OP- AMP

Operational Amplifier is a most prominent circuit consisting of differential amplifiers & is being used in variety of circuit applications.

Ideal OP AMP

• The input impedance is infinite.
• The output impedance is zero.
• The open-loop gain (A) is infinite.
• The bandwidth is infinite.
• The output voltage is zero when the input voltage difference is zero.

The Op-Amp produces an output voltage that is the difference between the two input terminals, multiplied by the gain A…

TYPES OF OP-AMP

• Low power

• Low noise

• Low offset

• High power

• High voltage

• High speed

• Single

• Dual

• Quad

OP-AMPs Applications:

• Amplify signals.

• Buffer signals.

• Integrate signals.

• Differentiate signals.

• Sum multiple signals.

• Make music very loud!

Some OP-AMP Circuits:

• SUMMING amplifier:

ü    Like Summing amplifier, we can have Subtractor also.

ü    The main point is where we are applying our core signals. i.e, on the inverting or Non inverting terminals.

• Instrumentation Amplifier:

• Differentiator:

MOSFET

Metal Oxide Semiconductor Field Effect Transistor

Also termed as Insulated Gate Field Effect Transistor

• MOSFET’s are unipoar conduction devices, conduction with one type of charge carrier, like a FET, but unlike a BJT.
• A MOSFET is a voltage controlled device like a FET. A gate voltage input controls the source to drain current.
• The MOSFET gate draws no continuous current, except leakage. However, a considerable initial surge of current is required to charge the gate capacitance.

MOSFET Structure

The MOSFET has source, gate, and drain terminals like the FET. However, the gate lead does not make a direct connection to the silicon.

The MOSFET gate is a metallic or polysilicon layer atop a silicon dioxide insulator.

In Figure (a) the MOS capacitor is placed between a pair of N-type diffusions in a P-type substrate. With no charge on the capacitor, no bias on the gate, the N-type diffusions, the source and drain, remain electrically isolated.

N-channel MOSFET (enhancement type): (a) 0 V gate bias, (b) positive gate bias.

MOSFET Capacitance

The gate bears a resemblance to a metal oxide semiconductor (MOS) capacitor in Figure.

• When charged, the plates of the capacitor take on the charge polarity of the respective battery terminals.
  • The lower plate is P-type silicon from which electrons are repelled by the negative (-) battery terminal toward the oxide, and attracted by the positive (+) top plate.
  • This excess of electrons near the oxide creates an inverted (excess of electrons) channel under the oxide.

  N-channel MOS capacitor: (a) no charge, (b) charged

  WORKING

  A positive bias applied to the gate, charges the capacitor (the gate).

  • An inversion region with an excess of electrons forms below the gate oxide.
  • The non-conducting, off, channel is turned on.

  The cross-section of an N-channel discrete MOSFET is shown in Figure

  N-channel MOSFET (enhancement type): (a) Cross-section, (b) schematic symbol.

  Following features makes MOSFETs majorly used in ICs :

  • High packing density
  • High Noise Margin
  • Low Power dissipation
  • High input resistance

  The doping profile is a cross-section, which may be laid out in a serpentine pattern on the silicon die. This greatly increases the area, and consequently, the current handling ability.

FET

The field effect transistor was proposed by Julius Lilienfeld in US patents in 1926 and 1933

A field effect transistor (FET) is a unipolar device, conducting a current using only one kind of charge carrier.

• The JFET source, gate, and drain correspond to the BJT’s emitter, base, and collector, respectively.
• Application of reverse bias to the gate varies the channel resistance by expanding the gate diode depletion region.

FET OPERATION

A voltage applied to the gate, input element, controls the resistance of the channel, the unipolar region between the gate regions.

• In an N-channel device, a heavy P-type region on both sides of the center of the slab serves as a control electrode, the gate. The gate is analogous to the base of a BJT.

• The depletion region extends more deeply into the channel side due to the heavy gate doping and light channel doping.
• The thickness of the depletion region can be increased by applying moderate reverse bias.
• Increasing the reverse bias V_GS will pinch-off the channel current. The channel resistance will be very high. This V_GS at which pinch-off occurs is V_P, the pinch-off voltage.
• The source and drain are interchangeable, and the source to drain current may flow in either direction for low level drain battery voltage (< 0.6 V).

JFET Circuit Symbol

Figure (b) shows the schematic symbol for an N-channel field effect transistor compared to the silicon cross-section at (a).

• The gate arrow points in the same direction as a junction diode.
• The “pointing” arrow and “non-pointing” bar correspond to P and N-type semiconductors, respectively.

Discrete FETs

Discrete devices are manufactured with the cross-section as shown in figure but only the FET part as above image shows the FET along with Battery terminals.

FET ICs

All three FET terminals are available on the top of the die for the integrated circuit version so that a metalization layer (not shown) can interconnect multiple components.

• Integrated circuit FET’s are used in analog circuits for the high gate input resistance.

TRANSISTOR AMPLIFIERS

There are four classes of operation for an amplifier. These are: A, AB, B and C.

