Electronics Circuits - Analog and Digital Electronics - Lecture Slides | EE 334, Exams of Electrical and Electronics Engineering

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EE 334

Analog Electronics

Midterm Exam Review

(Lectures 2-20)

Diode: Why we need to understand diode?

-^

The base emitter junction of the BJT behaves asa forward bias diode in amplifying applications.

-^

The behavior of the diode when reverse bias isthe key to the fabrication of the integratedcircuits.

-^

The diode is used in many importantnonamplifer applications.

Departure from ideal behavior

The four major reason why the actual diode do not correspond exactly to the ideal.1.

Ohmic resistance and contact resistance in series with the diode cause the VIcurve to become linear at high forward current.

Avalanche or Zener breakdown take place at high reverse voltage, causing anabrupt increase in reverse current.

Surface contaminants cause an ohmic layer to form across the junction, which is Increasing the reverse current as reverse voltage is increased.4.

Recombination of current carrier in the depletion region take place due to traps.

The purpose of modeling

-^

Nonlinear problems are much more difficult thanlinear ones. These problems could be impossible tosolve manually and could require huge amount of timeif solved on a computer.

-^

One possible solution of the above mentionedproblem is to approximate the nonlinear relationshipwith a model that has a linear relationship.

-^

The trust of nonlinear modeling is direct towards thisend.

-^

The modeling not only simplifies the solution, it alsoallows the designer to understand how the circuitbehaves. Modeling often increases the conceptualunderstanding of the circuit operation.

Schottky Barrier Diode

One semiconductor region of the pn

junction diode can be replaced by a non-ohmicrectifying metal contact.ASchottky contact is easily formedon

n

-type silicon. The metal region becomes the anode. An

n

+

region is added to ensure that thecathode contact is ohmic.

Schottky diodes turn on at alower voltage than

pn

junction

diodes and have significantlyreduced internal charge storageunder forward bias.

Reverse Breakdown

Increased reverse biaseventually results in thediode entering the breakdown region

,

resulting in a sharpincrease in the diodecurrent. The voltage atwhich this occurs is the breakdown voltage

,^ V

. Z

2 V < V

Z^

< 2000 V

Half Wave

Rectification

Full-Wave Rectifiers

Full-wave rectifiers cut capacitor dischargetime in half and require half the filtercapacitance to achieve a given ripplevoltage. All specifications are the same asfor half-wave rectifiers.Reversing polarity of the diodes gives a full-wave rectifier with negative output voltage.

Figure 2.7^ A full-wave bridge rectifier: (a) circuit showing the current direction for a positive input cycle,(b) current direction for a negative input cycle, and (c) input and output voltage waveforms

Half-wave rectifier with filter

Voltage regulation is the measure of circuit’s ability to maintaineda constant output even when input voltage or load current varies% regulation is used to measure how well the regulator isPerforming its function.

.

Bipolar Transistor

A Bipolar Transistor essentially

consists of a pair of PN JunctionDiodes that are joined back-to-back. This forms a sort of asandwich where one kind ofsemiconductor is placed inbetween two others. There are two kinds of Bipolar

sandwich, the NPNand PNPvarieties. The three layers of thesandwich are conventionallycalled the Collector, Base, andEmitter. The reasons for thesenames will become clear lateronce we see how the transistorworks. Some of the basic properties exhibited by a Bipolar Transistor are immediatelyrecognizable as being diode-like. However, when the 'filling‘ of the sandwich is fairlythin some interesting effects become possible that allow us to use the Transistor as anamplifier or a switch

Recombination current in the base•

Some of the free electronscrossing the base encounter ahole and ‘drop into it’. As aresult the base region losesone of its positive charges(holes) each time thishappens.

-^

For particle BJP only about 1%of the free electrons which tryto cross base region getcaught in this way. Hence wesee a base current, I

, which isB

typically around one hundredtimes smaller than the emittercurrent, I

.E

The large value ofVCE

decreases the effective base width W.Since I

is inverselyS^

propositional to W,which cause increase in I

.C

Bipolar NOR logic gate

Example 3.

Determine current and voltage in the circuit 3.43(b)

R^ c

=1K

Ω

R^ B

=20K

Ω

V^ BE

(on)=0.7V VCE(sat)=0.2V β=

Lecture #

The process by which the quiescent output voltage is caused to fallsomewhere the cutoff and saturated values is referred to as biasing.

Chapter 4

Small-Signal Modeling and

Linear Amplification

DC and AC Analysis

-^

DC analysis:–

Find dc equivalent circuit by replacing all capacitors by opencircuits and inductors by short circuits.

-^

Find Q-point from dc equivalent circuit by using appropriatelarge-signal transistor model.

-^

AC analysis:–

Find ac equivalent circuit by replacing all capacitors by shortcircuits, inductors by open circuits, dc voltage sources byground connections and dc current sources by open circuits.

-^

Replace transistor by small-signal model

-^

Use small-signal ac equivalent to analyze ac characteristics ofamplifier.

-^

Combine end results of dc and ac analysis to yield totalvoltages and currents in the network.

