Electronic – Op-amp circuit has lower gain at low frequencies

gainnon-invertingoperational-amplifier

I'm trying to design an amplifier, which amplifies line level audio to 5 Vpp, so I can sample it with a microcontroller. My plan is to do this in two steps: amplify the signal to 5 Vpp and then add a 2.5 V offset. I'm stuck at the first step.

I put together a simple inverting amplifier like so:

Op Amp Circuit

I'm using the LM324N op-amp. I'm powering it using a PC power supply (-12 V for negative rail and 12 V for positive rail). Ground, pictured in the circuit is 0 V.

I'm testing the setup with a sine wave (from my phone) as input. The amplifier works normally at higher frequencies, but the gain starts dropping at around 300 Hz. At 20 Hz, the gain is basically 1. For example:

1000 Hz:

Normal at 1000 Hz

40 Hz:

Lower gain at 40 Hz

What am I doing wrong? Is this normal behaviour for this op-amp? As I understand, op-amps start behaving abnormally at high frequencies, not at low. Is something wrong with my input?

Best Answer

If you ignore the input capacitor and look at the input impedance into your op-amp circuit from the left of the 200k resistor, the input impedance is the 200k resistor. This is because the op-amp is configured as a virtual earth amplifier.

In other words there is 200k loading your capacitor and the 3db high pass cut off point is when Xc = 200k ohms (in magnitude). This equates to: -

Frequency = \$\dfrac{1}{2\pi RC}\$ = 79.6 Hz

In other words below this frequency the signal level coming out of your amp falls at 6 dB per octave hence, at about 40 Hz the output signal will be about 6 dB down and at 20Hz the output signal will be 12 dB down.

At 20 Hz, the gain is basically 1.

Midband gain is about 5 (1 Mohm / 200kohm) and this, in decibel terms is about 14 dB hence, at 20 Hz there is a net gain of about 2 dB (1.26:1 in real numbers). Make C bigger in value.

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