Project Report

PROJECT REPORT ON STEREO BRICK AMPLIFIER

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Chapter 1 RF AMPLIFIER

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INTRODUCTION

An RF power amplifier is a type of electronic amplifier used to convert a low-power

radio-frequency signal into a larger signal of significant power, typically for driving the

antenna of a transmitter. It is usually optimized to have high efficiency, high output

Power (P1dB) compression, good return loss on the input and output, good gain, and

optimum heat dissipation.

Gain

The gain of an amplifier is the ratio of output to input power or amplitude, and is usually

measured in decibels. (When measured in decibels it is logarithmically related to the

power ratio: G (dB) =10 log (Pout / (Pin)). RF amplifiers are often specified in terms of

the maximum power gain obtainable, while the voltage gain of audio amplifiers and

instrumentation amplifiers will be more often specified (since the amplifier's input

impedance will often be much higher than the source impedance, and the load impedance

higher than the amplifier's output impedance).

* Example: an audio amplifier with a gain given as 20 dB will have a voltage gain of

ten (but a power gain of 100 would only occur in the unlikely event the input and output

impedances were identical).

If two equivalent amplifiers are being compared, the amplifier with higher gain settings

would be more sensitive as it would take less input signal to produce a given amount of

power.

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Bandwidth

The bandwidth of an amplifier is the range of frequencies for which the

amplifier gives "satisfactory performance". The definition of "satisfactory

performance" may be different for different applications. However, a common and

well-accepted metric is the half power points (i.e. frequency where the power goes

down by half its peak value) on the output vs. frequency curve. Therefore bandwidth

can be defined as the difference between the lower and upper half power points. This

is therefore also known as the −3 dB bandwidth. Bandwidths (otherwise called

"frequency responses") for other response tolerances are sometimes quoted (−1 dB, −6

dB etc.) or "plus or minus 1dB" (roughly the sound level difference people usually can

detect).

The gain of a good quality full-range audio amplifier will be essentially flat

between 20 Hz to about 20 kHz (the range of normal human hearing). In ultra high

fidelity amplifier design, the amp's frequency response should extend considerably

beyond this (one or more octaves either side) and might have −3 dB points < 10 and >

65 kHz. Professional touring amplifiers often have input and/or output filtering to

sharply limit frequency response beyond 20 Hz-20 kHz; too much of the amplifier's

potential output power would otherwise be wasted on infrasonic and ultrasonic

frequencies, and the danger of AM radio interference would increase. Modern

switching amplifiers need steep low pass filtering at the output to get rid of high

frequency switching noise and harmonics.

Efficiency

Efficiency is a measure of how much of the power source is usefully applied to

the amplifier's output. Class A amplifiers are very inefficient, in the range of 10–20%

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with a max efficiency of 25% for direct coupling of the output. Inductive coupling of

the output can raise their efficiency to a maximum of 50%.

Class B amplifiers have a very high efficiency but are impractical for audio

work because of high levels of distortion (See: Crossover distortion). In practical

design, the result of a tradeoff is the class AB design. Modern Class AB amplifiers are

commonly between 35–55% efficient with a theoretical maximum of 78.5%.

Commercially available Class D switching amplifiers have reported

efficiencies as high as 90%. Amplifiers of Class C-F are usually known to be very

high efficiency amplifiers.

More efficient amplifiers run cooler, and often do not need any cooling fans

even in multi-kilowatt designs. The reason for this is that the loss of efficiency

produces heat as a by-product of the energy lost during the conversion of power. In

more efficient amplifiers there is less loss of energy so in turn less heat.

In RF Power Amplifiers, such as cellular base stations and broadcast

transmitters, specialist design techniques are used to improve efficiency. Doherty

designs, which use a second transistor, can lift efficiency from the typical 15% up to

30-35% in a narrow bandwidth. Envelope tracking designs is able to achieve

efficiencies of up to 60%, by modulating the supply voltage to the amplifier in line

with the envelope of the signal.

Linearity

An ideal amplifier would be a totally linear device, but real amplifiers are only

linear within limits.

