The feedback resistor elements must be selected to provide the correct response within the audio bandwidth. In

order to achieve the correct frequency response, the passive components in Figure 8-2 must satisfy Equation 1,

Equation 2, and Equation 3:

R2×C2=3180 μs

(1)

R3×C3=75 μs

(2)

R2   R3 × C2+C3 =318 μs

(3)

R2, R3, and R4 must also be selected to meet the design requirements for gain. The gain at 1 kHz is determined

by subtracting 20 dB from gain of the circuit at very low-frequency (near dc), as shown in Equation 4:

A1kHz=ALF−20 dB

(4)

Therefore, the low frequency gain of the circuit must be 60 dB to meet the goal of 40 dB at 1 kHz and is

determined by resistors R2, R3, and R4 as shown in Equation 5:

ALF=1+R3+R2R4=1000 60 dB

(5)

Because there are multiple combinations of passive components that satisfy these equations, a spreadsheet or

other software calculation tool is the easiest method to examine resistor and capacitor combinations.

Capacitor C4 forces the gain of the circuit to unity at dc in order to limit the offset voltage at the output of the

preamplifier circuit. The high-pass corner frequency created by this capacitor is calculated by Equation 6:

FHP= 12πR4C4

(6)

The circuit described in Figure 8-2 is constructed with 1% tolerance resistors and 5% tolerance NP0, C0G

ceramic capacitors without any additional hand sorting. The large value of C4 typically requires an electrolytic

type to be used. However, electrolytic capacitors have the potential to introduce distortion into the signal path.

This circuit is constructed using a bipolar electrolytic capacitor specifically intended for audio applications.

OPA1655, OPA1656

SBOS901C – MARCH 2019 – REVISED SEPTEMBER 2022

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