Home Page of Dr. Dharmendra Kumar Pandey - Transistor Hybrid … · 2020. 7. 11. · from the DC...

Transistor Hybrid equivalent circuitand Single stage CE Amplifier

PPN College, Kanpur

Department of Physics, P. P. N. (PG) College,

Kanpur – 208 001, U.P., IndiaEmail: [email protected],

[email protected]: dkpandey.weebly.com

Dharmendra Kumar Pandey

27 May 2020 1

mailto:[email protected]

27 May 2020 PPN College, Kanpur 2

Transistor Configuration

On the basis of common terminal between input and output

• CB mode: Base is common between input and output

• CE Mode: Emitter is common between input and output

• CC mode: Collector is common between input and output

On the basis of Biasing

• R-R Bias: Cut-off mode

Input/Ist Junction- Reverse Bias; Output/IInd Junction- Reverse Bias.

• F-R Bias: Active mode

Input/Ist Junction- Forward Bias; Outtput/IInd Junction- Reverse Bias.

• F-F Bias: Saturation mode

Input/Ist Junction- Forward Bias; Output/IInd Junction- Forward Bias.

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27 May 2020 PPN College, Kanpur 327 May 2020 PPN College, Kanpur 3

Transistor Current and Voltage: Two-Port Network

I1: Input current I2 : Output current

V2: Output voltage

• Relation in V and I provides: Four type of parameters

Impedance Z, Admittance Y, Hybrid h, Inverse-hybrid g

Both currents and Both voltages can be related by four types

Transistor as two port or four

terminal network

V1: Input voltage

I2 I1

V1V2

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)V,V(fI 211

)V,V(fI 212

2

1

2221

1211

2

1

V

V

Y Y

Y Y

I

I

2121111 VYVYI 2221212 VYVYI


2. Transistor V-I relation in terms of Admittance Parameters

)I,I(fV 211

)I,I(fV 212

2

1

2221

1211

2

1

I

I

Z Z

Z Z

V

V

2121111 IZIZV 2221212 IZIZV

1. Transistor V-I relation in terms of Impedance Parameters

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3. Transistor V-I relation in terms of hybrid Parameters

)V,I(fV 211

)V,I(fI 212

2

1

2221

1211

2

1

V

I

h h

h h

I

V

2121111 VhIhV 2221212 VhIhI

4. Transistor V-I relation in terms of Inverse hybrid Parameters

)I,V(fI 211

)I,V(fV 212

2

1

2221

1211

2

1

I

V

g g

g g

V

I

2121111 IgVgI 2221212 IgVgV

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)V,I(fV 211

)V,I(fI 212

2

1

2221

1211

2

1

V

I

h h

h h

I

V

2r1i2121111 VhIhVhIhV

2o1f2221212 VhIhVhIhI

Transistor hybrid equivalent circuit

Transistor V-I relation in terms of hybrid Parameters

02V1

1i

I

Vh

Four characteristic curves, Four hybrid Parameters

Input characteristic curve: Graph between V1 and I1 at constants V2.

hi: input impedance or resistance

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02I2

1r

V

Vh

Reverse voltage transfer characteristic: Graph between V1 and V2 at constants I2.

hr: reverse voltage gain/ratio

02V1

2f

I

Ih

Forward current transfer characteristic : Graph between I2 and I1 at constants V2.

hf: forward current gain/ratio

02I2

2o

V

Ih

Output characteristic: Graph between I2and V2 at constants I1.

h0: output admittance

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2r1i1 VhIhV

2o1f2 VhIhI

Transistor hybrid equivalent circuit in CE mode

cerebiebe VhihV

ceoebfec Vhihi D

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Characteristic curves in CE mode

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Biasing of transistor

Biasing is the process by which a proper DC source (VBB,VCC)/potential difference can be provided to the input andoutput circuit of the transistor.

Type of Biasing1. Fixed Bias or Base Bias

2. Collector to Base Bias

3. Emitter Bias or Self Bias

4. Potential Divider Bias

Correct Biasing1. Q-point should not vary.