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When the signal drive to the amplifier is increased, the output also increases

until a point is reached where some part of the amplifier becomes saturated and cannot

produce any more output; this is called clipping, and results in distortion.

In most amplifiers a reduction in gain takes place before hard clipping occurs;

the result is a compression effect, which (if the amplifier is an audio amplifier) sounds

much less unpleasant to the ear. For these amplifiers, the 1 dB compression point is

defined as the input power (or output power) where the gain is 1 dB less than the small

signal gain. Sometimes this nonlinearity is deliberately designed in to reduce the

audible unpleasantness of hard clipping under overload.

The problem of nonlinearity is most often solved with negative feedback.

Linearization is an emergent field, and there are many techniques, such as feed

forward, predistortion, post distortion, in order to avoid the undesired effects of the

non-linearities.

Noise

This is a measure of how much noise is introduced in the amplification

process. Noise is an undesirable but inevitable product of the electronic devices and

components; also, much noise results from intentional economies of manufacture and

design time. The metric for noise performance of a circuit is noise figure or noise

factor. Noise figure is a comparison between the output signal to noise ratio and the

thermal noise of the input signal.

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Output dynamic rangeOutput dynamic range is the range, usually given in dB, between the smallest

and largest useful output levels. The lowest useful level is limited by output noise,

while the largest is limited most often by distortion. The ratio of these two is quoted as

the amplifier dynamic range. More precisely, if S = maximal allowed signal power

and N = noise power, the dynamic range DR is DR = (S + N) /N. [2]

In many switched mode amplifiers, dynamic range is limited by the minimum

output step size.

Slew rateSlew rate is the maximum rate of change of the output, usually quoted in volts

per second (or microsecond). Many amplifiers are ultimately slew rate limited

(typically by the impedance of a drive current having to overcome capacitive effects at

some point in the circuit), which sometimes limits the full power bandwidth to

frequencies well below the amplifier's small-signal frequency response.

Rise timeThe rise time, tr, of an amplifier is the time taken for the output to change from

10% to 90% of its final level when driven by a step input. For a Gaussian response

system (or a simple RC roll off), the rise time is approximated by:

tr * BW = 0.35, where tr is rise time in seconds and BW is bandwidth in Hz.

Settling time and ringing

The time taken for the output to settle to within a certain percentage of the final

value (for instance 0.1%) is called the settling time, and is usually specified for

oscilloscope vertical amplifiers and high accuracy measurement systems. Ringing refers

to an output variation that cycles above and below an amplifier's final value and leads to

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a delay in reaching a stable output. Ringing is the result of overshoot caused by an

under damped circuit.

Overshoot In response to a step input, the overshoot is the amount the output exceeds its final,

steady-state value.

Stability Stability is an issue in all amplifiers with feedback, whether that feedback is added

intentionally or results unintentionally. It is especially an issue when applied over

multiple amplifying stages.

Stability is a major concern in RF and microwave amplifiers. The degree of an

amplifier's stability can be quantified by a so-called stability factor. There are several

different stability factors, such as the Stern stability factor and the Linvil stability factor,

which specify a condition that must be met for the absolute stability of an amplifier in

terms of its two-port parameters.

Applications

The basic applications of the RF power amplifier include driving to another

high power source, driving a transmitting antenna, microwave heating, and exciting

resonant cavity structures. Among these applications, driving transmitter antennas is

most well known. The transmitter–receivers are used not only for voice and data

communication but also for weather sensing (in the form of RADAR). Microwave or

RF heating is an industrial application which is also benefiting our homes in the form

of microwave ovens. Exciting cavity resonators is quite a research lab and industrial

application of an RF source. Particle accelerators utilize RF sources extensively.

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ABSTRACT

If you are looking for a compact and inexpensive add-on amplifier for your

digital surround sound system or computer, then stereo brick amplifier is suitable for

your needs. It delivers 15w per channel in stereo mode and about 29w in bridge mono

mode from a box as small as a house brick.

