11 audio amplifier
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EECE208 INTRO TO EE LAB
Dr. Charles Kim
11. Audio Amp
Objectives:
The main purpose of this laboratory exercise is to design an audio amplifier based on the LM386
Low Voltage Audio Power Amplifier chip and to analyze the amplifier in terms of gain,
bandwidth, power consumption, and total harmonic distortion for various input levels.
Distortion, clipping, and output power will also be evaluated as a function of frequency.
Elements and Equipment:
LM 386 Audio Amplifier
Electret Mike
Speaker
Resistors
Ceramic Capacitors (Non-Polarity)
Electrolytic Capacitors (Polarity)
Power Supplier
Function Generator
Oscilloscope
LM386 Low Power Amplifier:
General Description of LM386
The LM386 is a power amplifier designed for use in low voltage consumer applications. The
gain is internally set to 20 to keep external part count low, but the addition of an external
resistor and capacitor between pins 1 and 8 will increase the gain to any value from 20 to 200.
The inputs are ground referenced while the output automatically biases to one-half the supply
voltage. The quiescent power drain is only 24 milliwatts when operating from a 6 volt supply,
making the LM386 ideal for battery operation.
Features
• Battery operation
• Minimum external parts
• Wide supply voltage range: 4V–12V or 5V–18V
• Low quiescent current drain: 4mA
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Voltage gains from 20 to 200
Ground referenced input
Self-centering output quiescent voltage
Low distortion: 0.2% (AV = 20, VS =6V,RL =8Ω,PO =125mW, f = 1kHz)
Available in 8 pin MSOP package
Applications
• AM-FM radio amplifiers
• Portable tape player amplifiers
• Intercoms
• TV sound systems
• Line drivers
• Ultrasonic drivers
• Small servo drivers
• Power converters
Gain Control of LM386
To make the LM386 a more versatile amplifier, two pins (1 and 8) are provided for gain control.
With pins 1 and 8 open, the internal 1.35 kΩ resistor sets the gain at 20 (26 dB). If a capacitor is
put from pin 1 to 8, bypassing the 1.35 kΩ resistor, the gain will go up to 200 (46 dB). If a
resistor is placed in series with the capacitor, the gain can be set to any value from 20 to 200.
Gain control can also be done by capacitively coupling a resistor (or FET) from pin 1 to ground.
Additional external components can be placed in parallel with the internal feedback resistors to
tailor the gain and frequency response for individual applications. For example, we can
compensate poor speaker bass response by frequency shaping the feedback path. This is done
with a series RC from pin 1 to 5 (paralleling the internal 15 kΩ resistor). For 6 dB effective bass
boost: R ≅ 15 kΩ, the lowest value for good stable operation is R = 10 kΩ if pin 8 is open. If pins
1 and 8 are bypassed then R as low as 2 kΩ can be used. This restriction is because the amplifier
is only compensated for closed-loop gains greater than 9.
Input Biasing of LM386
The schematic shows that both inputs are biased to ground with a 50 kΩ resistor. The base
current of the input transistors is about 250 nA, so the inputs are at about 12.5 mV when left
open. If the dc source resistance driving the LM386 is higher than 250 kΩ it will contribute very
little additional offset (about 2.5 mV at the input, 50 mV at the output). If the dc source
resistance is less than 10 kΩ, then shorting the unused input to ground will keep the offset low
(about 2.5 mV at the input, 50 mV at the output). For dc source resistances between these values
we can eliminate excess offset by putting a resistor from the unused input to ground, equal in
value to the dc source resistance. Of course all offset problems are eliminated if the input is
capacitively coupled. When using the LM386 with higher gains (bypassing the 1.35 kΩ resistor
between pins 1 and 8) it is necessary to bypass the unused input, preventing degradation of gain
and possible instabilities. This is done with a 0.1 µF capacitor or a short to ground depending on
the dc source resistance on the driven input.
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PRE-LAB-11
NAME
ID
1. First read the LM386 Data Sheet accompanied by this lab. Then, draw a connection diagram
for a Gain 20 audio amplifier. Attach a speaker at the output.
2. Draw a connection diagram for a Gain 200 audio amplifier. Attach a speaker at the output.
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LAB PROCEDURE-11
NOTE: This audio amp circuit will be again used for the joint lab (Mobile Studio Lab)
scheduled on Friday, April 22. Therefore, keep your circuit on the breadboard after today's lab.
Do not disrupt your board or loose elements on it.
1. Create an audio amplifier circuit as shown below. Remember that the capacitor at the very
end is polarized one so that we have to make sure the correct polarity of it.
(a) Using a function generator, apply a sinusoidal source with amplitude of 0.1V and frequency
of 500Hz to A-B terminal of the circuit.
(b) Using a scope, measure both the input and outputs signals.
(c) Sketch input and output signals below. And find the gain of Output/Input.
(d) Change the Vs (currently set at 9[V]) from 0 - 15[V] and observe the changes in the output.
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2. Now connect a speaker at the output.
(a) Using a scope, measure both the input and outputs signals, while listening to the sound.
(b) Sketch input and output signals below.
(c) Change the frequency of the sinusoidal signal to 200Hz, 1000Hz, and 2000Hz, and describe
the sounds from the speaker.
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3. Connect an Electret Microphone as in input device. Also connect a 10K resistor and voltage
supply of +5V as indicated.
(a) Blow a breath or whistle toward the microphone, and using a scope, measure both the input
and outputs signals.
(b) Sketch input and output signals below. And find the gain of Output/Input.
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4. Now connect the speaker at the output.
(a) Blow a breath or whistle toward the microphone, and using a scope, measure both the input
and outputs signals.
*NOTE: If the sound from speaker has noise, cover the speaker with your hand until it
quiets. And conduct your experiment with your hand covering the speaker.
(b) Sketch input and output signals below.
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