8.2.2.6 Input Capacitor, C_I

In the typical application, an input capacitor (C_I) is required to allow the amplifier to bias the input signal to the proper DC level for optimum operation. In this case, C_I and the input impedance of the amplifier (Z_I) form a high-pass filter with the corner frequency determined in Equation 3.

Equation 3.

The value of C_I is important, as it directly affects the bass (low-frequency) performance of the circuit. Consider the example where Z_I is 60 kΩ and the specification calls for a flat bass response down to 20 Hz. Equation 3 is reconfigured as Equation 4.

Equation 4.

In this example, C_I is 0.13 µF; so, one would likely choose a value of 0.15 μF as this value is commonly used. If the gain is known and is constant, use Z_I from Table 2 to calculate C_I. A further consideration for this capacitor is the leakage path from the input source through the input network (C_I) and the feedback network to the load. This leakage current creates a DC offset voltage at the input to the amplifier that reduces useful headroom, especially in high gain applications. For this reason, a low-leakage tantalum or ceramic capacitor is the best choice. When polarized capacitors are used, the positive side of the capacitor should face the amplifier input in most applications as the DC level there is held at 3 V, which is likely higher than the source DC level. Note that it is important to confirm the capacitor polarity in the application. Additionally, lead-free solder can create DC offset voltages and it is important to ensure that boards are cleaned properly.