Many low-cost DVD players available in the market do not have in-

built audio amplifiers. To enjoy stereophonic sound, you have to buy separate left and

right speakers. Moreover, to play a DVD in Dolby Digital format, you would probably

need five amplifiers and speakers, one for each of the channels.

Stereo brick amplifier can upgrade the audio system according to your

personal taste. It helps a low-cost DVD player to play excellent Dolby Digital (AC-3)

surround sound. Dolby Digital ‘5.1 channel’ system have separate channel outputs

with the centre-front signal quite different from the front ,left and right ones. Many

DVDs have main screen dialogues on the centre-front track, so to hear dialogues, you

need a centre front amplifier and speaker. Besides there are separate rear change

signals with Dolby Digital for MPEG and DTS encoding, and to hear them you need

two separate amplifiers and speakers. The subwoofer track (low frequency effect) is

quite discrete in Dolby Digital encoding. It delivers most of the grunt for explosions.

So if you watch an action movie without subwoofer, you miss out most of the sound

effects.

In brief in order to enjoy Dolby Digital format movies, one needs three

extra speakers and matching amplifiers apart from stereo system. Getting extra

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speakers at affordable price is not a problem, but it is not the case with add-on

amplifier for a stereo or bridged mono system. The circuit provides two 15W RMS

power amplifier channels, with low distortion noise and a good frequency response.

The input sensitivity is good, too. You need around 200mV input to produce full

output, suitable for use with line output on DVD players with in-built Dolby Digital

decoders.

In fact, one or two brick amplifiers can be used with the DVD player to

get an effective output from your surround sound system. This amplifier is also

suitable for enhancing multi media in a computer, allowing you to play external

speakers.

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Fig: stereo brick amplifier circuit

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Working

The brick amplifier circuit is simple. It consists of power supply and

audio power amplifier sections. The simple scheme of the two channels to be driven

from one of the inputs facilitates a hassle-free bridge mono mode operation. The

circuit of brick amplifier consists of TL072 low-noise. JFET dual operational

amplifier(IC1) and two LM1875(IC2 and IC3) power amplifiers.

In stereo mode, each half of the TL072 operates as a non-

inverting input amplifier, with a gain of about 2.8 times as determined by feedback

resistors R5 and R6. In front of each input amplifiers there is a low pass filter for RF

SUPRESSION (R1/C1 and R15/C12), followed by volume controls VR1 and VR2 and

blocking capacitors C2 and C13.

Switch S1 is used to select bridge mono mode operation. When it is

switched to mono position. ‘M’, the non-inverting input of IC1 (B) is connected to the

ground, making the left input of the brick ineffective. However, at inverting terminal

of IC1 (B), the cold end of resistor R13 is disconnected from the ground and

connected to R7. It is now converted into a unity gain inverting stage and the output of

IC1 (A) is connected to IC1 (B) through R7. This converts the front end to produce

dual phase drive signals from the right channel input. In stereo mode, each of the

amplifier stage drives the right and the left speaker connected across pin4 of IC2 and

IC3, respectively.

Each input stage drives one of the LM1875 power stages (IC2 and IC3)

with traditional circuitry. Coupling capacitors C7 and C14 provide DC blocking, with

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the power stage voltage gain set to around 18 by feedback networks R9/R10 and

R20/R21.

Resistors R8 and R19provide biasing for the non-inverting inputs, while

C8/C15 provides AC grounding for the feedback networks.

R11 and C9, and R22 and C18, are Zobel impedance stabilizing circuits

used across power amplifier output to ensure maximum AC stability. Output stages

operate from DC rails of +21V,-21V which are provided from the power supply, using

21V-0-21V, 2A centre-tapped transformer. The transformer drives the bridge rectifier

module BR1 and four 2200uF capacitors to provide +21V and -21V DC supplies. The

left and the right channel input stages formed by the TL072 op-amp require+12V,-

12V DC supplies for the operation. while +12V is obtained from resistor R12 and

zener diode ZD1,-12V is obtained from resistor R23 and zener diode ZD2.

