I'm using an LM358 to learn how to use op amps, I asked a question and everyone referred me to books so I've been reading up on that, but whenever I try to use them I never actually get a boost. Today I tried using this setup below but I only got -(1/2)Vin = Vout. I don't have much to work with so I'm using Vcc+ = 2.4v and Vcc- = -2.4v. RG = RG = 10K Vin = 1.2v. In this case Vout = -.6v, when I tried other resistances it didn't really match the -RF/RG equation. I'm probably not reading the data sheet right or the resistances aren't high enough. Not sure what's wrong. 
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\$\begingroup\$ Everything is as it should. Check the connections and resistance values (just use two apparently equal resistors). What is your input source? Is it a potentiometer? Maybe it has some internal resistance? \$\endgroup\$– Circuit fantasistCommented Dec 21, 2019 at 23:19
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\$\begingroup\$ what load is the amp driving? \$\endgroup\$– user16324Commented Dec 21, 2019 at 23:20
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\$\begingroup\$ With Rf = Rg you'd expect Vout = -Vin as long as Vout was somewhat more positive than Vcc-. The LM358 Vout will drive near but not all the way to Vcc- - a pull down helps. || What is the source of Vin? Have you measured Vin with a meter in operation? As Brian says - if you use a potentiometer to supply Vin its resistance will affect the results. \$\endgroup\$– Russell McMahon ♦Commented Dec 22, 2019 at 7:56
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1 Answer
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Congratulations on being keen enough t0 learn by experimenting!
The LM358 / LM324 have an input common mode range of Vcc+ - 1.5V.
This means Vin on either input needs to be 1.5V or more below Your V+ supply.
This is specified in the data sheet.
In this case V+ is 2.4V wrt ground.
2.4 V - 1.5 V = 0.9V wrt ground.
This is the highest that either input can be at without violating the Vcn constraint.
Your Vin of 1.2V exceeds this :-(.
Easier on the brain MAY be to start by operating the amplifier as a single supply amplifier with Vcc+ = 4.8V (5v?) and Vcc- = ground.
Then try a non inverting amplifier
- Input to In+,
- Rf1 from out to In- and
- Rf2 from in - to ground.
Gain = (Rf1+Rf2)/Rf2.
This makes all signal referenced to ground (Vcc-) and stays away from the Vcm_max limit initially.
Then try inverting amplifiers (as per your example) by biasing In+ above ground by say 1V with a resistive divider.
answered Dec 21, 2019 at 23:59
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\$\begingroup\$ A very useful guidance for a beginner... Only to note that in the inverting configuration, the common-mode voltage is zero because of the virtual ground at the op-amp inverting input. Thus we can drive it with thousands of volts but it will stay zero (of course if we keep the same ratio R1/R2). \$\endgroup\$ Commented Dec 22, 2019 at 7:18
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1\$\begingroup\$ @Circuitfantasist Hmmm. Obviously yes re Vcm and inverting input, which messes up my answer in this specific case. | This only applies while Vout is in controllable range - in this case slightly under 2.4V (as Vcc- = 2.4V and the '358 gets a bit weak around the knees when Vout is really close to V-. The OPs +1.6V should give -1.6V as shown which should be well within limits. \$\endgroup\$– Russell McMahon ♦Commented Dec 22, 2019 at 7:52
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1\$\begingroup\$ @Circuitfantasist There's a trap for young players in this. The LM324/358 is prone to phase reversal where a sufficiently negative voltage (outside the rated common mode range, negative wrt ground) on the inverting input will drive the output low, causing the output to latch low if it ever gets the chance. The TI SPICE model will allow this behavior to be simulated (in fact it will start up in a latched state). So some external parts should be added if the inverting configuration is used with inputs that are less than a few hundred mV. \$\endgroup\$ Commented Dec 22, 2019 at 8:07
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\$\begingroup\$ @RussellMcMahon, I said it just because of "teasing":) It is clear that for the purposes of the initial introduction to the op amps, the supply voltage should be normal (large enough)... and the experimenter should focus on the principal things (though this is also something quite basic). \$\endgroup\$ Commented Dec 22, 2019 at 8:16
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\$\begingroup\$ @ Spehro Pefhany, I remember a similar problem with the ancient 702 (or 709)... where two diodes had to be connected back to back in parallel to the input... \$\endgroup\$ Commented Dec 22, 2019 at 8:27