Diodes Waveform shaping Circuitsaries.ucsd.edu/NAJMABADI/CLASS/ECE65/12-W/Slides/ECE65_W12-Di… ·...

Diodes Waveform shaping Circuits

Lecture notes: page 2-20 to 2-31

Sedra & Smith (6th Ed): Sec. 4.5 & 4.6 Sedra & Smith (5th Ed): Sec. 3.5 & 3.6

F. Najmabadi, ECE65, Winter 2012

Two-port networks as building blocks


Recall: Transfer function of a two-port network can be found by solving this circuit once.

Concept of input resistance can be used to find vi/vsig (will be discussed in transistor amplifier section)!

We focus on finding transfer function, vo vs vi (circuit below) o “Open-loop” Transfer function (RL → ∞ or io = 0)

Rectifier Circuit


LoD

DiooDi

Do

Rvivvvvvv

ii

/ :Law :KVL

:KCL

=Ω−=→+=

=

00

0

0

and 0 :OFF Diode

DioiDD

DLo

DDD

VvvvVviRv

Vvi

<=−→<==

<=

0

0

0

0 0/

0 and :ON Diode

DiDioLoD

DiDio

DDD

VvvvvRviVvvvv

iVv

≥→≥−=→≥=−=−=

≥=

0 and OFF Diode ,For and ON Diode ,For

0

00

=<−=≥

oDi

DioDi

vVvVvvVv

Rectifier Circuit: vo is the positive portion vi


0 and OFF Diode ,For and ON Diode ,For

0

00

=<−=≥

oDi

DioDi

vVvVvvVv

Application of Rectifier Circuit: AC to DC convertor for power supply


Full-wave rectifier (converts all of AC input to DC value)

Half-wave rectifier (only converts half of AC input to DC value)

Each pair of diodes conduct only for half of the cycle


Clipper or Limiter Circuit (open-loop transfer function)


00 0R

DiDD

iooi

VvVvvvvv

<→<=→+×=

00

0

0/)( DiDiD

Do

VvRVviVv

≥→≥−==

0 and 0 :OFF Diode DDD Vvi <= 0 and :ON Diode 0 ≥= DDD iVv

ioDi

DoDi

vvVvVvVv

=<=≥

and OFF Diode ,For and ON Diode ,For

0

00

Clipper Circuit does not allow vo > VD0 to go through


ioDi

DoDi

vvVvVvVv

=<=≥

and OFF Diode ,For and ON Diode ,For

0

00

Impact of RL is discussed as an exercise problem

Rectifier & clipper circuits are the same but vo is taken at different locations


Half-wave Rectifier

Clipper

Clipper circuit limits vo when the diode is ON


By adjusting “VD0 ” we can adjust limiting voltage!

Limiting voltage can be adjusted


vo limited to ≤ VD0 + VZ vo limited to ≤ VD0 + VDC

Bottom portion of signal can also be clipped


vo limited to ≥ − VD0 − VDC

vo limited ≥ − VD0 −VZ

vo limited to ≤ VD0 + VDC1 and ≥ − VD0 − VDC2

Both top & bottom portions of the signal can be clipped simultaneously


vo limited to ≤ VD0 + VZ1 and ≥ − VD0 − VZ2

“Ideal” Peak Detector Circuit

Because vc cannot change suddenly, the state of diode will depend not only on vi but also on the “history” of the circuit (e.g., dvi/dt , vc at certain times,)


000

0

const.

DciDciD

co

VvvVvvvvv

+<→<−===

0 and 0 :OFF Diode DDD Vvi <=

Capacitor does not charge or discharge! vc (t) = vc0 where vc0 is the capacitor voltage at

the moment diode turned OFF!

“Ideal” Peak Detector Circuit (open-loop transfer function)


0 and :ON Diode 0 ≥= DDD iVv

constvvVvvVvvvVvv/dtdv

coDci

DicoDcii

==+<−==+=≥

00

00

OFF, Diode :For , ON Diode : & 0For

0 0

)( 0

0

≥→≥=

=−

===

−==

dtdvii

dtdvC

dtVvdC

dtdvCii

Vvvv

icD

iDiccD

Dico

Because state of diode depends on vc , we cannot produce a universal plot vo vs vi

Response of the “Ideal” Peak Detector (1)



coDci

DicoDcii

==+<−==+=≥

00

00


When vi = vc0 + VD0 = VD0 , diode turns ON (since dvi/dt > 0)

Capacitor starts to charge and vc tracks vi o vo = vc = vi - VD0

Start at t = 0 with vc= 0 For t > 0, dvi/dt > 0. For vi < vc0 + VD0 = VD0 ,

diode remains OFF. o vo = vc0 = 0




coDci

DicoDcii

==+<−==+=≥

000

00


Even when vi starts to increase (dvi/dt > 0) diode remains OFF as vo < vc0 + VD0 o vc0 + VD0 = V + − VD0 +VD0 = V + !

Diode turns ON vi = V + and immediately turns OFF vi starts to decrease (dvi/dt < 0)

Cap continue to charge until vi = V + (vc = V + - VD0 )

Afterward vi starts to decrease (dvi/dt < 0) and diode turns OFF. o vo = vc0 = V + − VD0



vo is the “peak” value of input waveform (V + – VD0 ): “Peak Detector” o Note vo did not “drop” after the peak was decreased in the 3rd cycle.

