Circuits lecture

7/25/2019 Circuits lecture

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Term Roadmap :• Introduction to Signal Processing

• Differentiating and Integrating Circuits (OpAmps)

• Clipping and Clamping Circuits(Diodes)

• Design of analog filters

• Sinusoidal Oscillators

• Multivibrators

• Sampling and Quantization techniques of analog signals

• DACs and ADCs• Data Acquisition Systems

• Introduction to discrete time transform and DSP

• The Z transform

• Design of Digital Filters

Materials Types

1. INSULATORS

• An INSULATOR is any material that inhibits

(stops) the flow of electrons (electricity).

• An insulator is any material with 5 to 8 freeelectrons in the outer ring. Because, atomswith 5 to 8 electrons in the outer ring are held(bound) tightly to the atom, they CANNOT beeasily moved to another atom nor make roomfor more electrons.

• Insulator material includes glass, rubber, andplastic



Materials Types2. CONDUCTORS

•

A CONDUCTOR is any material that easilyallows electrons (electricity) to flow.

• A CONDUCTOR has 1 to 3 free electrons inthe outer ring. Because atoms with 1 to 3electrons in the outer ring are held (bound)loosely to the atom, they can easily move toanother atom or make room for more electrons.

• Conductor material includes copper and gold

Materials Types3. SEMICONDUCTORS

• Any material with exactly 4 free electrons inthe outer orbit are called

SEMICONDUCTORS.

• A semiconductor is neither a conductor or insulator.

• Semiconductor material includes carbon,silicon, and germanium.

• These materials are be used in themanufacturer of diodes, transistors, andintegrated circuit chips.



Semiconductor Diode• Diode is formed by bringing these two material together p- and n-type.• Holes diffuse from the p side to the n side, leaving behind negatively

charged immobile negative ions.• Electrons diffuse from the n side to the p side, leaving behind positively

charged immobile positive ions.• Electrons and holes at joined region will combine, resulting in a lack of

carriers in the region near the junction ( depletion region )

Reverse-biased p-n junction

Reverse-Bias Condition (V D < 0V)



Reverse-Bias Condition (V D < 0V)

• The number of positive ions in the depletion region of n-typewill increase due to large number of free electrons drawn tothe positive potential.

• The number of negative ions will increase in p-type resultingwidening of depletion region.

• This region established great barrier for the majority carriersto overcome, resulting I majority = 0

• A very small amount of reverse current does flow, due to

minority carriers diffusing from the (p/n) regions into thedepletion region and drifting across the junction.

Forward-biased p-n junction

Forward-Bias Condition (V D > 0V)



Forward-Bias Condition (V D = 0V)

• A semiconductor diode is forward-biased when the

association p-type and positive voltage and n-typeand negative voltage has been established.• The application of forward-bias potential will pressure

the electrons in n-type and hole in p-type torecombine with ions near the boundary and reducethe width of depletion region

• The reduction in width of depletion region hasresulted in a heavy majority flow across the junction

Semiconductor Diodes

Figure 3.39 Simplified physical structure of the junction diode. (Actual geometries are given in Appendix A.)

Circuitsymbol



Diodes

Several types of diodes. The scale is centimeters

The i–v characteristic of a silicon diode.

Figure 3.7 The i – characteristic of a silicon junction diode.

Figure 3.8 The diode i – relationship with some scalesexpanded and others compressed in order to reveal details.



The i–v characteristic of a silicon diode.

• The Forward-Bias region:-

• In the forward region the i- v relationship is closely approximated by…..

• I s …….the reverse saturation current ( scale current)

– K = Boltzmann`s constant = 1.38*10-23 joules / kelvin

– T k = the absolute temperature in kelvins = 273 + temperature in °C

)1( k T kv

e I i s

The i–v characteristic of a silicon diode.• The Reverse-Bias region:-

• The exponential term becomes negligibly small compared to unity, and the diode current becomes…..

• That is, the current in the reverse direction is constant and equal to I s which tends tozero.

• The Breakdown Region:-

• The breakdown region is entered when the magnitude of the reverse voltage exceeds athreshold value that is specific to the particular diode, called the breakdown voltage .

s I i



Ideal Diode

Figure 3.1 The ideal diode: (a) diode circuit symbol; (b) i – characteristic;(c) equivalent circuit in the reverse direction; (d) equivalent circuit in theforward direction.

Figure 3.2 The two modes of operation ofideal diodes and the use of an external circuitto limit the forward current (a) and the reversevoltage (b) .

