audio amplifier coupled by infrared - polito.it · fig.1 audio amplifier coupled by infrared...

MINI-PROJECT OF ANALOG ELECTRONIC

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AUDIO AMPLIFIER COUPLED BY INFRARED POLITECNICO DI TORINO

ANALOG AND TELECOMUNICATION ELECTRONICS

June, 2016 Professor: Students: Dante Del Corso Martinez Rojas Alejandro – S225265


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I. INTRODUCTION

Necessary guideline and restrictions are established to realize design of an audio amplifier coupled infrared, it will

have two stages based on operational amplifiers and transistors. Then the simulation supports to corroborate the

data calculated for the circuit implementation is showed, specifications of each component assembly will be

designed. Furthermore, graphs and results obtained will be presented.

II. METHODOLOGY

Design is started from second stage back to ensure the output power of audio amplifier needed. Desired output power must be set, since it is needed to calculate the bias voltage circuit which Pre‐Amplifier and Driver should be amplifier. Current mirror topology in complementary arrangement is selected in order to intensify the output current and provide a low output resistance. After that, characteristics of the signal at its maximum excursion are determined by means of components. Finally, elements of the preamplifier stage are computed by using operational amplifiers (guarantee a high input impedance) connected to current mirror to guarantee a current enough to polarized the Infrared Diode.

Fig.1 Audio amplifier coupled by infrared schematic

III. DESIGN STAGE

Amplifier design will be performed by step; hence last stage will be the first stage which will be design in order to

guarantee the requirement chosen.

III.A. Power stage (complementary class ab amplifier arrangement)

Current mirror is used in order to minimize the output resistance and voltage load effect. Furthermore, it avoids the

crossover distortion.


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Fig. 2. Power amplifier schematic with Current mirror to avoid crossover

Darlington transistors were chosen to guarantee a better development with high current. In Fig.2, two transistors

are also short‐circuited in order to avoid crossover distortion, since if we do not use them, upper transistor will be

OFF when the input signal is fewer than 1.4V (threshold voltage), therefore output signal is zero until lower

transistor is ON, so with these short‐circuited transistors, transistors will be ON when input signal is lower than ‐

1.4V.

They are short‐circuited to cover them in diodes and guarantee the same thermal behavior.

Maximum output power V * IcSAT

Pout =(max)2

cc

Power amplifier should be able to generate an output around 5W. Then remembering that:

2

,

2O P

out

VP

R

Output voltage Output voltage necessary to obtain this output power (Psal >=5W). Considering RL=8 ohm, it will be the speaker resistance.

, 2(8 )(5 ) 9O PV W Vp

Output voltage is set in 10VP to guarantee 5W including losses; hence in real condition, saturation or non‐linearity

takes place, then in order to avoid saturation due to amplifier, 10% peak‐peak signal is introduced as a safety

margin, therefore to guarantee the output voltage set, the supply source selected is:

( ) ( )VCC VEE 10%Vo Vo

VCC VEE 2 20 22p p p p

V V V

VEE=VCC= 12V are chosen.

In the circuit, a single dual source dc is used for the two stages. V1=12v. Output current Output current with Vo=10v is:

( )

2 2

10 / 20.883

8

106.25

2 16

o RMS po

L

out Pout

L

V VI A

R

V VP W

R

Hoped value in the output will be: Pout=6.25W, Iout=0.883A and Vout=10Vp


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Saturation current is:

121.5

8SATL

Vcc VIc A

R

Saturation current DC is considered when 0Vce V . DISSIPATION POWER IN EACH TRANSISTOR

Sal( )

12 *1.59

2|

9 125

MAX

D D Especificada FabricantTransistor

D Transistor

V AP W

P P

P W W

Fig.3 Datasheet of transistor TIP142 to check the maximum power dissipation.

Power delivered by the source Maximum power delivered by source is determined by the following equation where =0.785 is the class AB

amplifier performance with current mirror.

(m a x ) 9

1 1 .4 6 50 .7 8 5

oD C D C

P WP P W

Current mirror (dc analysis)

Fig.4 Schematic of current mirror

Current mirror is performed in order to eliminate crossover distortion and achieve thermal stability, power devices Darlington TIP142 and TIP147 were selected, as well as for transistors which will be short‐circuited base‐collector terminals and used as diodes, this will achieve great reliable complementarity. Furthermore, trans‐conductance curves will be the same for transistor and diode. To polarize the diode is necessary to apply a current around 1% of output current.