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List of components

Semi conductors

IC1 - TL072 dual op-amp

IC2,IC3 -LM1875 Power amplifier

ZD1, ZD2 -12V,1W Zener diode

BR1 -W04, 1.5A bridge rectifier module

LED1 - 5mm light emitting diode

Resistors

R1,R5,R7,R15,R13 -2.7-kilo-ohm

R2,R16 -100-ohm

R3,R4,R17,R18 -270-kilo-ohm

R6,R14 -5.1-kilo-ohm

R8,R19,R10,R21 -82-kilo-ohm

R9,R20 -4.7-kilo-ohm

R11,R22 -2.7-ohm

R12,R23 -470-ohm

VR1,VR2 -100-kilo-ohm log

potentiometer

Capacitors

C1,C12 - 470 Pf

C2,C13 - 100uF,50V electrolytic

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C3,C5,C10,C16,C19,C21 -0.1 uF ceramic

C4,C20 -1000uF,50V electrolytic

C7,C14 - 1uF ceramic,25V electrolytic

C8,C15 - 4.7uF,50V electrolytic

C6,C17,C11,C22 - 220uF,50V electrolytic

C9,C18 - 0.22uF ceramic

C23,C26 - 2200uF,50V electrolytic

Miscellaneous

S1 - DPDT switch

S2 - On/off switch

F1 -2A power fuse

X1 - 230V AC primary to 21V-0-21V,2A secondary centre-tapped

transformer

-Metal case

-Finned heatsink

-RCA connectors

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DESCRIPTION OF COMPONENTS

9.1 RESISTENCE

The jobs done by resistors include directing and controlling current, making

changing current produce changing voltage (as in a voltage amplifier) and obtaining

variable voltages from fixed ones (as in a potential divider). There are two main types

of resistor-those with fixed values and those that are variable.

Resistance is the opposition of a material to the current. It is measured in Ohms (). All

conductors represent a certain amount of resistance, since no conductor is 100% efficient.

To control the electron flow (current) in a predictable manner, we use resistors.

Electronic circuits use calibrated lumped resistance to control the flow of current.

Broadly speaking, resistor can be divided into two groups viz. fixed & adjustable

(variable) resistors. In fixed resistors, the value is fixed & cannot be varied. In variable

resistors, the resistance value can be varied by an adjuster knob. It can be divided into (a)

Carbon composition (b) Wire wound (c) Special type. The most common type of resistors

used in our projects is carbon type. The resistance value is normally indicated by color

bands. Each resistance has four colors, one of the band on either side will be gold or

silver, this is called fourth band and indicates the tolerance, others three band will give

the value of resistance (see table). For example if a resistor has the following marking on

it say red, violet, gold. Comparing these colored rings with the color code, its value is

27000 ohms or 27 kilo ohms and its tolerance is ±5%. Resistor comes in various sizes

(Power rating). The bigger the size, more is the power rating of 1/4 watts. The four color

rings on its body tells us the value of resistor value as given below.

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When choosing a resistor there are three factors which have to be considered, apart

from the stated value.

The Tolerance

Exact values cannot be guaranteed by mass-production methods but this is snot a

great disadvantage because in most electronic circuits the values of resistors are not

critical. The tolerance tells us the minimum and maximum values a resistor might

have, e.g. one with a stated (called nominal) value of 100 and a tolerance of +-10%

could have any value between 90 and 110

The Power Rating

If the rate which a resistor changes electrical energy into heat exceeds its power rating,

it will overheat and be damaged or destroyed. For most electronic circuit 0.25 Watt or

0.5 Watt power ratings are adequate. The greater the physical size of a resistor the

greater is its rating.

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The Stability

This is the ability of a component to keep the same value as it ‘ages’ despite

changes of temperature and other physical conditions. In some circuits this is an

important factor.

