A public address system is connected to a microphone that has a maximum output voltage of 10mV. The microphone is connected to a 10 watt audio amplifier system that is driving an 8 Ohm speaker. The voltage amplifier is a non-inverting op-amp circuit. Calculate the maximum voltage gain for the voltage amplifier stage and determine the resistor values to obtain the desired gain. Assume the lower amplifier stage has a voltage gain of 1. -

Mumbai University > Computer Engineering > Sem 3 > Electronic Circuits and Communication Fundamentals

Marks: 5 Marks

Year: May 2014

The formula for Power P is given by,

$$P=\dfrac{V^2}{R}$$

Solving for V we end up with:

$$V=\sqrt{R(Power)}V=\sqrt{8Ω(10 Watts)}=8.94 V$$

The voltage gain for the amplifier is:

$$A_v=\dfrac{V_{out}}{V_{in}} =\dfrac{8.94 V}{10 mV}=894$$

In selecting the resistor values for the non-inverting amplifier, set the feedback resistor to 33kΩ. The formula for the voltage gain of the non-inverting amplifier is:

$$A_{VN_1}=1+\dfrac{R_f}{R_i}$$

Solving for $R_i$ we get the following result:

$$R_i=\dfrac{R_f}{A_V-1}=\dfrac{33K \Omega}{894-1}\cong 37 \Omega \\ A_V=-\dfrac{R_f}{R_i}$$

The current flowing through $R_i$ is

$$I=\dfrac{V_{in}}{R_i}$$

The output voltage is

$$V_{out}=IR_i$$