ILab week 7

B.     Construction of the Audio Amplifier

 

 

1.   Design the audio monitor circuit. Show all calculations clearly in your “Design Method” on the worksheet.

Note: The pin 1 and pin 8 terminals are not connected at this time. These pins are used to provide additional feedback to the IC as described in the data sheet.

 

2.   Connect a signal function generator as the V1 input. The circuit input signal should be a sine wave initially set at 1 kHz. Turn on the circuit power and then the generator power. While monitoring the amplifier output (test point D), adjust the generator amplitude and the volume control potentiometer until the speaker output waveform clips. Now, reduce the amplifier input signal amplitude until no clipping occurs on the amplifier output.

Briefly enjoy the sound produced from your speaker. Vary the generator frequency and appreciate the “music.” Indicate on the worksheet if you could hear the sound as you changed the frequency.

 

 

3.   Now, disconnect the 8 W speaker replacing it with an 8 W, 1 W (or higher) non-inductive resistor. This prevents the inductive speaker from affecting your readings.

 

Measure the frequency response of the amplifier from 10 Hz to 100 kHz inclusive. A sequence of 10, 20, 50, and 100 … is recommended. Prior to recording your measurements, check to make sure that the amplifier output is not distorted. Record your oscilloscope measurements taken at test points A (signal input), B (volume control input), C (amplifier input), D (amplifier output), and E (load voltage). Record all data in Table 8 on the worksheet.

 

4.   Using the measured data, calculate the gain for each stage both in units of Volts per Volt and dB (decibels). The input coupling capacitor stage gain is calculated by dividing the voltage reading at test point B by the voltage reading at test point A. The volume control gain is calculated by dividing the test point C voltage reading by the test point B voltage reading. The LM386 amplifier gain is found by dividing the test point D voltage by the test point C voltage. The output coupling capacitor stage gain is determined by dividing the voltage at test point E by the voltage at test point D. The total system gain of the amplifier circuit is calculated by dividing the output signal amplitude at test point E by the input signal amplitude at test point A.

 

5.   Graph the overall gain (in dB) as a function of the signal frequency using semi log graph provided on the worksheet (you may copy the template provided in the worksheet, paste it onto MS Paint, and add your graph).

 

6.   Using the capacitive coupling design rule, calculate the coupling capacitor output voltage for an input voltage of 1 V_rms at the lowest frequency of interest (100 Hz).

Calculate the coupling capacitor output voltage for an input voltage of 1 V_rms at 10 kHz.

Compare the measured amplifier voltage gain to the LM386 data sheet voltage gain. Subtract the theoretical value from the measured value, expressing the result in dB.