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OBJECTIVE: To design an Op Amp based Stereo Earphone Amplifier.
IC, LM 386 Op Amps, Low Voltage Amp, 0.25W @6V
Holder Battery 9V PC Mount
Capacitors, 100uF, 25V, 20%, Aluminum Radial Electro.
Capacitors, 0.1uF, 50V, 20%, MONO CERAMIC
Capacitors, 100pF, 50V, 20%, MONO CERAMIC
Capacitors, 2.2uF, 50V, 20%, Aluminum Radial Electro.
Capacitors, 0.047uF, 50V, 20%, MONO CERAMIC
Diode, 1N4007, SIL RECT, 1000PRV, 1A
LED, Red Diffused, T1, ¾
Jack, DC PWR, Male, 2.5MM, PC MOUNT
Stereo Jack, PC MOUNT
Resistor, 270K OHM, 1/4W, Carbon Film
Resistor, 10K OHM, 1/4W, Carbon Film
POT, TRIM, 1 Turn, 10K LIN, 3/8” SQR, PC MOUNT
Switch, Slide, SPDT, ON-ON, PCB MNT
4”x4”, Small Single Sided PCB
Battery, 9 Volt
U1, U3
C1, C5, C8, C9, C11
C3, C6
C4, C7
C10, C12
J2, J3
R1, R3
R5, R6, R7
R2, R4
Portable CD player and a pair of Headphones (for Input and Output respectively)
Operational Amplifiers are active circuit devices (they need external power to
function). They have high gain and are used to perform a variety of useful functions
when used with passive circuit elements. When operational amplifiers were first built,
they were constructed with vacuum tubes to carry out analog mathematical operations.
Subtraction was the fundamental building block made possible by differential amplifiers.
Addition is just subtracting an inverted signal. Multiplication is just successive addition;
division is successive subtraction with a remainder. A strategically placed capacitor in
the feedback path turns the amplifier into an integrator or a differentiator. Some of the
early computers like Eniac were huge, house-sized systems built to calculate ballistic
tables for naval artillery.
The op-amps were going to use are much smaller. A common packaging is an 8pin chip but you can find them in just about every packaging style including surface
mount. The symbol in the schematic form is a triangle with 8 leads coming from the
triangle. Entire books have been written about the different uses that op-amps can fill.
We’re going to use ours as a simple amplifier. Several assumptions make analyzing
op-amp circuits easier. Check to see if there is any negative feedback. In other words,
is there a connection between the output pin and the inverting input. If that is the case,
you can use what some authors call the “summing point constraint,” i.e. the input
current at both the inverting and non-inverting terminal is forced to zero, and the
differential voltage between the inverting and non-inverting inputs is forced to zero.
These two characteristics make it possible to use basic circuit analysis techniques to
develop the transfer functions for some fairly complicated circuits. Negative feedback in
the circuit also extends the stable frequency response of the amplifier.
Operational amplifiers are simple to use, inexpensive and offer a very large
amount of gain. These IC's come in a variety of packages with 1, 2 or 4 complete Op
amps in a single IC. Op Amps have two inputs called the non-inverting and the
inverting, designated by the plus and minus sign, respectively. Op Amps are actually
differential amplifiers because they amplify the difference between the inverting and the
non-inverting inputs. Op amps will normally operate from any supply voltage in the 6 to
15 volt range. In our design we will use an LM386. Its designed specifically for
operation in the audio frequency range. For most folks the audio spectrum runs from
about 20 Hz to 20 kHz. The pin diagram for LM386 op amp is shown below in Figure 1.
LM386 is a power amplifier designed for used in low voltage consumer
applications. The gain is internally set to 20 to keep the 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
The following schematics and diagrams are just one example of a amplifier. You
can approach the experiment in a different way, and still have a complete Stereo
Earphone Amplifier.
STEP 1: Setting the Supply Voltage.
The supply voltage in the Stereo Earphone Amplifier is shown in Figure.2. It consists on
a 9V Battery (B1) that is placed in parallel with D1 and J1. This specific schematic is
designed so the battery supplies the 9V when the switch is turned on and there isn’t an
external voltage supply. D1 is placed in such a way that the current flows from the
positive end of the external DC supply through the diode, to the rest of the components.
