Question B.24:  In the circuit of Figure B.15, which value is closest to v3...

In the circuit of Figure B.15, which value is closest to v_{3} if R_{1}=2.2  \mathrm{k} \Omega, R_{2}=1.5  \mathrm{k} \Omega, R_{3}=18  \mathrm{k} \Omega, v_{1}=120  \mathrm{mV} and v_{2}=-40  \mathrm{mV} ?
a. -250  \mathrm{mV}      b. 500  \mathrm{mV}        c. -500  \mathrm{mV}       d. 1.46 \mathrm{~V}        e. -1.46 \mathrm{~V}
b.15
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Using the summing amplifier equation in Chapter 12, we calculate:

v_{3}=-\frac{R_{3}}{R_{1}} v_{1}-\frac{R_{3}}{R_{2}} v_{2}=-\frac{18}{2.2}(120)-\frac{18}{1.5}(-40)=-501.8  \mathrm{mV}

Thus, the nearest answer is \mathrm{c}.

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