Class A Operation

• Output signal is a 100% (or 360°) copy of the input signal
• Current in the output circuit must flow for 100% of the input signal time.
• Amplifier current flows for 100% of the input signal.

Class AB Operation

• Current flows in the device for 51% – 99% of the input signal, the amplifier is operating class AB.

• Class AB amplifiers have better efficiency and poorer fidelity than class A amplifiers.

Class B Operation

Class B amplifier operates for 50% of the input signal.

Class C Operation

• In this, Only a small portion of the input signal is present in the output signal.

• Since the transistor does not conduct except during a small portion of the input signal, this is the most efficient amplifier.
• It also has the worst fidelity.
• The output signal bears very little resemblance to the input signal.

• Class C amplifiers are used where the output signal need only be present during part of one-half of the input signal.

BJT

BJT

The bipolar transistor (BJT) was named because its operation involves conduction by two carriers: electrons and holes in the same crystal.

• The first bipolar transistor was invented at Bell Labs by William Shockley, Walter Brattain, and John Bardeen in 1947

The device in Figure has a pair of junctions, emitter to base and base to collector, and two depletion regions.

• It is customary to reverse bias the base-collector junction of a bipolar junction transistor as shown in Figure.

Note: This increases the width of the depletion region. There is no current flow, except leakage current, in the collector circuit.

Working of n-p-n TRANSISTOR

Current Amplification in a BJT

Few electrons injected by the emitter into the base of an NPN transistor fall into holes. Also, few electrons entering the base flow directly through the base to the positive battery terminal.

Modulating the small base current produces a larger change in collector current. If the base voltage falls below approximately 0.6 V for a silicon transistor, the large emitter-collector current ceases to flow.

• 100% emitter current splitting between the base as 1% and the collector as 99%. The emitter efficiency is known as α = I_C/I_E.

Transistor IC

The BJT die, a piece of a sliced and diced semiconductor wafer, is mounted collector down to a metal case for power transistors. Multiple transistors may be fabricated on a single die called an integrated circuit.

Let us do the basics of the BJT

• Bipolar transistors conduct current using both electrons and holes in the same   device.
  • Operation of a bipolar transistor as a current amplifier requires that the collector-base junction be reverse biased and the emitter-base junction be forward biased.
  • A transistor differs from a pair of back to back diodes in that the base, the center layer, is very thin. This allows majority carriers from the emitter to diffuse as minority carriers through the base into the depletion region of the base-collector junction, where the strong electric field collects them.
  • Thus TOTAL CURRENT  I_E = I_B + I_C
    • Emitter efficiency is improved by heavier doping compared with the collector. Emitter efficiency: α = I_C/I_E, 0.99 for small signal devices
    • Current gain is β=I_C/I_B, 100 to 300 for small signal transistors.

Breakdown Diode

Breakdown DIODES

‘Zener diode’ and ‘avalanche diode’ are terms often used interchangeably, with the former much more common. Both refer to breakdown of a diode under reverse bias.

• When a diode is reverse biased, very little current flows, and the diode is to a first order approximation an open circuit. As the reverse voltage is increased, a point is reached where there is a dramatic increase in current.

“Avalanche breakdown is caused by impact ionization of electron-hole pairs”.

o The efficiency of the avalanche effect is characterized by a so-called multiplication factor M that depends on the reverse voltage.

• Avalanche breakdown occurs in lightly-doped pn-junctions where the depletion region is comparatively long.

• The doping density controls the breakdown voltage.

• As the temperature increases, so does the reverse breakdown voltage.

“Zener breakdown occurs in heavily doped pn-junctions.”

• The heavy doping makes the depletion layer extremely thin.  So thin, in fact, carriers can’t accelerate enough to cause impact ionization.

• The temperature coefficient of the Zener mechanism is negative

• In a ‘Zener’ diode either or both breakdown mechanisms may be present

• At low doping levels and higher voltages the avalanche mechanism dominates.
• At heavy doping levels and lower voltages the Zener mechanism dominates.

At a certain doping level and around 6 V for Si, both mechanism are present with temperature coefficients that just cancel.

• Neither Zener nor avalanche breakdown are inherently destructive in that the crystal lattice is damaged. However, the heat generated by the large current flowing can cause damage, so either the current must be limited and/or adequate heat sinking must be supplied.

Junction Diodes

JUNCTION DIODES

PN junctions are fabricated from a monocrystalline piece of semiconductor with both a P-type and N-type region in proximity at a junction.

• Point contact diodes have superb high frequency characteristics, useable well into the microwave frequencies.
• Junction diodes range in size from small signal diodes to power rectifiers capable of 1000’s of amperes.
• The level of doping near the junction determines the reverse breakdown voltage. Light doping produces a high voltage diode.
• Even to this day, the point contact diode is a practical means of microwave frequency detection because of its low capacitance.
• Point contact diodes, though sensitive to a wide bandwidth, have a low current capability compared with junction diodes.