Modeling the diode forward characteristic

The Piecewise-linear Model

Figure 3.12 Approximating the diode forwardcharacteristic with two straight lines: the piecewise-linear model.

Figure 3.13 Piecewise-linear model of the diode forward characteristic and itsequivalent circuit representation.

Figure 3.14 The circuit ofFig. 3.10 with the diodereplaced with its piecewise-linear model of Fig. 3.13.




The Piecewise-linear Model

Example:

Given: V DD = 5V, V DO= 0.65, r D = 20 ,R= 1K

Thus mA I D 26.4

02.0165.05

VD = VDO+IDr D

= 0.65+4.26x0.02=0.735V


The Constant-voltage-drop Model

Figure 3.15 Development of the constant-voltage-drop model of the diode forwardcharacteristics. A vertical straight line (B) is usedto approximate the fast-rising exponential.Observe that this simple model predicts V D towithin 0.1 V over the current range of 0.1 mA to10 mA.

Figure 3.16 The constant-voltage-drop model of the diode forwardcharacteristics and its equivalent-circuit representation.




The Ideal Diode Model

Figure 3.1 The ideal diode: (a) diode circuit symbol; (b) i – characteristic; (c) equivalent circuit in thereverse direction; (d) equivalent circuit in the forward direction.

Operation in The reverse BreakdownRegion- Zener Diodes

Figure 3.20 Circuit symbol for a zener diode.

Figure 3.21 The diode i – characteristic with the breakdown region shown in somedetail.

Figure 3.22 Model for the zener diode.



Figure 3.23 (a) Circuit for Example 3.8. (b) The circuit with the zener diode replaced with its equivalent circuit model.

Operation in The reverse BreakdownRegion- Zener Diodes

Example: Find I ?

Diode Applications



AND/OR Gates AND and OR gates represent basic components of computers thatare used to implement Boolean algebra.

OR-Gate AND-Gate1 2 3

0 0 0

0 1 1

1 0 1

1 1 1

1 2 3

0 0 0

0 1 0

1 0 0

1 1 1

If logic “ 1” is represented by + 10 (+5 ) V and logic “ 0” isrepresented by 0 V, the OR and the AND gates can berepresented by the following diode combinations;

AND/OR GatesFor the OR gate; – D1 ON – D2 OFF – V0=10V ( logic 1)For the AND gate; – D1 OFF – D2 ON – V0=0V ( logic 0)



Diodes Applications: Rectifier Circuits

Figure 3.24 Block diagram of a dc power supply.

• So far, we have considered time invariant signals only (DC).• Now, diode circuit analysis will be extended to include circuits containing time

varying signals (AC).• The simplest diode application that uses AC signals is the HWR signal shown.• To simplify the analysis, we’ll assume that the diodes used are ideal.

• Note that, the DC content of the input waveform is zero, Why?• During time interval t=0 T/2, diode is ON.• Since we are using an ideal diode model, v 0 =v i .• During the time interval t=T/2 T, diode is OFF ; v 0 =0.• Now, what is the value of the DC level in the output waveform? (V dc=0.318V m)

Sinusoidal Inputs; Half-wave Rectification(Ideal diode Model)

Ti m

0 s 1 . 0 s 2 . 0 s

V ( R 5 : 2 ) V ( V 5 : + )

- 5 . 0 V

0V

5 . 0 V



HWR with Const Voltage Drop Diode Model

• In case of using the constant voltage drop diode model, during the conduction period diode will be replaced with a constant voltage source V D0.

• Thus, the peak of the output waveform will decrease from V s by V D0.

• In addition, the conduction period of the diode will be slightly less than T/2.

• In this case, the DC content of the output waveform becomes;• Vdc 0.318(V s-VD0) (Note:0.318V s =V s/ ) (Vs i.e Vm , V D0 i.e VT)• Peak Inverse Voltage (PIV)• Definition: PIV is the value of the maximum reverse voltage that is expected

to apply to the diode in during its operation.• PIV: Peak Inverse Voltage in this case = V s, Thus, PIV rating >V s

The rectifier with a filter capacitor



The rectifier with a filter capacitor

Full-wave Rectifier

• DC level can be improved to 100% of that obtained in HWR, by using the full-waverectifier configuration shown.

