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I was having trouble designing a small headphone amplifier that used lm358 as input stage. After verifying the push-pull output stage was correct and that every DC point was correct too (output resting at VCC/2, bias at correct voltage etc), I started to question the opamp's functionality.

I tested a simple circuit using lm358 in multisim with a single 9V supply, and I found out that, somewhy, the output voltage swings only from 6.6V to 2.4V (This is, VCC/2 +-2.1V).

Is that supposed to be like that? I didn't expect it to go all the way from 9V to 0V, but I expected it could at least swing from 8V to 1V.

Here's a screenshot of my circuit, showing the oscilloscope output for it. I'm using an AC input with 0.4V amplitude, so the output was expected to swing between 8.5V and 0.5V, but it only goes from 6.6V to 2.4V:

enter image description here

The datasheet emphasizes the large output swing for this chip. So I expected it to go further. Is this correct?

Thanks!

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  • \$\begingroup\$ Did you read the datasheet? Could you add the link to datasheet? \$\endgroup\$ – Chupacabras Mar 7 '18 at 20:27
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    \$\begingroup\$ I'm not surprised at the top number, it is not a rail to rail op-amp. The low end seems high for a 358 though. \$\endgroup\$ – Trevor_G Mar 7 '18 at 20:28
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    \$\begingroup\$ So you are expecting this. The LM358 should only go up to 7.5 V (9V -1.5V), not 8 V. It should however go down close to 0 V. - Have you tried with other op-amps? \$\endgroup\$ – Harry Svensson Mar 7 '18 at 20:32
  • \$\begingroup\$ Yes, @HarrySvensson , I was expecting some at least a bit closer to that. 2.1V amplitude is too little for what I want. \$\endgroup\$ – user2934303 Mar 7 '18 at 20:38
  • \$\begingroup\$ Look at figure 10 and figure 11 in the datasheet and also tell me what the current through the 10kohm resistor is. \$\endgroup\$ – τεκ Mar 7 '18 at 20:39
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Here is what is probably a better simulation using LTspice and a TI model for the LM358. I modified the circuit slightly, using 5.6K/3.9K for the bias divider to center it near the middle of the no-load output range (~3.7V). Input is 370mV.

You can see it saturating badly on the first cycle, then it settles down towards the middle of the range.

enter image description here

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