Analytical approach for designing a BJT amplifier: Part 2

From this article, I am going to describe the design considerations of the output speaker system. So the output circuitry is an LC high passfilter as shown in the following figure.

source: https://industrial.panasonic.com/ww/ds/ss/technical/b4

Let’s move on to the calculations of this circuitry.

Equations for determine the L and C values.

For example, consider an 8-ohms @ 1kHz speaker, then Z = 8-ohms and f = 1kHz. And we take the cut-off frequency fc = 500Hz (by refering the speaker Resonant Frequency) for this calculation.

fc= 1 / 2π√LC

500Hz = √1000 / 2π√1.273C

C = 80 uF

The next move is to calculate the input voltage of this filter.

Equation to calculate the output and input voltage

In this formula, Vout can be obtained by calculating the speaker's power consumption. If the rated input power is 2W then,

P=V²/ Z

2W = V²/ 8-ohm

4V = Vout

Vin = 5V

2W = IV

0.5A = Iin

So, in here, Vin refers to the output voltage we needed to obtain from our BJT amp.

Now we have determined the values for the output circuitry. Next, we need to calculate the small signal circuit model to obtain the Collector Resistor and voltage divider resistors values.