Question 9.6: Objective: Design a difference amplifier with a specified ga...

The differential gain is given by
$\frac{R_{2}}{R_{1}} = \frac{R_{4}}{R_{3}} = 30$
From Appendix C, we can use standard resistors of
$R_{2} = R_{4} = 390  k\Omega$      and       $R_{1} = R_{3} = 13  k\Omega$
These resistor values are obviously less than 500 kΩ and will give an input resistance of $R_{i} = 2R_{1} = 2(13) = 26  k\Omega$.
Trade-offs: Resistor tolerances must be considered as we have in other designs. This effect is considered in end-of-chapter Problem 9.62. Resistor tolerances also affect the common-mode rejection ratio, as analyzed in the following example.
Comment: This example illustrated one disadvantage of this differential amplifier design. It cannot achieve both high gain and high input impedance without using extremely large resistor values