Color Coding

Black-----------------------------------------------------0

Brown----------------------------------------------------1

Red-------------------------------------------------------2

Orange---------------------------------------------------3

Yellow---------------------------------------------------4

Green-----------------------------------------------------5

Blue-------------------------------------------------------6

Violet-----------------------------------------------------7

Grey------------------------------------------------------8

White-----------------------------------------------------9

The first rings give the first digit. The second ring gives the second digit. The third

ring indicates the number of zeroes to be placed after the digits. The fourth ring gives

tolerance (gold ±5%, silver ± 10%, No color ± 20%).In variable resistors, we have the

dial type of resistance boxes. There is a knob with a metal pointer. This presses over

brass pieces placed along a circle with some space b/w each of them.

Resistance coils of different values are connected b/w the gaps. When the knob is

rotated, the pointer also moves over the brass pieces. If a gap is skipped over, its

resistance is included in the circuit. If two gaps are skipped over, the resistances of both

together are included in the circuit and so on.

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A dial type of resistance box contains many dials depending upon the range,

which it has to cover. If a resistance box has to read upto 10,000ohms, it will have three

dials each having ten gaps i.e. ten resistance coils each of resistance 10ohms. The third

dial will have ten resistances each of 100ohms.The dial type of resistance boxes is

better because the contact resistance in this case is small & constant.

Capacitors

A capacitor stores electric charge. It does not allow direct current to flow

through it and it behaves as if alternating current does flow through. In its simplest

form it consists of two parallel metal plates separated by an insulator called the

dielectric. The symbols for fixed and variable capacitors are given in fig. Polarized

types must be connected so that conventional current enters their positive terminal.

Non-polarized types can be connected either way round.

The capacitance (C) of a capacitor measures its ability to store charge and is

stated in farads (f). The farad is sub-divided into smaller, more convenient units.

1 microfarad (1uF) = 1 millionth of a farad = 10-6 F

1 nanofarad (1 nF) = 1 thousand- millionth of a farad = 10-9 F

1 picofarad (1pF) = 1 million-millionth of a farad = 10-12 F

In practice, capacitances range from 1 pF to about 150 000 uF: they depend on

the area A of the plates (large A gives large C), the separation d of the plates (small d

gives large C) and the material of the dielectric (e.g. certain plastics give large C).

When selecting a particular job, the factors to be considered are the value

(again this is not critical in many electronic circuits), the tolerance and the stability.

There are two additional factors.

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The working voltage

The largest voltage (d.c or peak a.c) which can be applied across the capacitor

and is often marked on it, e.g. 30V wkg. If it is exceeded, the dielectric breaks down

and permanent damage may result.

The leakage current

No dielectric is a perfect insulator but the loss of charge through it as leakage

current’ should be small. Fixed Capacitors

Fixed capacitors can be classified according to the dielectric used; their

properties depend on this. The types described below in (i), (ii) and (iii) are non-

polarized, those in (iv) are polarized.

(i) Polyester: Two strips of polyester film (the plastic dielectric) are wound

between two strips of aluminum foil (the plates). Two connections, one to each strip of

foil, form the capacitor leads. In the metalized version, films of metal are deposited on

the plastic and act as the plates. Their good all-round properties and small size make

them suitable for many applications in electronics. Values range from 0.01uF to 10uF

or so and are usually marked (in pF) using the resistor color code. Polycarbonate

capacitors are similar to the polyester type; they have smaller leakage currents and

better stability but cost more.

(ii) Mica: Mica is naturally occurring mineral, which splits into very thin

sheets of uniform thickness. Plates are formed by depositing a silver film on the mica

or by using interleaving sheets of aluminum foil. Their tolerance is low ( + 1% ),

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stability and working voltage high, leakage current low but they are used in radio

frequency tuned circuits where low loss is important and are pictured in figs.

Polystyrene capacitors have similar though not quite so good properties as mica types

but are cheaper.

(iii) Ceramic. There are several types depending on the ceramic used. One

type has similar properties to mica and is used in radio frequency circuits. In another

type, high capacitance values are obtained with small size, but stability and tolerance are

poor; they are useful where exact values are not too important. They may be disc, rod- or

plate-shaped. A disc-shaped capacitor is shown below

Diode

The simplest semiconductor device is made up of a sandwich of P-type semiconducting

material, with contacts provided to connect the p-and n-type layers to an external circuit.

