Lab Four –Audio Amplifiers

ECT246 Electronic Systems III with Lab

DeVryUniversity Online

Part A: Audio Amps

TCO #4:

Given an audio amplifier application that uses an integrated amplifier, examine the overall amplifier’s performance and verify the operation by using suitable input and output circuits.

Investigation of the LM386 Audio Amplifier takes place during week 4.  The LM386 is a versatile special purpose op-amp used to amplifier audio frequencies.  The device is contained in a single 8-pin dip package and has a gain of up to 200 and a power output of 1-watt.

A.   Use an Internet search utility, such as Google, find an audio amplifier such as a LM386 download the specification sheet and determine its general characteristics such as its general description, the package type and pin configuration.

a.    Usethe Internet to search for a LM386 data sheet.  Try www.national.com or www.datasheetarchive.com.  Print the datasheet.

b.    From the data sheet find the following information and record.

What are its general characteristics?

Features

Applications

Package type

Pin configuration

B.   Use the specification sheet to determine its electrical characteristics and operating values.

a.    Record the electrical values of the LM386.

Absolute Maximum Ratings:

          Supply Voltage

          Package Dissipation

          Input Voltage

Electrical Characteristics:

Operating supply voltage

Output power

Voltage gain

Bandwidth

Input Resistance

C.   Using information from the data sheet or other sources and an application such as Multisim, Visio, or a CAD program draw the schematic and verify the component values of an audio amplifier application using the device.

a.    Using the Typical Applications section or other source find an amplifier with a gain equal to 20.

b.    Use an application such as Multisim, Visio, or a CAD program to create the circuit schematic.  Label the schematic, LM386 Audio Amplifier. 

c.    Label the pin numbers on the LM386 and give each component a name and value.

d.   Explain briefly how the gain of the LM386 can be changed.

D.   Construct the audio amplifier on your breadboardwith various gains (20 and 200).  Using a frequency generator as an input, verify the amplification of the audio amplifier. Use 10mV peak as the input signal and keeping all the component connecting wired as short as possible. Also keep all the components close together on the breadboard.

a.    For measurements purposes, prototype an amplifier similar to the one below. Remember keep all component connecting wires as short as possible, and keep all components as close to the audio amplifier as possible. The 10mV peak input is set between the 1k resistor and C6.

b.    Complete the tablebelow.

R         C­­₅                 Expected      Measured

          —       —                          _____                   _____         _____                   _____

                               _____                   _____         _____                   _____

          —                            _____                   _____         _____                   _____

c.    Beginning with pin 1 an 8 open and the frequency generator set to 10mVpat 1 kHz, use the oscilloscope to measure the voltage values.  Calculate the gain of the amplifier, .  Record the results on the sheet. 

d.   Continue this procedure until you have determined the three gain values.

e.    Were there any problems with the different gain values?  If so, explain the symptoms and how to correct the problem.

E.   Determine and plot the audio amplifier’s frequency response.  Determine midband gain, 3-dB bandwidth, critical frequencies and roll-off rate. Compare the results to the data sheet.

a.    Using the information above, set the gain of the amplifier to 20.

b.    Set the output of the frequency generator to a sine wave with amplitude of 100 mV at 1 (one) Hz.

c.    Complete the table below.

          1                 _____                   _____                   _____

          10               _____                   _____                   _____

          100             _____                   _____                   _____

          1k               _____                   _____                   _____

          10k             _____                   _____                   _____

          100k           _____                   _____                   _____

          500k           _____                   _____                   _____

d.   With the frequency set to 1 Hz, use the oscilloscope to measure the above values and calculate the gain.  Note that very low or high frequencies may be difficult to measure.

e.    Continue changing the input frequency by a power of 10 (10, 100, etc.) until you reach 500kHz.

f.     Use a piece of graph paper to plot the results.  A work sheet can be found in Doc Sharing, week 4 — “graph_paper.doc”.

F.    Connect an input circuit such as a frequency generator and a microphone and a load such as a speaker verify the operation and troubleshoot any problems.

a.    Prototype the advance amplifier circuit for this step.  Set the gain to 20.

b.    Connect a frequency generator set to a sine wave at 400 mV peak and 100 Hz to the input of the circuit.  Connect an 8-ohm speaker to the output of the circuit.  Set  to ithighest value.

c.    Write down your observations.Record the information below.

d.   Apply power and record what you hear.  Slowly decrease  and record any changes.  Continue to decrease.  Use an oscilloscope to observe the output signal.  What is happening to the output waveform?

e.    Change the input frequency to 1 k Hz and 10 kHz and record the change.

f.     Change the frequency generator to a square wave and repeat the steps above.  Record the results.

g.   Remove the frequency generator and connect a dynamic microphone to the input.  Test the circuit and record the results.  Vary  for maximum output.  Can you get maximum gain from the circuit without oscillation?

Part B – More LM386

1.   The LM386 may be used to amplify voice inputs.  Dynamic or Condenser (Electret) microphone are two common input devices.  Dynamic microphones can be connected directly to the input of the LM386 amplifier and uses resistance variation within the microphone to convert voice fluctuations into small electrical signals.  The condenser type microphone use capacitance to detect these changes and requires a bias voltage applied to the microphone.

The specifications of a typical condenser microphone are shown below.

FET

2 to 10V DC

1K ohm impedance

50 to 8 KHz

Using the information above, create a bias circuit for the microphone.  Draw the schematic and label each component.