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LM741: Differences between LM741 and LF356/TL071 with respect to high frequency noise.

Part Number: LM741
Other Parts Discussed in Thread: LF356, TL071

I am using the attached circuit.

It is a standard inverting amplifier circuit.

There is a class D switching amplifier nearby and the switching frequency is 51 kHz.

Noise from the switching amplifier seems to be on the input and output of the LM741.

The supply voltage is ±15V.

The input voltage is ±50mV to ±200mV and the frequency is 40 to 55Hz.

When the output of the switching amp is turned up, the negative output of the LM741 is inverted.

This is an unintended behavior.

If I replace it with an LF356 or TL071 with the same pin assignment in the same circuit, the output is normal.

This is strange.

My questions are

(1) Are there any possible causes?

(2) Is there any difference between the LM741 and the LF356/TL071 with respect to high frequency noise?

I would appreciate it if you could answer my questions.

  • Hi Hyasaki,

    provided the LM741 are not damaged, this looks like a massive EMI issue. It seems that there's not enough shielding between the class-D amplifier and the 741 circuit. Also, the 741 circuit could profit from some low pass filtering.

    I would replace the 741 by fresh ones and check again. If the problem persists, I would improve the shielding and add low pass filtering. I'm sure your circuit wil profit from a 10nF cap in parallel to the 33k resistor and 5k1 resistor. You could also decrease the 1M and 33k resistors by a factor of three or so (-> 300k and 10k instead of 1M and 33k). Then I would mount 10...33pF NP0 caps in parallel to the 10k feedback resistors (from the -input to the output of OPAmp). And you may want to add a passive low passive filter to the output of second OPAmp, or at least an isolation resistor.

    Another idea is to additionally use a considerably faster OPAmp like the LF356 or TL071.

    Can you give us a more complete schematic, which also shows what is connected to the input and output of shown 741 circuit?


  • Hello Hayasaki-san,

    My I assume that op amp supply is +15V, -15V? How much noise is on the supplies?

    LM741 is substantially different than LF or TL device. So noise rejection is likely different. 

    The inversion of half he signal doesn't make sense to me. What is the purpose of the  -1 * -1 = 1 gain?

  • Hi Kai

    Hi Ron

    Thanks for the reply.

    Replacing LM741 by fresh ones does not change this.

    Also, LM741 is not broken.

    I connected RC LPF, but not change this.

    Gain is fixed at 1x. Do not worry about the gain.

    It is difficult to measure the noise level with an oscilloscope.

    It is difficult to provide a complete schematic.

    LF356 and TL071 do not malfunction.

    Please tell me why the faster OPAMP does not malfunction.

  • Hi Hayasaki,

    as Ron already mentioned, this inversion of negative half waves makes no sense and either means that there's something terribly going wrong in the circuit or that the LM741 is stressed by extreme high EMI caused by insufficient shielding and/or low pass filtering.

    A faster OPAmp may better be able to handle the high EMI frequencies. A slower OPAmp will more likely suffer from non-linear demodulation and rectifying effects caused by the dynamically overstrechting. Keep in mind that an OPAmp which is not fast enough and cannot follow the injected EMI will respond very weirdly by showing unexplainable symptoms like varying DC-offset, lock-up, latch-up, etc.