This is a junction Diode. If the positive terminal of the battery is connected to the p-type

material (cathode) and the negative terminal to the N-type material (Anode), a large

current will flow. This is called forward current or forward biased.

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If the connections are reversed, a very little current will flow. This is because under this

condition, the p-type material will accept the electrons from the negative terminal of the

battery and the N-type material will give up its free electrons to the battery, resulting in

the state of electrical equilibrium since the N-type material has no more electrons. Thus

there will be a small current to flow and the diode is called Reverse biased.

Thus the Diode allows direct current to pass only in one direction while blocking it in the

other direction. Power diodes are used in concerting AC into DC. In this, current will

flow freely during the first half cycle (forward biased) and practically not at all during the

other half cycle (reverse biased). This makes the diode an effective rectifier, which

convert ac into pulsating dc. Signal diodes are used in radio circuits for detection. Zener

diodes are used in the circuit to control the voltage.

Fig: Symbols & representation of diodes

ZENER DIODE:-

A zener diode is specially designed junction diode, which can operate

continuously without being damaged in the region of reverse break down voltage. One of

the most important applications of zener diode is the design of constant voltage power

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supply. The zener diode is joined in reverse bias to d.c. through a resistance R of suitable

value.

PHOTO DIODE:-

A photo diode is a junction diode made from photo- sensitive semiconductor or

material. In such a diode, there is a provision to allow the light of suitable frequency to

fall on the p-n junction. It is reverse biased, but the voltage applied is less than the break

down voltage. As the intensity of incident light is increased, current goes on increasing

till it becomes maximum. The maximum current is called saturation current.

LIGHT EMITTING DIODE (LED):-

When a junction diode is forward biased, energy is released at the junction diode

is forward biased, energy is released at the junction due to recombination of electrons and

holes. In case of silicon and germanium diodes, the energy released is in infrared region.

In the junction diode made of gallium arsenate or indium phosphide, the energy is

released in visible region. Such a junction diode is called a light emitting diode or LED.

POWER SUPPLYIntroduction

In alternating current the electron flow is alternate, i.e. the electron flow

increases to maximum in one direction, decreases back to zero. It then increases in the

other direction and then decreases to zero again. Direct current flows in one direction

only. Rectifier converts alternating current to flow in one direction only. When the anode

of the diode is positive with respect to its cathode, it is forward biased, allowing current

to flow. But when its anode is negative with respect to the cathode, it is reverse biased

and does not allow current to flow. This unidirectional property of the diode is useful for

rectification. A single diode arranged back-to-back might allow the electrons to flow

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during positive half cycles only and suppress the negative half cycles. Double diodes

arranged back-to-back might act as full wave rectifiers as they may allow the electron

flow during both positive and negative half cycles. Four diodes can be arranged to make a

full wave bridge rectifier. Different types of filter circuits are used to smooth out the

pulsations in amplitude of the output voltage from a rectifier. The property of capacitor to

oppose any change in the voltage applied across them by storing energy in the electric

field of the capacitor and of inductors to oppose any change in the current flowing

through them by storing energy in the magnetic field of coil may be utilized. To remove

pulsation of the direct current obtained from the rectifier, different types of combination

of capacitor, inductors and resistors may be also be used to increase to action of filtering.

Need of power supply

Perhaps all of you are aware that a ‘power supply’ is a primary requirement for the ‘Test

Bench’ of a home experimenter’s mini lab. A battery eliminator can eliminate or replace

the batteries of solid-state electronic equipment and the equipment thus can be operated

by 230v A.C. mains instead of the batteries or dry cells. Nowadays, the use of

commercial battery eliminator or power supply unit has become increasingly popular as

power source for household appliances like transreceivers, record player, cassette players,

digital clock etc.