Basic Diode

Diodes are non-linear devices. This means that superposition does not apply to circuits containing diodes. When a voltage is applied across a resistor, current flows in proportion to the voltage where the proportionality factor is constant. The current-voltage (IV) characteristic for a resistor is shown in Figure and is expressed analytically by .

• For a diode, the I-V characteristic looks like Figure and is described analytically by the formula .

SEMICONDUCTOR MATERIALS AND PROPERTIES

Classification of Materials

1. Conductors : Good conductors of electricity
2. Insulators    : Bad conductors of electricity
3. Semiconductors : Conductivity lie between conductors and insulators

Materials

Silicon is the most proffered material for semiconductor materials:

• Ease of availability
• Cost of processing
• Higher Temperature range
• High resistivities than available counterparts
• Low leakage currents (dealt later)

TYPES OF SEMICONDUCTORS:

1. Intrinsic : Semiconductor in extremely pure form.
2. Extrinsic : Doped semiconductor material is known as extrinsic semiconductor

a)      N-type semiconductor – have pentavalent impurity (like- Arsenic)

b)      P-type semiconductor – have trivalent impurity (like Boron)

Conductivity and Mobility of INTRINSIC Semiconductor:

The current density in an intrinsic semiconductor, due to movement of electrons:

Jn = q.n.µ_n.E

Where,

q = charge on electron

n = electron concentration in intrinsic semiconductor

µ_n= mobility of electrons in semiconductor

E = applied electric field

Similarly, The current density in an intrinsic semiconductor, due to movement of holes:

Jp = q.p.µ_p.E

Where,

q = charge on electron

n = electron concentration in intrinsic semiconductor

µ_p= mobility of electrons in semiconductor

E = applied electric field

The Total Current Density ,

J = Jn  +  Jp

Using abve mentioned eqns :

The current density in an intrinsic semi conductor, due to movement of electrons:

J = q.n.µ_n.E + q.p.µ_p.E

J = qE(n.µ_n. + p.µ_p )

MASS ACTION LAW

Under thermal equilibrium for any semiconductor, the product of no. of holes and no. of electrons is constant and independent of the amount of donor and acceptor impurity.

n . p = n_i²

where,

n = number of free electrons per unit volume

p = number of holes per unit volume

n_i = the intrinsic concentration

TOTAL CURRENT

The total current in a semiconductor material is the summation of the drift current as well as the diffusion current associated with the sample.

Thus,

Total current  =  Drift current  +  Diffusion current

ELECTRONICS

Electronics is that branch of science which deals with the motion of electrons in gases,liquids and solids under different operating conditions.

Brief HISTORY

1890  Hertz experimented on generation of electromagnetic waves

1894  J.C. Bose discovered the propogation of radio waves

1895  H.A. Lorentz postulated the existence of electron

1897  J.J. Thomson verified the existence of electron

1904  Fleming invented the diode

1947 Bardeen invented point contact transistor

1960  SSI (<100 components per chip)

1966  MSI (>100 but <1000)

1969  LSI (>1000 but <10000)

1975 VLSI (>10000)

ELECTRONIC  COMPONENTS

PASSIVE COMPONENTS

1. Resistors

a) Linear Resistors

eg. Carbon Resistors

b) Non Linear Resistors

eg. Thermistors

1. Capacitors

a)      Fixed Capacitors        eg.Ceramic Capacitor

b)      Variable Capacitors   eg.Varicap

1. Inductors

a) Fixed Inductors

eg.air core inductors

b) Variable Inductors

eg.ferrite core inductors

Inductance of a Coil

L = µ0 µr AN²

l

where ,

l   = length of core

A = Area of cross-section of core

N = Number of turns of coil

µ0  = Permeability of free space = 4∏ X10^-7

µr = Relative permeability of the core material

Active Components

1. Tube type devices

eg. Vaccum diodes, vaccum triodes

2.  Semiconductor devices

eg. Junction diodes, BJT

VOLTAGE AND CURRENT SOURCES

BATTERY

Battery is made up of two or more number of similar cells. A cell may be defined as a fundamental source of energy. It contains a combination of some particular materials which produce d.c. electrical energy from internal chemical reactions.

REGULATED D.C. SUPPLIES

A regulated supply consists of a rectifier, filter and a voltage regulator.

For example, the battery eliminator used with a transistor radio or calculator is a regulated d.c. supply source.

A GENERATOR

A generator is a device which changes mechanical energy into electrical energy by the principle of electro-magnetic induction.

OSCILLATOR or SIGNAL GENERATOR

An oscillator or a signal generator may be defined as the equipment which supplies a.c. voltages. Also the frequency of the a.c. signal can be varied over a wide range of frequency and is capable of producing triangular or square wave outputs in addition to sinusoidal output.

CONCEPT OF VOLTAGE SOURCE

I = V

R

Such type of source are called as ideal sources

But normally the sources themselves have some of the resistance associated with them known as the internal resistance of the source.

In that case the total resistance is given by:

I =    V           _

R_int +  R_l

Where,

Rint = It is the internal resistance of the source

Ideal v/s Practical Voltage sources:

Ideal v/s Practical current sources