Audio Amplifier Report9

8/8/2019 Audio Amplifier Report9

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DESIGN AND BUILD YOUR OWN AUDIOAMPLIFIER

Reshed HusseinKevin Wade

Final ReportENEE 417-0105

University of Maryland @ College Park


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Introduction

The purpose of this project was to design and build your own audio amplifier

from scratch. Good audio amplifiers take a small input signal and magnify it several

times, without distorting it, so that it can be heard at the speakers. In order to design our

audio amplifier we used the scheme provided by professor Yang as a starting point, and

made a few changes in order to meet the required specifications of high input impedance,

low output impedance, and a gain of about 10.

The project was very instructive because it not only helped us solidify our

theoretical knowledge acquired in Microelectronics classes, but it also gave us an insight

into the procedures an engineer must take from a design on paper to a final working

product. Another benefit of this project is the satisfaction that is gained after completing

such a challenging task. Even though our design satisfies all requirements there are many

other ways to design a good audio amplifier; either by using op-amps or JFET instead of

the BJT used in our design.

The design of our audio amplifier was done on PSPICE, where we made sure that

all requirements were met. After completing the design we built the circuit on the bread

board and compared the observed DC voltages and currents with the ones obtained in

PSPICE. We then designed a PCB layout on Microsoft Paint and built the PCB board.

Few changes were made from the initial design in order to improve the performance of

our circuit. This report contains all design schematics and measurements, as well as

observed measurements done on the PCB board.

Circuit design

Our audio amplifier consists of two parts: a differential stage built using 2 BJTs

and an emitter follower. The differential pair is used to amplify the input signal while the


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emitter follower is used as a power amp stage providing the necessary current to the load.

Figure 1 below shows the circuit built in PSPICE.

Figure 1: Final Audio Amplifier Schematic


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Figure 2: DC Voltages in PSPICE

Figure 3: DC Currents in PSPICE.


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Figure 2 and 3 above, show the various DC voltages and currents that were obtained in

PSPICE.

Figure 4 below shows the voltage gain as a function of frequency. From the figure we can

see that the gain is approximately 10 across the audio range (from 10 Hz to 20 KHz).

Frequency

1.0Hz 10Hz 100Hz 1.0KHz 10KHz 100KHz

V(Vout)/ V(Vin:+)

1.0

10

100

Figure 4: Voltage Gain as a function of Frequency

Frequency

1.0Hz 10Hz 100Hz 1.0KHz 10KHz 100KHz 1.0MHz

P(V(Vout)/V(Vin:+))

-20d

0d

20d

40d

60d

Figure 5: Phase as a function of Frequency


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As it can be seen from Figure 6 below the input impedance in the audible frequency

range is approximately 100K. In the ideal case the input impedance would be infinite,

but since our circuit is far from being ideal 100K is an acceptable value.

Frequency


ABS(V(V5:+)/I(V5))

10K

100K

1.0M

Figure 6: Input Impedance vs. Frequency (Amplitude)

Frequency


P(V(V5:+)/I(V5))

140d

150d

160d

170d

180d

Figure 7: Input Impedance vs. Frequency (Phase)

As it can be seen from the figure below the output impedance is approximately zero,

which is the desired value.


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Frequency


ABS(V(V6:+)/ I(V6))

1.0e-30

3.0e-30

5.0e-30

7.0e-30

9.0e-30

Figure 8: Output impedance vs. Frequency (Amplitude)

Frequency


P(V(V6:+)/I(V6))

-1.0ud

-0.5ud

0d

0.5ud

1.0ud

Figure 9: Output impedance vs. Frequency (Phase)

Figure 10 shows the input vs. the output as simulated in PSPICE.


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Time

0s 0.5ms 1.0ms 1.5ms 2.0ms 2.5ms 3.0ms 3.5ms 4.0ms

V(Vout) V(V8:+)

-10V

0V

10V

Figure 10: Input vs. Output

Figure 11 below shows the PCB layout for our circuit.

Figure 11: PCB layout


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Although we came up with a scheme for the Tuner we were not able to test it or build it

due time restrictions. The figure below shows the schematic for the Tuner.

R1

1k

R2

320

R3

160

R4

10k

R5

10k

R6

320

R7

160

C1.1uF C2

.1uF

C3

.1uF

C4

100uF

R8

320

C5

.1uF

C6

.1uF

0 00

00

0

0

V1

0Vdc

0

V2

1.5Vac

0Vdc

0

R9

320

R10

1k

+3

-2

V+

7

V-

4

OUT6

OS11

OS25

U5

LM741

+3

-2

V+

7

V-

4

OUT6

OS11

OS25

U6

LM741

Figure 12:Tuner Schematic

Experiment

The actual built circuit is shown in Figure 13. As it can be seen almost all

voltages are close to the ones simulated in PSPICE. The output voltage is -4.2 mV;

ideally this would be equal to zero but -4.2 mV is small enough not to burn the speaker.