Bridge Rectifier A more widely used full-wave rectifier circuit is the bridge rectifier. It requires

four diodes instead of two, but avoids the need for a centre-tapped transformer. During

the positive half-cycle of the secondary voltage, diodes D2 and D4 are conducting and

diodes D1 and D3 are non-conducting. Therefore, current flows through the secondary

winding, diode D2, load resistor RL and diode D4. During negative half-cycles of the

secondary voltage, diodes D1 and D3 conduct, and the diodes D2 and D4 do not conduct.

The current therefore flows through the secondary winding, diode D1, load resistor RL

and diode D3. In both cases, the current passes through the load resistor in the same

direction. Therefore, a fluctuating, unidirectional voltage is developed across the load.

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Filtration

The rectifier circuits we have discussed above deliver an output voltage that

always has the same polarity: but however, this output is not suitable as DC power

supply for solid-state circuits. This is due to the pulsation or ripples of the output

voltage. This should be removed out before the output voltage can be supplied to any

circuit. This smoothing is done by incorporating filter networks. The filter network

consists of inductors and capacitors. The inductors or choke coils are generally

connected in series with the rectifier output and the load. The inductors oppose any

change in the magnitude of a current flowing through them by storing up energy in a

magnetic field. An inductor offers very low resistance for DC whereas; it offers very

high resistance to AC. Thus, a series connected choke coil in a rectifier circuit helps to

reduce the pulsations or ripples to a great extent in the output voltage. The fitter

capacitors are usually connected in parallel with the rectifier output and the load. As,

AC can pass through a capacitor but DC cannot, the ripples are thus limited and the

output becomes smoothed. When the voltage across its plates tends to rise, it stores up

energy back into voltage and current. Thus, the fluctuations in the output voltage are

reduced considerable. Filter network circuits may be of two types in general:

TL072 dual op-amp

DescriptionThe JFET-input operational amplifiers in the TL07x series are designed as

low-noise versions of the TL08x series amplifiers with low input bias and offset

currents and fast slew rate. The low harmonic distortion and low noise make the

TL07x series ideally suited for high-fidelity and audio preamplifier applications. Each

amplifier features JFET inputs (for high input impedance) coupled with bipolar output

stages integrated on a single monolithic chip.

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The C-suffix devices are characterized for operation from 0°C to 70°C. The I-

suffix devices are characterized for operation from –40°C to 85°C. The M-suffix

devices are characterized for operation over the full military temperature range of –

55°C to 125°C.

FeaturesLow Power Consumption

Wide Common-Mode and Differential Voltage Ranges

Low Input Bias and Offset Currents

Output Short-Circuit Protection

Low Total Harmonic Distortion ... 0.003% Typ

LowNoise

    Vn = 18 nV/ Hz Typ at f = 1 kHz

High Input Impedance . . . JFET Input Stage

Internal Frequency Compensation

Latch-Up-Free Operation

High Slew Rate . . . 13 V/µs Typ

Common-Mode Input Voltage Range Includes VCC+

LM187520W Audio Power Amplifier

General Description

The LM1875 is a monolithic power amplifier offering very low

distortion and high quality performance for consumer audio

applications.

The LM1875 delivers 20 watts into a 4or 8load on ±25V

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supplies. Using an 8load and ±30V supplies, over 30

watts of power may be delivered. The amplifier is designed

to operate with a minimum of external components. Device

overload protection consists of both internal current limit and

thermal shutdown.

The LM1875 design takes advantage of advanced circuit

techniques and processing to achieve extremely low distortion

levels even at high output power levels. Other outstanding

features include high gain, fast slew rate and a wide

power bandwidth, large output voltage swing, high current

capability, and a very wide supply range. The amplifier is

internally compensated and stable for gains of 10 or greater.

