMmin = 0.3 + 0.7 = 1V

Uper limit of common mode voltage:

When Vce of Q1 or Q2 = 0.3V

The Vbe of BJT = 0.7V

Vce = Vcb + Vbe

Vcb = 0.3 - 0.7 = -0.4V

Vbc = 0.4

As the IEE = 0.25mA

So current flowing through Rc = 0.25/2 mA = 0.125

So output common mode voltage =5 - 0.125*10*10^{-3}*10^{3} = 3.75V

VCMmax = 0.4 + output common mode voltage = 4.15V

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