How to choose coupling capacitor sizes for an audio amplifier?

Choosing the correct coupling capacitor size for an audio amplifier involves balancing low-frequency response, impedance matching, and size constraints. Here are the key considerations:

1. Understand the Role of the Coupling Capacitor

Coupling capacitors block DC while allowing AC (audio signals) to pass through. They are positioned between amplifier stages or at the input/output to prevent DC bias from affecting subsequent circuits.

2. Determine the Cutoff Frequency

The coupling capacitor and the input impedance of the next stage form a high-pass filter. The cutoff frequency (fc) is calculated as:

fc=1/2πRC​

Where:

• fc = Desired cutoff frequency (e.g., below 20 Hz for full-range audio)
• R = Input impedance of the next stage (in ohms)
• C = Coupling capacitor value (in farads)

3. Select the Input Impedance (R)

Find the input impedance of the next amplifier stage or load. For example, if the input impedance is 10 kΩ, use this value in the equation.

4. Choose the Desired Frequency Response

Decide the lowest frequency the amplifier should handle without significant attenuation. For audio applications, 20 Hz is often the target.

5. Calculate the Capacitor Value

Rearrange the formula to calculate C:

C=1/2πRfc​

For example, if R=10 kΩ and fc=20 Hz

C=1/2π×10,000×20=0.8 μF

6. Choose a Practical Capacitor

• Select the next standard capacitor value above the calculated value (e.g., 1 μF if $0.8\mu \text{F}$ is calculated).
• Consider using film or electrolytic capacitors, depending on the circuit's requirements. Film capacitors are preferred for audio due to their stability and low distortion.