0.01 0.01*(0.883 ) 9D DI Io I A mA

Base current computing 10

1.25* 1000 * 8

Pb b

ac L

VVcci i mA

R

ac is the minimum current gain of the transistor in accordance Darlington, the worst condition( ac =1000) is

chosen to guarantee a minimum current.

2 4

2

1 2 1 .41 0 3 4

1 0 .2 5

(1 0 .2 5 ) * 1 0 3 4 0 .1 0 9

B E

d io d e b a s e

R

V c c V v vR R R

i i m A

W m A W

Commercially R = 1.2k ohm was taken at 0.5W.

Load is a speaker which has an RL = 8 Ohm with an allowable power P = 5W power and Pr = 10W break.


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Specification of power devices

Power devices were implemented with respect to Darlington transistor, TIP142(NPN) and TIP147(PNP), then its

characteristics and parameters were specified.

( )

( )

1 2

( ) 1 .51 .9 1

0 .7 8 5

1 .9

C M A X

D M in

V c e V

I c s a t AI A

P W

Input resistor The input resistance is calculated from the equivalent circuit of Fig.4, shorting the capacitor reflecting RL to the base as B * RL. The diode has a resistance associated macroscopic Rf (Pol. Direct).

Fig.5. Equivalent circuit in the output

Output resistor

2 2

0.026 * ln2

2 ' 0.1350.883

2 * 0.135 0.27m

out out L

IoutIdRout re r e

g A

R R R

Voltage output is:

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* 0.98 *10 9.78 0.27 PVsal Vin V V , this will be the output voltage.

III.B. DRIVER STAGE Driver stage is implemented before the power stage in order to avoid a very high gain in preamplifier and avoid

non‐linear amplification, this is also needed to amplify the voltage signal after photo‐transistor. This is performed

based on operational amplifier.


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Fig.6 Schematic of driver circuit taking a phototransistor as a voltage supply

Assuming an input voltage around 280mV (25% of output voltage pre‐amplifier) to guarantee a reliable input taking

on a large loss between LED and photo transistor. 10

25 R in=3.3 K 400

25 * (3.3 K ) 82.5

P F

P IN

V RAv

mV R

Rf K

Rf value commercially 82K is selected.

Resistor R4 is approximately equal to Rin 3.3 k

Resistor R3 is positioned to avoid any damage to the photo‐transistor.

3

3 3

ie=3 mA is selected as emitter current enought to phototransistor

124 4.7

3

e

VccR

i

VR K R k

mA

Frequency Response

Operational amplifier has a passband filter behavior; hence cut‐off is needed to compute. Cut‐off frequency depend

on two factors: Gain and operational amplifier bandwidth.

MAX477 is chosen as operational amplifier, since it has a great bandwidth to guarantee a flat gain between audible

range frequency (20 to 20kHz).

BW=300MHz at Vout =2Vpp

300= =12MHz

25c

BW MHzf

G

III.C. PRE‐AMPLIFIRE STAGE


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Fig.7 Schematic of Pre‐amplifier.

In order to obtain an output voltage around 1.2Vrms, Vin=25mVrms is assumed.

48Vo Rf

AvVi Rin

Rin=2.7k was selected

Commercially is 150K 55.5Rf

AvRin

In order to avoid errors and compensate the grounding current, R1 is located whose value is:

5 5|| 2.7IN FR R R R K

An extra stage of current amplification is used in order to activate the IR diode LED. This stage will be a bidirectional

power amplifier with current mirror to avoid crossover distortion.

Bidirectional amplifier design

IR diode LED IR333‐A works between 2‐20mA, so desired current at the output will be approximately 15mAp with the maximum input voltage allowed (Vin=50mVp), this will be the worst condition, since this voltage is amplified until 1.5Vrms. Current bidirectional amplifier is used to amplify the negative signals and positive coming from the excitation source and current mirror to guarantee which transistors are always ON. Transistors are 2N3904 and 2N3906. Current mirror resistor

2

120.01 0.01* 1.2

100R DL

Vdd vI I mA

R

1.30.13

* 100 *100b bac L

Vo Vi i mA

R

Base current DC will have the pre‐amplifier circuit.

ac =100 is the minimum current gain of the transistor in order to guarantee the minimum current in the worst

condition, since normally this value will be a bit larger.

Resistor

It is used to limit the current which flows thought diode and base of transistor.


