revision

Half-wave Rectifier

Bi-phase Rectifiers

Bridge Rectifiers

Operation of Bridge Rectifiers

(cont.)

Operation of Bridge Rectifiers

Summary of Types Of Rectifiers using non-Ideal Diode(Silicon)

Type of rectifier Half-wave Bi-phase (Centretap)

Bridge Rectifier

1 Output Peak (Vp (out))

2 Output Average(VAVG)

3 Peak Inverse Voltage (PIV)

4 Output Frequency(f) Equal Input Frequency

Double Input Frequency


)out(pV

)out(pV2

)out(pV2

7.0V )s(p 7.02

V )s(p 4.1V )s(p

)s(pV 7.0V )s(p 7.0V )s(p

7.0V )out(p 7.0V2 )out(p 7.0V )out(p

Summary of Types Of Rectifiers using Ideal Diode

Type of rectifier Half-wave Bi-phase (Centretap)

Bridge Rectifier

1 Output Peak (Vp (out))

2 Output Average(VAVG)

3 Peak Inverse Voltage (PIV)

4 Output Frequency(f) Equal Input Frequency



)out(pV

)out(pV2

)out(pV2

)s(pV2

V )s(p)s(pV

)s(pV )s(pV )s(pV

)out(pV )out(pV2)out(pV

For Full Wave Rectifier With Capacitance Input Filter

)rect(VCfR

1V p

L)pp(r

)rect(VCfR2

11V p

L)DC

Note: f is the output frequency

Series Diode Clippers

Series Negative Clipper

Series Positive Clipper

Parallel Diode Clippers

Parallel Negative Clipper

Parallel Positive Clipper

Biased Diode ClippersBiased Negative Clipper

Biased Positive Clipper

Biased Double-Diode Clippers

Positive Clamper

Negative Clamper

Biased Clamper

Zener Diode08. How does the zener impedance affect the voltage across the terminals of the device?

9. (a) shows the original circuit. (b) Zener diode represented using the second approximation. What is the max and min IZ and VZ?

10. This is a typical loaded voltage regulator. Do you know the value of IZ?

BJT Schematic Symbols

NPN PNP

BJT Biasing

BJT Biasing

NPN Biasing PNP Biasing

Common Emitter

Common Emitter

• It is called the common-emitter configuration because the emitter is common or reference to both the input and output terminals (in this case common to both the base and collector terminals).

Common Collector

Common Base

Configuration Characteristics

Current Gain• The dc current gain produced by an amplifier is the ratio

of output current to input current, i.e.,

• In the case of a transistor operating in common emitter mode, the input current is the base current, IB, whilst the output current is the collector current, IC.

(Cont.)

in

outDC I

I

B

CDC I

I

DC usually designated as hFE on transistor data sheets.

• h is derived from an ac hybrid equivalent circuit.• The subscript FE is derived from forward-current

amplification and common-emitter configuration.

• Typical values of DC range from less than 20 to 200 or higher.

Current Gain

DC is a very important BJT parameter.

DC is not truly constant but varies with both collector current and with temperature.

• A transistor data sheet usually specifies DC at specific IC values.

Current Gain

• If a steady bias current is superimposed with an a.c. current, this will produce a collector current which varies above and below its d.c. current value respectively.

• The small signal ac current gain is then given by,

B

Cac I

I

Ratio of dc collector current to dc emitter current, DC (Common Base

Mode)DC = IC/IE

• Typically, values of DC range from 0.95 to 0.99 or greater but it is always less than 1.

• The small signal ac ratio is then given by,

E

Cac I

I

• The relationship of DC and DC is given as,

DCDC

DC

1

DC Equivalent of a BJT

Output Characteristics

Cutoff

Midpoint Bias

• Without an ac signal applied to a transistor, specific values of IC and VCE exist.

• The IC and VCE values exist at a specific point on the dc load line.

Base Bias

Base Bias

RB

Base Bias

• For silicon transistors, VBE equals 0.7V.

• The collector circuit is represented as a current source whose value is dependent only on the values of DC and IB.

• Collector supply voltage variations will have little or no effect on the collector current, IC.

Current and Voltage Analysis

B

BEBBB

BEBBBB

R

VVI

VRIV

0

CCCCCE

CECCCC

RIVV

VRIV

0

BECECB

BECECB

VVV

VVV

0

In the active region (not operating in saturation or cutoff)

VV

II

BE

BDCC

7.0

The collector circuit acts as a current source with a high internal impedance.

Base Bias

• More practical to use VCC as a single bias source.

• The base supply voltage, VBB has been omitted and RB is connected to the positive (+) terminal of VCC.

