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I am trying to amplify the signal from an electret condenser microphone with an LM741 op amp. I don't care about preserving the negative half of my input signal, so I am powering the IC with a single 9V battery. Here is the schematic of my design:enter image description here The microphone outputs about 20mV peak-peak and I can see the correct waveform at the junction of C1 and R1. However, the op amp outputs a steady 9v when I measure its output. I have tried two different LM741s and still have the same problem.

What is causing the output to behave like this?

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    \$\begingroup\$ Your key take-away from this should be that the main use of the 741 today is to teach students about all the ways a real op-amp can be different from an ideal op-amp. \$\endgroup\$
    – The Photon
    Commented Mar 22, 2017 at 5:41

2 Answers 2

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R1 biases the noninverting input at 0 V.

But the negative supply of the 741 is also at 0 V, and the common mode input range of the 741 only goes down to 2 or 3 V above the negative supply.

You can avoid this problem if you use the + and - 15 V supplies the 741 is designed to work with. Or if you use an op-amp with rail-to-rail input and output capability (and not too bad of saturation behavior) instead of the 741.

Second problem is that 100x is likely more gain than you're going to be able to get out of a single op-amp stage, but if your bandwidth requirement is very small, maybe you'll get away with it.

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enter image description here

With a few additions/upgrades you could make this work.

R7,C6 form an RC filter for the electret's supply reducing noise to the microphone. Any noise on the supply will be amplified by the op amp.

R5,R6,C4 form a smoothed potential divider given a DC offset at the input of 4.5V.(allows maximum swing)

C2 gives the op amp a gain of 1 at DC allowing the 4.5V offset to appear at the output (for maximum swing). At audio frequencies the gain will be controlled by the values of R3 and R4.

C5 only allows the AC signal through and blocks the 4.5V DC at the output.

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  • \$\begingroup\$ One other point I noticed last night is the 741 GBW is not well specified (at least on the TI datasheet). With >100x gain, it looks likely that higher-frequency portions of the audio band could be cut-off (and the cut-off frequency could vary device-to-device). \$\endgroup\$
    – The Photon
    Commented Mar 22, 2017 at 15:09
  • \$\begingroup\$ @ThePhoton Very true - any modern op amp would preferable to the 741 which is way way past its use-by date. I totally agree with your initial comment about its use as a comparison with an ideal op-amp. I remember (back in the 70s) that gain was usually no more than 10x per op amp stage in pre-amp designs because the bandwidth was so limited. \$\endgroup\$ Commented Mar 22, 2017 at 15:40

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