I have built a simple inverting amplifier circuit with LM4565 operational amplifier. But only half of the signal appears at the output of the circuit and the gain looks wrong as well. Could anyone give me a tipp what is the problem? Thanks.


enter image description here

Expected behavior, with 500mVp-p signal:

enter image description here

Expected behavior, with 100mVp-p signal:

enter image description here

Measured circuit with 500mVp-p signal: enter image description here

  • \$\begingroup\$ Scope says you're feeding it with 440 mV pk-pk. With a gain of 10 you are hoping for 4.4 V pk-pk out but you only have a 3.3 V supply. \$\endgroup\$ – Transistor May 4 at 15:56
  • \$\begingroup\$ I know, this is why I wrote the gain looks wrong. I tried with smaller signals as well but the phenomea was the same. And I took pic from the scope only with this 440mV signal. But my problem probably is the power supply issue what JRE pointed out. \$\endgroup\$ – D_Dog May 4 at 16:15
  • \$\begingroup\$ With overdrive on an under-voltaged op-amp you'll see all sorts of weird effects as the amplifier will be completely wonky. \$\endgroup\$ – Transistor May 4 at 16:19

The datasheet of the LM4565 shows that it is not rail to rail on output or input.

Input range is 14V when operated on 15V, so one volt less that the rail voltage.

The output range is somewhere above 11V when operated on 15V. So, output range is a couple of volts less than rail voltage.

It says it's rated for operation from +-2V to +-18V. So, single sided 4V to 36V.

Your power supply is outside the specified limits, and even on 4V it would be of little use.

This is the relevant section of the datasheet:

enter image description here

Look at the lines marked "Maximum Output Voltage" and "Input Common-mode Voltage Range," and note that those voltages are for when the IC is operating on one 15V rail.

  • \$\begingroup\$ Thanks! I missed that parameter... I will try a different OP amp. \$\endgroup\$ – D_Dog May 4 at 16:13
  • \$\begingroup\$ I had 10 different op amps in stock which could be fitted to the PCB. And only one is compliance the power supply requirement. The LMH6619. And it is working! Thank you for your fast comment and solution! \$\endgroup\$ – D_Dog May 4 at 19:19

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