This operation is possible only if the charge on the anode is 0.7V more positive than the
charge on the cathode. On the other hand, if no voltage is supplied to the power jack,
the battery will supply the voltage. A LED (D2) is placed after the switch, so the user
can detect if the power is on or off. The function of R5 is to reduce the current traveling
through the LED, as we only need a small current to light the LED.
FIG 2: Stereo Earphone Amplifier Supply Voltage
STEP 2: Setting the Input Jack and Potentiometer.
Figure 3 shows the connections from the input jack to the potentiometer. The
input jack has three different connectors: the Sleeve (Ground), the Tip (Left Channel)
and the Ring (Right Channel). This schematic is symmetric, so we will concentrate on
one channel. The connections go from the Stereo Jack to a 10K Variable Resistor
(Potentiometer) to control the amount of voltage going to the LM368 chip. There is a
270K resistor (R1 or R3) between those two components to lower the input signal
coming from the device. R1 and R3 should be replaced with a jumper if the input
source has low signal amplitude, because a resistor will make it even lower. The
resistors are only needed when using high signal amplitude devices.
FIG 3: Input Jack and Potentiometer.
STEP 3: Setting the LM386 chip.
Fig. 4 shows the schematic of the 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 1.35 kΩ
dB). In this case, we will place a 2.2uF capacitor from pin 1 to 8 that will bypass the 1.35
and set the gain up. 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.
We can also 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
kresistor). For 6 dB effective bass boost: R. 15 k, the lowest value for good stable
operation is R = 10 kif pin 8 is open. If pins 1 and 8 are bypassed then R as low as 2
kcan be used. This restriction is because the amplifier is only compensated for
closed-loop gains greater than 9.
When using the LM386 with higher gains (bypassing the 1.35 kW 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.
FIG 5: LM386
In our schematic, the input signal comes from pin 3. Pins 4 and 7 goes to ground
(Pin 7 is set up to bypass, so C9 is place between the pin and ground). Pin 6 inputs the
voltage supply, while pin 5 output the signal. The LM386 has a Voltage Divider
integrated, so the output will be one half the input voltage. See Fig.5
C5 and C8 block the DC power at the output pin 5 and let the AC power pass.
This capacitor is large enough to pass the audio frequency.
STEP 4: Putting it together
Fig 6 (below) illustrates the wiring connections for amplifier (both L-R channels along
with the battery supply).
FIG 6: Stereo earphone Amplifier
Assemble a complete circuit on a Single sided PC board using the basic
information explained above, along with the schematic provided above (fig 6)
A) Creating Schematic
Fig. 6 shows an example of a schematic, but your schematic doesn’t have to be
exactly the same. There is a lot of room for improvement, BE CREATIVE. The
schematic should be created in Eagle.
Eagle can be found in the computer lab or can be download a free copy on There is also a library set up for this class, where all the necessary
components can be found. The library name is EE3305. Students that won’t use the
computer lab, need the library extension with the components. Please ask the TA
B) Creating a Board
The board should also be design using the software Eagle. You are encouraged
to use the component in the library (others component wont match with the component
selected for this course. This board shouldn’t be bigger that 4”x4” PCB. Your design
will be use to create a single sided board.
Students will need to create a board extension (.brd), and turn it in to the TA, so
the board can be created.
C) Finishing project
Once the board is created, put all the components together.
A) Project Timeline: Four Weeks
Week 1:
a) Understanding Design requirements and procedures.
b) Designing and constructing headphone amplifier for one audio channel on
c) Testing the amplifier circuit for designed “noise-free” amplification (use
Week 2:
a) Constructing on breadboard and testing portable dual power supply for the
Headphone amplifier. Construct dual power supply circuit according to the design
described in STEP 3. Use an oscilloscope to test for exact +/- 4.5 VDC at the power
supply output terminals.
b) Begin constructing complete headphone amplifier circuit design using Eagle.
Week 3:
a) Complete constructing headphone amplifier circuit design using Eagle and make sure
your design is approved, so it can be produced.
Week 4:
a) Solder all components on the board and test your headphone amplifier. It is
recommended to place the headphone amplifier in an enclosure for future use.