DC Equivalent for BJT Amplifier

-^

All capacitors in original amplifier circuits arereplaced by open circuits, disconnecting

v^ I

,

R

,I

and

R

3

from circuit.

AC Equivalent for BJT Amplifier

k^

Ω

100 k^

Ω (^3). 4 3

k^ Ω 30 k^

Ω

10 2 1

=

=

=

=

R C R R

R R

B R

•Find ac equivalent circuit by replacing all capacitors by short circuits,

To obtained linear amplifier, the ac current and voltages must a small enoughTo insure a linear relationship between the ac signals. To meet this objectiveThe Time-varying signals are assume to be small signals.

Hybrid parameter I: diffusionresistance/input impedance

-^

The diffusion resistance r

is define as theπ

reciprocal of the i

-vB^

BE

curve, which can be find as,

Output terminal characteristics of the bipolar

transistor: transconductance

-^

If we assume constant collector-emitter voltage the,

• As we know -^

By using the two hybrid parameters (r

π

, g

), we canm

develop a simplified small signal hybrid-

π- equivalent

circuit for the npn transistor.

•Voltage -controlled current source

g

vm

be

can be transformed into current-

controlled current source,

From equivalent circuit we can write as

Voltage gain

-^

If we include the early effect then collector currentin terms of early voltage as,

•

then

Summary of hybrid-

π

-model parameters

Diffusion resistance^ transconductance

Current gain

Output resistance

Characteristics of a CE amplifier^ •

It has moderately low input impedance (1K to 2K)

-^

Its output impedance is moderately large(50K or so)

-^

Its current gain is high

-^

It has very high voltage gain of the order of 1500 or so

-^

It produce very high power gain of the order of 10,000 times or40dB

-^

It produce phase reversal of input signal

Uses: many applications because ofLarge gain in voltage, current and power

(by voltage dividerRule)

Small-Signal Analysis of Complete C-E Amplifier: AC

Equivalent

-^

Ac equivalentcircuit isconstructed byassuming that allcapacitances havezero impedance atsignal frequency anddc voltage source isac ground.

-^

Assume that Q-point is alreadyknown

.

2 1

R R B R

=

Small-Signal Analysis of Complete C-E Amplifier: Small-

Signal Equivalent

3 R RC ro L R^

=

If we include an emitter resistance in thecircuit, the Q-point of the circuit will beless dependant on the transistor currentgain

β

.

In order to determine the inputimpedance R

, which is the resistanceib

looking into the base of the transistor.We can write the following loop equation^ The overall input impedance to the amplifier is now

Voltage gain is lessdependant on

β

The voltage gain isSubstantially reducedWhen an emitter resistoris included!!

How

can we Improve the voltage gain?

A

common collector amplifier has following chracteristics: 1

High input impedance (20-500K) 2

Low output impedance (50-2000 Ohms) 3

High current gain (50-300) 4

Voltage gain of less than 1 5

Power gain of 20 to 20dB 6

No phase reversal between input and output signals

Apply KVL around the base emitter loop

Using above equations we can

write voltage gain as

Input impedance

Clearly the voltage gain is less than 1and no phase reversal

Input and output impedance for EF

Very large compared to

CE

-^

Common base amplifier has

-^

Very low input impedance (30-

Ω

)

-^

Very high output resistance (500K)

-^

Current gain

<

-^

Large voltage gain of about 1500

-^

Power gain of upto 30dB

-^

No phase reversal between input and output

-^

Uses: for matching low impedance circuit to high impedancecircuit

Chapter 5

Field-Effect Transistors

The MOS Transistor

Polysilicon

Aluminum

-^

It is to be noted that the V

DS

measured relative to the source

increases from 0 to V

DS

as we travel along the channel from

source to drain. This is because the voltage between the gateand points along the channel decreases from V

GS

at the source

end to

V^ GS

-V

DS

.

-^

When V

DS

is increased to the value that reduces the voltage

between the gate and channel at the drain end to V

that is ,t

-^

VGS

-V

DS

=V

t^

or

V^

DS

= V

GS

-V

t^

or V

DS

(sat)

≥

V

GS

-V

t

Concept of Asymmetric Channel

NMOS Transistor: Saturation Region

TN

GS

DS

TN

GS

D^

V v

v

V v W L n K i^

−

≥

−

=^



^ 

for

'^2

2

v

DSAT

=

v

GS

−

V

TN

is called the saturation or pinch-off voltage

Depletion-Mode MOSFETS

-^

NMOS transistors with

-^

Ion implantation process is used to form a built-inn-type channel in the device to connect sourceand drain by a resistive channel

-^

Non-zero drain current for

v^ GS

= 0; negative

v^ GS

required to turn device off.

VTN

≤ 0

Common source circuit with couplingcapacitance Cc, which act an an opencircuit to the dc

DC equivalent circuit.

Gate

PMOS common source circuit

If the device is biased in saturationregion

Pinchoff at the drain terminal

⇒

⇒

⇒

⇒ We see

⇒

Chapter 6