Features

Up to 30 watts output power

Low distortion: 0.015%, 1 kHz, 20 W

Wide power bandwidth: 70 kHz

Protection for AC and DC short circuits to ground

Thermal protection with parole circuit

High current capability: 4A

Wide supply range 16V-60V

Internal output protection diodes

94 dB ripple rejection

Plastic power package TO-2

Applications

High performance audio systems

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Bridge amplifiers

Stereo phonographs

Servo amplifiers

Instrument systems

PCB DESIGN AND LAYOUT

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Fig 1:A single-side,actual-size PCB layout for stereo brick amplifier

PCB Layout:

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Fig2 Component layout for the PCB

CONSTRUCTION

Construction steps involve PCB design, mounting of components on the PCB,

cabinet design, connecting the power supply to PCB and extending wires from the PCB

to the front and rear panels. Having conventional circuitry, the amplifier has to be

squeezed into a low-cost brick shaped box.21V-0-21V/2A power transformer occupies

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almost half of the internal space. To make room for the PCB of amplifier circuit in the

brick cabinet, part of power supply section is mounted on a separate PCB so that it can

be fixed on the rear side of the cabinet. Note that proper finned heatsinks should be used

for power amplifier ICs(LM1875) While designing PCB layout, there is need to

determine a proper location on the board to mount the heatsinks for IC2 and IC3.

A single-side, actual-size PCB is shown in PCB Fig1. and its component

layout in Fig.2 .The PCB for the power supply section can be separated from the

amplifier section by cutting along the straight line shown in Fig.1 and 2 layouts. The

steps should be followed sequence Mount the PCB terminals and connectors, followed

by low-profile components(resistors and zeners), then smaller capacitors such as

ceramics, and finally larger electrolytic capacitors. Ensure correct placement of zener

diodes in the PCB as per schematic diagrams. In fact, you may like to fit a good

quality 8 pin DIL socket to the broad to allow the amplifier IC1 to be conveniently

plugged in later on.

The PCB can fit inside the box by proper drilling, reaming and cutting

holes on the front and rear panels and the base of the cabinet. Note that besides

mounting holes in the rear panel for the speaker terminal, there should be clearance

holes for other terminals also . On the front panel, the holes for volume control,

Mono/stereo switch and RCA input sockets should be large enough to accept plastic

insulating sleeves. Two holes have to be drilled for volume controls on the front panel.

Larger holes for the mains fuse holder (F1) and power switch(S2) are irregular. So, if

you prepare them by hand careful drilling is required besides a nibbling tool and/or

small jeweller’s files. Once the wires are connected, carefully place the PCB inside the

cabinet use the insulation sleeves, flat washers, lock washers and nuts, so that

everything is in place before screws are tightened. Mount volume control knobs into

their respective holes on the front panel. With the PCB fitted at the bottom of the

cabinet, bring speaker wires from connecting points, marked as LS1 and LS2,

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respectively, on the PCB, to the rear panel. Fit four screws along with suitable spacers

into the holes located in four corners of the cabinet bottom to tighten up the PCB

firmly in its place. You can place the power transformer inside the cabinet and tighten

it to the cabinet bottom with nuts and bolts.

Now take a pair of tweezers or needle nose pliers and carefully push the tip of

each wire through holes for left and right RCA input sockets and stereo/mono switch

firmly to the cabinet by tightening the respective nuts. Make sure that the wires are not

twisted together or unduly strained. Also, make sure that sufficient length of wires from

the PCB to the cabinet are provided for speakers, switches, volume controls and power

supply. Following these steps, solder each wire to the PCB pad and clip off any excess

parts. This essentially completes the wiring.

Testing brick amplifier

As there are no adjustments to be made, connect the amplifier to a pair of

speakers, source of power and provide mono or stereo signals, before listening to the

sound. It is good idea to perform a functional check of the power supply before IC

TL072 (IC1) is inserted into its socket on the PCB. So, turn on the power, then

measure the voltages across the supply, which should be +12V and -12V, respectively.

If all the four voltages fulfill the measurement criteria, then turn off the power, unplug

the mains cord, insert IC1 in the socket and fit the top of the cabinet.

If you are concerned about the proximity of the sleeved transformer’s

primary connections to the side of the cabinet, you may attach a protective inverted-L

cover of ‘elephant hide’ or a similar fireproof insulating sheet over the side facing the

transformer.

The brick amplifier is now ready for use.

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