Difference amplifier

The purpose of this section is to introduce precision amplifiers and to learn to distinguish differential and common mode signals.

Ref: Simpson, Ch. 9-10, esp. Sec. 9.8.7, 10.4; Faissler, Ch. 31 (review); Malmstadt et al., Ch. 8.1.

$\parbox{2.0in}{\raisebox{-1.5in}{\par \hbox{\hskip 0in \vbox to 1.5in{\includegraphics[height=1.5in]{FIGS/fig4.5.ps}\vfill}}}}$ $\parbox{3.0in}{% Wire up the difference amplifier as shown: \par Bal... ... (several values between $+1$V and $-1$V) and measure \mbox{$V_{\rm out}$}. }$

Calculate the average gain of the amplifier. In this measurement, which components determine the gain of the amplifier? How does the measured value compare with the theoretical one?

Connect $V_{2}$ to a constant +1V source and repeat the above two steps.

Connect both $V_1$ and $V_{2}$ to the same variable voltage source; measure $V_{\rm out}$ for several values of $V_{1} = V_{2}$ between $+1$V and $-1$V.

Plot $V_{\rm out}$ vs. $V_{1}$ and determine the value of the common mode gain from the plot.

Interpret your data in terms of the imbalance of the resistance ratios of the two pairs of resistors determining the gain, for the inverting and for the non-inverting input. Which pair has the higher gain and by how much? How could this common mode gain be reduced?

Calculate the common-mode rejection ratio (CMRR) for your difference amplifier. Calculate the maximum common-mode signal the amplifier can accept if a 100 mV signal is to be amplified with an error of less than 0.1%.