Exercise: Show that if the diode direction is reversed, circuit detects the “negative” peak value, −V − (i.e., lowest voltage of the wave form which should be negative)

Practical Peak Detector Circuit (1)


00

00

)( ])/( exp[ )(

DciDciD

cco

VtvvVvvvtt-vtvv

+<→<−=−== τ

0 and 0 :OFF Diode DDD Vvi <=

Capacitor discharges into the resistor with a time constant of τ = RC

A resistor is added in parallel to the capacitor! (It can be the load for the circuit)

Practical Peak Detector Circuit (2)


0 and :ON Diode 0 ≥= DDD iVv

])/( exp[ )( OFF, Diode :For , ON Diode : ,& 0For

000

00

τtt-vtvvVvvVvvvVvv/dtdv

ccoDci

DicoDcii

−==+<−==+=≥

0 0

)( 0

0

≥→≥=

=−

===

−==

dtdvii

dtdvC

dtVvdC

dtdvCii

Vvvv

icD

iDiccD

Dico


000

00


ccoDci

DicoDcii

−==+<−==+=≥

Response of the Practical Peak Detector (1)


When vi = vc0 + VD0 = VD0 , diode turns ON (since dvi/dt > 0)

Capacitor starts to charge and vc tracks vi o vo = vc = vi - VD0

Start at t = 0 with vc= 0 For t > 0, dvi/dt > 0. For vi < vc0 + VD0 = VD0 ,

diode remains OFF. o vo = vc0 = 0


000

00


ccoDci

DicoDcii

−==+<−==+=≥



Even when vi starts to increase (dvi/dt > 0) diode remains OFF as long as vo < vc + VD0

Diode turns ON when vi = vc + VD0 and charges capacitor until vi = V + is reached)

Cap continue to charge until vi = V + (vc = V + - VD0 )

Afterward vi starts to decrease (dvi/dt < 0) and diode turns OFF. Capacitor discharges:

])/( exp[ )( 00 τtt-vtvv cco −==



Shape of output signal depends on the ratio of τ/T “ideal” peak detector: τ/T → ∞ “Good” peak detector: τ/T >> 1 As τ/T decreases, the circuit departs from a peak detector. For τ/T << 1, capacitor discharges very fast and circuit resembles a rectifier

circuit

Decreasing τ/T

Peak detector is used in AM receivers


Carrier wave amplitude is modulated with the sound data (sound signal is the “envelop” of the carrier wave)

soundcarrier TRCT <<=<<τ

Peak-Detector with a “load”


A clipper circuit with a load RL is similar to the open-loop clipper with R → R || RL

Examples of Design Choices: As a peak detector (want τ/T → ∞) R is NOT needed and we should set

C RL to be large (>>T). o Peak detector circuit is used to “smooth” out the output voltage of a

rectifier for the power supply circuit (Need a large C!). For applications such as AM receiver when the peak detector is used as

separate the signal from a carrier, R and C should be chosen such that

Lsoundcarrier RRTRCT <<<<=<< and τ

Clamp Circuit


“Ideal” peak detector: vo = vc = V + − VD0

)(

0

0

DiciDo

Dc

VVvvvvvVVv

−−=−==

−=+

+

vo is equal to vi but shifted “downward” by − (V + − VD0)

Clamp circuit: vo = vD

If amplitude of vi (V + ) changes, the shift would changes and vo becomes distorted!

Clamp Circuit with a Load


Capacitor charges when the diode is ON:

vc = V + − VD0 Capacitor remains charged

when diode is OFF.

Capacitor charges when the diode is ON:

vc = V + − VD0 Capacitor discharges into RL

when diode is OFF. As long as τ = RLC >> T

capacitor discharges little and clamp circuits works fine!

Voltage shift in a clamp circuit can be adjusted!


Peak detector circuit: vc = V +A − VD0

vc = V + − VDC − VD0

vo is equal to vi but shifted

“downward” by − (V + − VDC − VD0)

vA = vi − VDC

V + : peak of vi

V +A : peak of vA

V +A = V + − VDC

)(

0

0

DDCicio

DDCc

VVVvvvvVVVv

−−−=−=

−−=+

+

)( 0DZio VVVvv −−−= +

Clamp circuit can also introduce a “positive” shift


Peak detector (diode is reversed): vo = vc = − (V − − VD0)

)(

)(

0

0

DiciDo

Dc

VVvvvvvVVv

−+=−==

−−=−

−

vo is equal to vi but shifted “upward” by (V − − VD0)

Clamp circuit (diode reversed): vo = vD

The positive shift can also be adjusted.


)( 0DZio VVVvv −−+= − )( 0DDCio VVVvv −−+= −

How to find response of clipper or clamp circuits: Assume diode is ON and calculate vc .

o If vc = +vi …, replace vi with V+ (peak positive value) o If vc = −vi …, replace vi with −V− (peak negative value)

If clipper, vo = vc . If Clamp, use KVL to find vo (e.g., , vo = vi − vc )

Diodes Waveform shaping Circuitsaries.ucsd.edu/NAJMABADI/CLASS/ECE65/12-W/Slides/ECE65_W12-Di… ·...

Documents

Transcript of Diodes Waveform shaping Circuitsaries.ucsd.edu/NAJMABADI/CLASS/ECE65/12-W/Slides/ECE65_W12-Di… ·...