1
\$\begingroup\$

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

\$\endgroup\$
3
\$\begingroup\$

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.

\$\endgroup\$
6
  • \$\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

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.