• For t=0 T/2, D1 and D2 ON while D3, and D4 OFF.• For t=T/2 T, D3, D4 ON, while D1 and D2 OFF.• As seen form the waveform generated, the DC level for that configuration is twice that of

the HWR.• Vdc(FWR) =2 Vdc(HWR) = 2 0.318V s , ideal diode model.• =2 0.318(V s-2V D0) simplified• PIV| rating >V s



Center Tapped Transformer FWR

• For t=0 T/2, D1 ON while D2, OFF.• For t=T/2 T, D2 ON, while D1, OFF• PIV| rating >2V s

• Example 2.19

Clipper Circuits

Figure 3.33 Applying a sine wave to a limiter can result in clipping off its two peaks.

Clipper circuit is the circuit which clip off a portion ofthe input signal without distorting the remainingpart of the signal



Time

0s 100ms 200ms 300ms 400ms

V (V s: +) V( RL :2 )

-10V

0V

10V

Clippers• Clipper circuits is used to remove one part of the signal without distorting the remaining part.• The orientation of the diode determines the part of the signal that is removed, while the value

of the DC controls the level of clipping.• It is usually consists of , a diode, a resistance and a DC source.• Clippers have two major configurations;

– Series Configuration, where the diode is connected in series with the the source. – Parallel Configuration; where the diode is connected in parallel with the output port.

• Single Side Clippers Double Side Clippers

• Series Clippers

– The output voltage is given by KVL as; – v o= v s-V- V D such that the voltage at the diode input has to be greater than V T for the

diode to conduct. Otherwise, the diode will be off and v o will be zero.

Lec(3-1)

Parallel ClippersUpper Side Clippers:

Lower Side Clipper

Time

0s 100ms 200ms 300ms 400ms 500msV (V :+ ) V (R L: 2)

-10V

0V

10V

ON is Dif V v

OFF is Dif vv

D

i0

Tim e

0s 50ms 1 00ms 1 5 0ms 20 0 ms 250 ms 3 00ms

V (R 2: 2) V (V 1: +)

-5 . 0 V

0V

5 .0V

Time

0s 50ms 100ms 150ms 200ms 250msV( V1 0: +) V( V1 :+)

-5.0V

0V

5.0V

Double Side Clipper



Clipper Circuits



Example:



Clampers• A clamping circuit is the circuit that is used to clamp a signal to a certain

DC level.• It must contain a capacitor, a diode and a resistive element.

• The value of the discharging time constant of the capacitor, dis=RC>>T/2has to be large enough to ensure that the capacitor doesn’t discharge duringthe OFF period of the diode.

• The very small resistance of the diode R D makes the charging time constantch=R DC so small that its can be considered that the diode charges in zero

time.

Clampers• Rules:

– The Direction of the diode’s arrow determines whether the signal isclamped up or down.

– The value of the DC source connected to the diode’s anode determinesthe max or the min of the clamped signal respectively.

• Operation – For t=0 T/2, D is ON

Resistance R1 isshort-circuited by the diode.

– C charges to V m inzero time (theoretically).

– For t=T/2 T,D is OFF. – C discharges through R1 – The Value of the output voltage

is – [V s(Vm)+V c(Vm)]



Clampers• Rules:

– The Direction of the diode’s arrow determines

whether the signal is clamped up or down. – The value of the DC source connected to the

diode’s anode determines the max or the min ofthe clamped signal respectively.

• Operation

Example:



VOLTAGE MULTIPLIER CIRCUITS

• Voltage multiplier circuits are used to maintain low transformer peakvoltage and stepping up the voltage to 2, 3,or 5 times this value

• Voltage Doubler ( Half-Wave)

During +ive half cycle, D1 ON, D2 OFF, C1 charges to+V m

– During -ive half cycle, D1 OFF, D2 ON, C2 charges to+2V m as following;-VC2+V c1+V m=0VC2=Vc1+V m=2V m



The Voltage Doublers

Figure 3.38 Voltage doubler: (a) circuit; (b) waveform of the voltage across D 1.

Dr. Mohamed Hassan

Full-wave Voltage Multiplier

• During (+)ive half cycle, D1 ON, D2 OFF,C1 charges to +V m

• During (-)ive half cycle, D1 OFF, D2 ON,C2 charges to +V m.

• So the total output voltage applied to theload is 2V m.



Voltage Tripler and Quadrupler

• During (+)ive half cycle , D1 ON, C1charges to V m.

• During (-)ive half cycle , D1 OFF, C2charges to 2V m.through C1 and thesecondary winding of transformer.

• During the next (+)ive half-cycle, D3conducts, and C2 charges C3 to 2Vm

Thank You

Circuits lecture

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