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2

1 2 0 .78 .6 9 8 .2

1 .3

(1 .3 ) * 8 .2 0 .0 1 4

B E

b a se D io d e

R

V c c VR k R k

i i m A

W m A k W

Output resistor Finally, for this extra circuit, a load resistance is added to avoid the LED is burned when input signal has the maximum value Vin=50mVp

2.1 / 298

15

100

poL

o

L

VVR

I mA

commercially R

Frequency Response

MAX477 is chosen as operational amplifier again to this stage

BW=300MHz at Vout =2Vpp and Gain=55.5

300= =5.4MHz

55.5c

BW MHzf

G

So, frequency response of the whole system will have the lowest cut‐off frequency between each stage, therefore

cut‐off frequency of system is Fc=5.4MHz. This is enough to have a flat gain.

Fig.7 Radiation pattern diagram.

IV. SIMULATIONS

PREAMPLIFICADOR

Simulation.1. Output voltage at Vi=25mV and 500Hz


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Simulation.2. Output voltage signal and input signal(left) and Bode diagram(right).

DRIVER

Simulation.3. Output voltage at Vi=280mV and 1kHz

Voltage source was used to simulated the photo transistor configured in common‐collector (gain=1).

Simulation.4. Output voltage signal and input signal(left) and Bode diagram(right).


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HARMONIC ANALYZE

Simulation.5. Harmonic results of pre‐amplifier circuit in current(right) and voltage(left)

Simulation.6. Harmonic results of DRIVER circuit in Voltage(left) and Current(right)

V. RESULTS

En la Tabla I se encuentra la confrontación de los parámetros obtenidos con los valore simulados del circuito y los

valores teoricos.

Parameter Computed Simulated Error

Gain 55.5 53.7 3.2%

Iout 15mA 1.8mA 88%

Vout 1.38Vrms 1.32Vrms 4.3%

Fcut‐off 5.4MHz 6.34MHz 14.8%

Table 2. Pre‐amplifier circuit results


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Parameter Computed Simulated Error

Gain 25 23 8%

Power 6.25W 5.74W 8.2%

Vout 10VP 9VP 10%

Fcut‐off 12MHz 3.53MHz 70.5%

Table 3. Driver circuit results

Based on table 2 and 3, the parameters with a high performance were in preamplifier gain with an error of 3.2%,

although there is one with 88%, but that is due to the fact that internal resistance which the diode is simulated,

however this good perforce is in spite of pre‐amplifier is working with small signal.

The worst performance was at driver circuit, since it was working with large signal which induces distortion or non‐

linearity.

VI. CONCLUSION

Output voltage (table.3) shows a small error with respect to others, nevertheless this is a bad signal, in other

word the top of signal is being saturated as we can observe in simulation.4.

Diode transmitter should be in opposite direction with photo transistor to avoid large losses, since we can

see in fig.7 the receiver will be portion of signal if this is not completely addressed to photo transistor.

it is observed that the pre‐amplifier was designed successfully because the output waveform is almost pure

sinusoidal(simulation.2) with little distortion due to LED, and the magnitude value is close to that expected.

Preamplifier stage has a low‐pass behavior. Audio signals have a range between 20 Hz and 20 Khz and our

design could amplify this range with losses.

It is also clear that the circuit is strongly influenced by harmonics due to the fact that large signal used,

nonetheless simulated value THD was low and harmonic components magnitudes too(simulation.6),

therefore it will not have some terrible problem refer to non‐linearity, although we have not considered the

distortion which could contribute infrared coupling, since it photo diode and photo transistor are non‐linear

devices, thus supply a great number of harmonics as we can see simulation.5.

Frequency response results have great error; it is due to the fact that output signal voltage in each amplifier

reduce the ideal bandwidth, in other word if output signal increases, ideal bandwidth decrease, therefore

frequency response of driver has an error larger than pre‐amplifier, since ideal bandwidth was taken to

Vout=2Vpp.


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VII. REFERENCES

[1] Electronic design, C. J. Savant, 3ª Edition, (Editorial Prentice‐Hall [2] electronic devices and circuit theory, Boylestad, Robert L. & Nashelsky, Louis , 4ª ed. 1989 [3] Integrated electronics, Millman, Jacob & Halkias, Christos C, McGraw‐Hill Book Company New york (EE.UU.)

audio amplifier coupled by infrared - polito.it · fig.1 audio amplifier coupled by infrared...

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