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I have a circuit in which a square wave with smaller amplitude should be amplified with an opamp. Before, I used a MAX492 which has G/B-product of 500 kHz. With this circuit I manage to easily amplify the signal, Vcc is single supply 3.3V:

schematic

simulate this circuit – Schematic created using CircuitLab

This gives me a nice amplification of around 5 (1 + 40k/10k) and the input is very high impedance.

Now here's my problem: I bought some LF356 Opamps with GBP of 5Mhz and tried to put it in instead of the MAX492. However, I noticed that

  1. it doesn't seem to work below 5V at all
  2. it pulls up the positive input to some DC voltage, overlaying the signal if I don't use a decoupling cap.

Is this how this old opamp is supposed to operate? Is this because I'm using a single supply?

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  • \$\begingroup\$ Have you checked the necessary supply voltage of the 356 and are you applying it accordingly? \$\endgroup\$ – PlasmaHH Oct 25 '16 at 9:04
  • \$\begingroup\$ @PlasmaHH In the datasheet it just says Max supply voltage +- 15V. So I'm putting +15 at Vcc and 0 and Vee. \$\endgroup\$ – Felix S Oct 25 '16 at 9:12
  • \$\begingroup\$ +-15V means to use +15 Vcc and -15 Vee. \$\endgroup\$ – JRE Oct 25 '16 at 9:23
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    \$\begingroup\$ The configuration shown doesn't give a gain of 4. \$\endgroup\$ – Brian Drummond Oct 25 '16 at 9:38
  • \$\begingroup\$ @BrianDrummond - good spot! \$\endgroup\$ – Andy aka Oct 25 '16 at 9:55
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Vcc is single supply 3.3V

Minimum recommended operating conditions for the LM356B is +/- 15 V power rails. However the data sheet does indicate that +/- 5V is likely for the LM156 version (figure 5). It won't at all properly work on a single 3.3V rail and ground.

I see in a comment that you are also trying it with +15V and 0V - this will partly work but still cause you problems. Look at the DC characteristics and note that the input common mode voltage range is specified as typically -12V (when using a +/-15V rail) at the low end. Now move that up 15V to see what it will be when 0V is the negative power rail and you should not expect any decent performance with inputs lower that +3V.

The output voltage swing will also clamp at a volt or so above the negative rail (0V in your set up) so this will be another problem. Use a rail-to-rail amplifier.

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  • \$\begingroup\$ Thanks. I already suspected something like this. Is there any "standard" modern rail to rail opamp with GBP around 5Mhz you could recommend? \$\endgroup\$ – Felix S Oct 25 '16 at 9:53
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    \$\begingroup\$ The AD8605 springs to mind but I'm sure if you went to any op-amp makers (AD, LT, Maxim, TI) they will have search engines that can narrow your requirements down. \$\endgroup\$ – Andy aka Oct 25 '16 at 9:54

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