You must have had the following problems in amplifier circuit design. For example - you have failed to decouple a single-supply operational amplifier circuit in the correct way. Today we will discuss this problem and give the correct decoupling method for single-supply amplifier circuits.
Single-supply op-amp circuits require biasing the input common-mode level to handle positive and negative oscillating AC signals. When a resistive voltage divider supply method is used to provide the bias, sufficient decoupling must be performed to maintain constant power supply rejection (PSR).
A common but wrong approach
A common, but erroneous, practice is to supply VS/2 bias to the in-phase side of the op-amp via a 100 kΩ/100 kΩ voltage divider circuit with a 0.1 μF bypass capacitor. If these values are used, the power supply decoupling often seems inadequate because its pole frequency is only 32 Hz.
Recommendations for the correct method
When the circuit is operating in an unstable environment, Figure 1 (in-phase amplification) and Figure 2 (inverting amplification) give how to obtain the best results with the VS/2 decoupling bias circuit.
Figure 1 Proper decoupling method for single-supply in-phase amplifier circuits
Midband Gain = 1 + R2/R1
Figure 2 Proper decoupling method for a single-supply inverting amplifier circuit
Midband Gain = -R2/R1
In both cases, the bias function is provided by the in-phase input, the feedback gives the same bias to the inverting input, and the unit DC gain biases the output to the same voltage. The coupling capacitor C1 is consistent with BW3 and rolls down the band gain.
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