B

BECCDCC

CCCCCE

B

BECCB

R

VVI

RIVV

R

VVI

Eg :

(Cont.)

CBE

BDCC

III

II

B

BEBBB R

VVI

CCCCCE RIVV

BECECB VVV

Base Bias

• For base bias, IC is dependant on DC.• But DC varies with temperature and also

varies from one transistor to another.• Variations in DC causes IC and VCE to change

thus changing the Q point of the transistor (near or at cutoff or saturation).

• This might cause distortion in the output signal.

• Base bias provides a very unstable Q point.

DC Load Line• The DC load line is a graph that allows us

to determine all the possible combination of IC and VCE for a given amplifier.

• A specific point on the DC load line gives a fixed value of IC and VCE is called the Q point.

• Q stands for quiescent currents and voltages with no ac input signal.

Base Bias

• For silicon transistors, VBE equals 0.7V.

• The collector circuit is represented as a current source whose value is dependent only on the values of DC and IB.

• Collector supply voltage variations will have little or no effect on the collector current, IC.


B

BEBBB

BEBBBB

R

VVI

VRIV

0

CCCCCE

CECCCC

RIVV

VRIV

0

BECECB

BECECB

VVV

VVV

0

In the active region (not operating in saturation or cutoff)

VV

II

BE

BDCC

7.0

The collector circuit acts as a current source with a high internal impedance.

Base Bias

• More practical to use VCC as a single bias source.

• The base supply voltage, VBB has been omitted and RB is connected to the positive (+) terminal of VCC.

B

BECCDCC

CCCCCE

B

BECCB

R

VVI

RIVV

R

VVI

Eg :

(Cont.)

CBE

BDCC

III

II

B

BEBBB R

VVI

CCCCCE RIVV

BECECB VVV

Voltage Divider Bias

• This is the most popular way to bias a transistor.

• Transistors biased in this manner are stable.


ECCCCCE

CE

E

EE

BEBE

CCB

RRIVV

II

R

VI

VVV

VRR

RV

21

2

The dc load line

• The dc load line intersects the IC axis at a the saturation point where IC is maximum and VCE is almost 0.

EC

CEsatC

CCoffCE

ECCCCCE

RR

VI

VV

RRIVV

DC Load Line

DC Load Line for Voltage Divider Bias

Emitter Bias

• If both positive and negative power supplies are available, emitter bias gives a solid Q-point that is fixed (fluctuates very little with temperature variation and transistor replacement).

Emitter Bias

• The emitter supply voltage, VEE, forward-biases the emitter-base junction through the emitter resistor, RE.

• The base voltage, VB=0V, because the IBRB voltage drop is very small due to the small value of base current, IB, which is typically only a few microamperes.

EC

EECCsatC

EECCoffCE

ECCEECCCE

EC

DC

BE

BEEEE

RR

VVI

VVV

RRIVVV

II

RR

VVI

Emitter Bias

• If both positive and negative power supplies are available, emitter bias gives a solid Q-point that is fixed (fluctuates very little with temperature variation and transistor replacement).

Emitter Bias

• The emitter supply voltage, VEE, forward-biases the emitter-base junction through the emitter resistor, RE.

• The base voltage, VB=0V, because the IBRB voltage drop is very small due to the small value of base current, IB, which is typically only a few microamperes.

EC

EECCsatC

EECCoffCE

ECCEECCCE

EC

DC

BE

BEEEE

RR

VVI

VVV

RRIVVV

II

RR

VVI

Collector Feedback Bias• This type of

biasing is more stable than the base bias.

Collector Feedback Bias

• The base resistor, RB, is connected to the collector, rather than to the supply voltage, VCC, as in the case with base bias.

• Collector-feedback bias is much more stable than the bias bias.


• Assume DC increases due to temperature.

• This produces an increase in the collector current, IC, which, in turn, increases the voltage dropped across Rc.

• This causes the VCE to decrease, thus decreasing the voltage drop across the base resistor, RB.


• Then, this reduces the base current, IB, which causes IC to decrease by an amount that almost completely offsets the original increase in the current, IC.


• Assume DC decreases.

• This causes the IC to decrease. This, in turn, causes VCE to increase, which then cause IB to increase due to the increased voltage drop across the base resistor, RB.

• The increase in IB causes IC to increase.

• This almost completely offsets the original change in IC caused by the reduction in DC.


• RB is usually chosen such that the Q point is placed in the middle of the dc load line.

• To satisfy this condition, choose RB to equal DC Rc . CCCCCE

DC

B

BECC

C

RIVV

RRC

VVI

04/12/23 6-78

SummaryBasic AC h-parameters

hi - input impedance (resistance) with output short circuited.

hr - reverse voltage transfer function with input open circuited.

hf - forward current transfer function with output short circuited.

ho - Output admittance (conductance) with input open circuited.