There were several problems associated with the original circuit so we had to change the

biasing resistors in order to achieve the desired output. After obtaining a good result on

the bread board we built our circuit on the PCB board. The most challenging part of our

project was probably troubleshooting the PCB board. We had to build the circuit twice

and make sure that we did not have a cold solder anywhere. In fact after rebuilding the

circuit the second time the circuit was not working because one side of the 47uF

capacitor was not soldered correctly. One of our recommendations for future student is

that they should make sure that every single item is properly soldered.


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R1

1k

R2

1k

R4

100k

R5

510

R6

1k R7

1.5k

R8

10k

R9

100k

R10

1

R11

1k

R12

1

C1

2.2uF

C2

47uF

Q1Q2N3904

Q2

Q2N3904

Q3

Q2N3906

R13

10kSET = 6.5k

Q4

TIP29

Q5

TIP30

1

2

D1

1N4500

1

2

D2

1N4500

1

2

D3

1N4500

V1

12V

V2

12V

V3

FREQ = 1000HzVAMPL = 1VVOFF = 0

0

0

0

0 0

R14

8

0

12 V

-12 V

11.24 V

10.85V

-950 mV

-280 mV

-1.01 V

-1.70 V

-300 mV -4.2 mV

10.61 V

940 mV

310 mV

-290 mV

280 mV

-920 mV

-290 mV

Figure 13: Actual DC Voltages.

Figure 14: Gain Amplitude vs. Frequency.


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Figure 15: Gain Phase vs. Frequency

Figure 14 and 15 represent the Amplitude and Phase of the Gain plotted against

the Frequency. The plots were done using LabView and as it can be seen the log-log plot

of the Amplitude increases smoothly and settles around 21; while the linear-log plot of

the phase has a steep slope, but since the circuit is far from being ideal this is an

acceptable result.

Input and Output Impedance

In order to measure the input impedance we used Kirchoff's current law after

inserting a 10K resistor at the input. The picture shown below shows the scheme that was

used:

V41Vac

0

0

R15

10k

Rin

x

V0

V1


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The procedure we followed was to measure the V0 and V1 shown in the picture and use

the following equation to solve for Rin:

(V0 V1) / 10K = V0 / Rin

Rin = (V0*10K) / (V0 V1)

For input amplitude of 200mV and frequency of 1000 Hz the input impedance we

obtained was: Rin = (580 mV * 10K) / (630 mV 580 mV) = 116 K. This value is

very close to the one obtained using PSPICE which was 100K.

The following table list input impedances measured at different frequencies.

Frequency (Hz) Input Impedance (K)

15 109

200 112

5400 115

10256 115

40000 116

Table 1: Input Impedance as a function of Frequency

The Output Impedance was measured using a similar scheme as the one used to measurethe input impedance. The pictures shown below show the scheme that was used:

Rout

x

R1

y

V1

V0

0

Rout

x

R2

y

V2

V0

0

By measuring V1 and V2 and using known resistor values R1 and R2, we plugged the

values in the following equations and solved them for Rout:

1. (V0 V1) / Rout = V1 / R1 2. (V0 V2) / Rout = V2 / R2

After equating equation 1 and 2, we solved them for Rout. The result we obtained is the

following:

Rout = (V1- V2) / [(V2/R2) (V1/R1)]


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Table 2 shows the Output Impedance for several values of input Frequency:

Freq (KHz) R1() V1(mV) R2() V2(mV) Rout()

1.040 13.6 59.9 5.6 57.8 0.355

5.295 13.6 56.9 5.6 54.9 0.356

9.927 13.6 58.3 5.6 56.2 0.365

18.684 13.6 58.8 5.6 57.5 0.219

Table 2: Output Impedance as a function of Frequency

As it can be seen from the table above all Output Impedance values are very close to zero

as desired.

The following are pictures of the PCB circuit that we built.

Figure 16: Actual Audio Amplifier (top view)


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Figure 17: Actual Audio Amplifier (side view)

Discussions

The main obstacle that we encountered with our circuit was not with the design

but with its real world application. We found that PSPICE can really only be a guide or

reference tool used to give the general expected results. When the design gives the

desired results in PSPICE, the challenge is achieving those results on the breadboard.

Once we built our circuit on the breadboard we spent a considerable amount of

time tweaking our design to get the desired gain at the output. Most of our problems

rooted in voltage and current biasing problems. We went through a series of labs testing

a seemingly infinite amount of resistor value configurations until we achieved a

functional output. The only remaining problem lied in an output voltage that was too

large. We remedied this problem by placing a potentiometer before the emitters in our

differential amp stage which made the granular adjustment needed to correct our circuits

biasing currents.


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With pleasing breadboard results, we moved on to building our PC board. After

constructing a malfunctioning channel in route to our final working board, the main thing

we learned was that the physical quality of the board is what characterizes your results. It

is of optimal concern to draw a layout that allows clean copper paths followed by solid

solder connections in order to attain the breadboard results.

Overall, this lab gave us a real opportunity to experience what it is like to

design and build a real world circuit. It gave us the necessary benefit of realizing the

circuits we have been studying only in textbooks in previous classes at the University.

Future projects may have involved building amplifiers of different classes or selective

tuners like those used in AM/FM radios.

Documents

Audio Amplifier Report9