Blocking filter for Audio Amplifier

The input signal to an audio amplifier must be coupled from the DC supply on which audio signal is riding on. The audio frequency is from 20Hz to 20kHz and so we can use a blocking filter to block the dc but allow other frequency to pass through it. That is why we can place a high pass filter centered at 5Hz which will block the dc signal(0V) but allow the audio signal to pass through it.

The following shows a circuit diagram of using RC high pass filter before the MOSFET audio amplifier circuit.

In the circuit diagram, the capacitor C1=3.3uF and resistor R3=10kOhm constitute the RC HPF with cutoff frequency of 5Hz. This high pass filter acts as a DC blocking capacitor or circuit that will block the dc component of the signal if any entering the audio amplifier biased with BJT voltage divider biasing method biased around the BC547 transistor Q4. What follows the BJT amplifier is the drain feedback bias used on the MOSFET transistor IRF540N. Actually, the resistor R3 is same as the input impedance of the BJT amplifier. If the input impedance of the amplifier is know, we can use it to calculate the capacitor value for desired cutoff frequency

You can design both the BJT amplifier and the filter using online calculator. For BJT amplifier design see BJT transistor amplifier design calculator and for MOSFET transistor see online MOSFET biasing calculator. You can use the online RC filter calculator to calculate the R and C component values for desired cutoff frequency.

The video below demonstrates how the filter works by showing how the signal amplitude before and after the blocking filter behaves with changes in frequencies.