-1
\$\begingroup\$

I have a circuit like this: Multisim circuit

I am using OP07CP op-amp . V+ and V- are supplied by two 12.2V adapters (which are quite stable!) V1 are supplied using Function Generator

According to what I have learned so far, Vout is equal to V1*2.2/1 = 2.2V1 . Multisim also shows this result . But the problem is that I got the wrong result.

For example :
If V1 is 0 , the output is ~ -0.56
V output is the same for V1=2.5V
What did I miss here ?

\$\endgroup\$
  • 1
    \$\begingroup\$ Your circuit is configured as an inverting amplifier. Its gain is -2.2V1 not +2.2V1 \$\endgroup\$ – brhans Apr 27 '17 at 17:42
  • \$\begingroup\$ Other than what @brhans said it should work as you indicated. Check your connections. \$\endgroup\$ – Trevor_G Apr 27 '17 at 17:44
  • 1
    \$\begingroup\$ Your positive supply shows "12.2V" right near the symbol and "VCC" off to the side. The negative supply is the other way around. Are you sure you're actually supplying -12.2 V to pin 4? \$\endgroup\$ – The Photon Apr 27 '17 at 17:44
  • 1
    \$\begingroup\$ Sorry for the elementary question, but do you have the other sides of the 12.2V adapters connected to ground or just to each other? \$\endgroup\$ – Spehro Pefhany Apr 27 '17 at 18:10
  • 1
    \$\begingroup\$ Is the common between the two adapters connected to your circuit ground? eg. to pin 3 of the OP-07 \$\endgroup\$ – Spehro Pefhany Apr 28 '17 at 4:06
1
\$\begingroup\$

You need to connect the common between the two power supplies (adapters in your case) to your circuit ground:

schematic

simulate this circuit – Schematic created using CircuitLab

Otherwise current through R1/R2/V1 and input bias currents will cause your ground to move around relative to the common point of the two power supplies and will cause problems with common mode input range and output range.

\$\endgroup\$
0
\$\begingroup\$

You have the power supplies reversed:

Or, you have the opamp schematic symbol defined with the power supplies backwards. Either way, there is a serious problem that needs to be fixed.

\$\endgroup\$
  • \$\begingroup\$ Instead, they are correct: pin 4 is Vee, and pin 7 is Vcc, see datasheet (also this pinout is very common in single op-amp, 8 pin package). This happens if you "flip" vertically your symbol, to have the inverting input on the top. (see also the numbers 2, 3, 6 , which are below the pin). \$\endgroup\$ – next-hack Sep 13 '17 at 20:00
  • \$\begingroup\$ @Nex: Like I said, either the power supplies are reversed or the symbol is flipped. Something is wrong either way. Even if the pinout in the symbol is actually correct, showing the negative supply at top and the positive at bottom is really bad schematic drawing, to the point of being wrong. \$\endgroup\$ – Olin Lathrop Sep 14 '17 at 0:35
  • \$\begingroup\$ I agree, it creates a lot of confusion, and it is source of errors. \$\endgroup\$ – next-hack Sep 14 '17 at 6:51

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.