The question says:

Design an op amp circuit with inputs \$v_1\$ and \$v_2\$ such that \$v_o = 3v_2-5v_1\$

One way to realize this circuit is to utilize difference amplifier which satisfies this equation $$ v_o = \frac{R_2(1+R_1/R_2)}{R_1(1+R_3/R_4)} v_2 - \frac{R_2}{R_1}v_1 $$ for this circuit enter image description here

Unequivocally, \$\dfrac{R_2}{R_1}=5,\ \ \dfrac{R_2(1+R_1/R_2)}{R_1(1+R_3/R_4)} =3 \implies R_2=5R_1, R_3=R_4\$. If \$ v_1=6, v_2=2 \implies v_o=3v_2-5v_1=3(2)-5(6)=-24\$. Using PSpice to realize this circuit, I'm getting different result

enter image description here


enter image description here

With the input voltages present, the output would need to be -24 volts but, it's likely that the power rails applied in OrCAD are limiting the output to +/- 15 volts hence, you get negative clipping.

Try inputting 0.6 volts and 0.2 volts to get -2.4 volts on the output.

  • \$\begingroup\$ This is exactly what I've done. Notice though, the hand calculation using nodal analysis yields -24 as well. \$\endgroup\$ – CroCo Aug 17 '20 at 15:39
  • \$\begingroup\$ This is from the book. What do you mean by it is not difference op amp? Why OrCAD gives -15 while hand calculation using nodal analysis gives -24? \$\endgroup\$ – CroCo Aug 17 '20 at 15:43
  • \$\begingroup\$ OK, it will work but your power supply range won't permit a negative voltage of -24 volts. I'll edit my answer to make that clearer. \$\endgroup\$ – Andy aka Aug 17 '20 at 15:44
  • \$\begingroup\$ Changing VNEG and VPOS to -24 and -24 in property editor yields the correct answer. Previously, the op amp operates in the saturated mode. \$\endgroup\$ – CroCo Aug 17 '20 at 15:47
  • \$\begingroup\$ This is truly a difference op amp. Thanks though for pointing out the saturated case. \$\endgroup\$ – CroCo Aug 17 '20 at 15:48

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