04/12/23 6-79

Complete h-parameter equivalent circuit

04/12/23 6-80

Common emitter h-parameter equivalent circuit

~

hie

hreVouthfe Ib hoe

Vout

Ib

E

CB

04/12/23 6-81

Approximate hybrid equivalent circuit

hie hfe Ib Vout

Ib

E

CB

04/12/23 6-82


• Since hr is normally a relatively small quantity, its removal is approximated by and , resulting in a short-circuit equivalent for the feedback element as shown.

• The resistance determined by 1/ho is often large enough to be ignored in comparison to a parallel load, permitting its replacement by an open circuit equivalent for the CE and CB models.

0rh0orVh

04/12/23 6-83

Common-Emitter Fixed Bias Configuration

VI

V0

__

Zin

Z0

II

Io

04/12/23 6-84

SummaryBasic AC h-parameters

hi - input impedance (resistance) with output short circuited.

hr - reverse voltage transfer function with input open circuited.

hf - forward current transfer function with output short circuited.

ho - Output admittance (conductance) with input open circuited.

04/12/23 6-85

Complete h-parameter equivalent circuit

04/12/23 6-86

Common emitter h-parameter equivalent circuit

~

hie

hreVouthfe Ib hoe

Vout

Ib

E

CB

04/12/23 6-87


hie hfe Ib Vout

Ib

E

CB

04/12/23 6-88


• Since hr is normally a relatively small quantity, its removal is approximated by and , resulting in a short-circuit equivalent for the feedback element as shown.

• The resistance determined by 1/ho is often large enough to be ignored in comparison to a parallel load, permitting its replacement by an open circuit equivalent for the CE and CB models.

0rh0orVh

04/12/23 6-89

Common-Emitter Fixed Bias Configuration

VI

V0

__

Zin

Z0

II

Io

04/12/23 6-90

Equivalent Circuit

RB

RC

Input

OutputB

E

C

04/12/23 6-91

AC equivalent circuit

voutvin

iin

iout

RB hie hfeib 1/hoeRC

Zi

Z0

IbIC

04/12/23 6-92

Zi and Z0 (Input Impedance and Output Impedance)

oeCC

ieBi

hRRZ

hRZ

10

iin

iout

RB hie hfeib 1/hoe RC

Zi

Z0

04/12/23 6-93

Voltage Gain AV

Cbfe

CCC

RIh

RIRIV 00

V0

iout


IC

04/12/23 6-94

Cie

ife

ie

ib

Rh

VhV

h

VI

0

Vi

iout


Ib

V0

04/12/23 6-95

ie

Cfe

iV

Cie

ife

h

Rh

V

VA

Rh

VhV

0

0

The negative sign in the resulting equation for AV reveals that a 180o phase shift occurs between the input and output signals.

04/12/23 6-96

Current Gain Ai

fei

oi

ifebfeC

ib

ieB

hI

IA

IhIhII

and

II

thenhR

0

,

Assuming

04/12/23 6-97

Eg: Find Zin, Z0, Ai and Av

RB = 330kRC = 2.7k hfe=120hie=1.175khoe=20A/V

04/12/23 6-98

Voltage Divider Bias

• Note that R1 and R2 remain part of the input circuit while R3 is part of the output circuit.

• The parallel combination of R1 and R2 is defined by RP

21

2121 //

RR

RRRRRP

04/12/23 6-99

R1R2

R3

Input

OutputB

E

C

04/12/23 6-100

Rp

R3

Input

OutputB

E

Cibiin

iout*RP = R1 // R2

04/12/23 6-101

AC equivalent circuit

vin voutRp hiehfeib 1/hoe R3

iin ioutib

04/12/23 6-102

Zi and Z0

oeCC

iepi

hRRZ

hRZ

10

iin

iout

Rp hie hfeib 1/hoe RC

Zi

Z0

RP = R1 // R2

04/12/23 6-103

AV

Cbfe

CCC

RIh

RIRIV 00

V0

iout

Rp hie hfeib 1/hoe RC

IC

04/12/23 6-104

Cie

ife

ie

ib

Rh

VhV

h

VI

0

Vi

iout

Rp hie hfeib 1/hoeRC

Ib

V0

04/12/23 6-105

ie

Cfe

iV

Cie

ife

h

Rh

V

VA

Rh

VhV

0

0

04/12/23 6-106

Ai

ieP

Pfei hR

RhA

revision

Documents

base current

input current

dc emitter current

current source

current value

dc current gain

current gain dc

ce cb v v v v v v