While building a microphone preamplifier, I noticed that the output was clipping even at low input levels. The problem turned out not to be the gain but the input bias.

Problem

The circuit was powered from a single 5 V supply.

My microphone output was centered around 0 V, but the non-inverting input of the op-amp needed to be biased around half the supply voltage.

Without doing this, the amplifier could not reproduce the negative half of the waveform correctly.

A single-supply op-amp still needs its input signal to remain within its common-mode input range.

Experiment

1. Connected the microphone directly.
2. Measured the output.
3. Observed clipping.
4. Added a 10 µF coupling capacitor.
5. Added a voltage divider to bias the input at 2.5 V.

Circuit

Mic
 |
 |---||----+------> +IN
     10µF  |
            |
           100k
            |
           2.5V

                      0 0  0 0 0  0 0 0 00 000 0000000      
    000 0 00 0  0 0 0                                00     
0000                                                  00    
 0                                                     00   
 0                                                      00  
 0                                                       0  0000000000000000000000000000
  0     ─────────────────────►this is very easy imo      0  
  0                                                      00 
  0                                                       0 
  0                                                       0 
  0                                                       0 
  000                                                      0
0000                  ┌─────┐                              0
0                  ───┤     ├──                            0
0                  ───┤     ├──                            0
0                  ───┤     │                              0
0                  ───┤     │                              0
0                     └─────┘                              0
0                                                        000
0                                                     000   
00                                              000000      
  00000 00 0 0 0 0000000000 0 00 0000 00000000000           

Results

Code

float gain = 1.0 + (R2 / R1);

Observations

• The capacitor blocks DC.
• The voltage divider establishes the DC operating point.
• The amplified waveform is now centered around 2.5 V.
• The clipping disappeared completely.

Things to Remember

• Every capacitor blocks changes in DC, not AC itself.
• Single-supply amplifiers usually need a bias voltage.
• Always check the op-amp's common-mode input range.

References

• LM358 Datasheet
• Op-Amps for Everyone