2. Stability factor should be minimum.

3. There should not be thermal runaway.

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Stability Factor and Thermal runaway

c

B0C

C

dI

dI1

1

dI

dIS

0CBC I)1(II

1. T: increases ; ICO: Increases; IC: Increases

2. If IC : increases then temp: increases and further ICO:Increases

3. IC and Temp : continuously increases

4. Due to heating: the self destruction of unstabilizedtransistor is known as thermal runaway.

S :Minimum; Stability: High1. varies for same type/number of transistor2. Temperature causes change in leakage current

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Q-point or operating point

Q-point is an acronym for Quiescent point. Q-point is the operating pointof the transistor (ICQ,VCEQ) at which it is biased. The concept of Q-point isused when transistor act as an amplifying device and hence is operated inactive region of input output characteristics. To operate the BJT at a pointit is necessary to provide voltages and currents through external sources.

Why stabilization of operating point is needed?In practice the operating point varies shifts due to drift in temperature e.t.c. Astemperature increases Ico, β, Vbe gets affected. The reverse saturation currentalmost doubles for every 10 degree rise in collector junction temperature. Thebase to emitter voltage decreases by 2.5 milli volts for every one degree rise intemperature. Hence the operating point should be stabilized against thevariations in temperature so that it remains stable. To achieve this biasing circuitsare introduced.

The operating point of a device, also known as a bias point, quiescentpoint or Q-point, is the steady-state DC voltage or current at a specifiedterminal of an active device such as a transistor with no input signalapplied.

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Q-point or operating point : DC Load Line

1. (Vcc, 0) is cut off point where transistorenters in to cut off region from active region2. (0, Vcc/Rc) is saturation point where the

transistor enters saturation region.

CCCECC RIVV

C

CCCE

CC

R

VV

R

1I

Best Q-point position

2

VV CCCE

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Q-point or operating point : AC Load Line

It is straight line through thequiescent operating point buthaving slope corresponding toAC load resistance. The AC loadseen by the amplifier is differentfrom the DC load, and so theslope of AC load line is different.The AC load line indicates themaximum possible output-voltage swing, called the peak-to-peak output voltage (Vpp) for agiven amplifier configuration.This maximum Vpp is oftenreferred to as the compliance ofthe amplifier.

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Q-point or operating point : AC Load Line

1C

1C1C

RR

RRR IIRRac

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Potential Divider biasing of transistorDC equivalent of PotentialDivider biasing of transistor

Potential Divider Biasing of TransitorThe voltage divider bias method is the most prominent method for of providingbiasing and stabilization. Here, two resistors R1 and R2 are employed, which areconnected to VCC and provide biasing. The resistor RE employed in the emitterprovides stabilization.

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Voltage across R2 = V2=VTh

From the above expression, it is evident that IC doesn’t depend upon β. VBE isvery small that IC does not get affected by VBE at all. Thus IC in this circuit isalmost independent of transistor parameters and hence good stabilization isachieved. Stability factor =1

Potential Divider Biasing of Transitor

Applying KVL in base emitter junction

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Potential Divider Biasing: Numerical

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Potential Divider Biasing: Numerical

Example: Convert this previous emitter-bias example to voltage divider bias.Given that-

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CE amplifier

B

Ci

i

i(ac) A

in

LV

R

RA

in

L2P

R

RA

======================

b

be

21

21in

i

V

RR

RRR

LC

LCac

RR

RRR

in

acV

R

RA

in

ac2P

R

RA

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CE amplifier: Numerical

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CE amplifier

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CE amplifier : Hybrid equivalent circuit

R1 and R2: Biasing resistancewhich forms potential divider toprovide source (V2=VTh=VBB) toinput circuit.

RC : Biasing resistance toprovide appropriate source tooutput circuit.

RE : Stabilization resistance.

CB : Base capacitor whichforward only ac voltage of inputsignal for the amplification.

CE : By pass capacitor whichbypasses ac voltage through itto reduce the potential dropthrough RE.

CC : Collector capacitor whichforwards only amplified acvoltage in output.

VCC : Power dc source

vv

vv

vv

vv

vv

vv

vv

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CE amplifier hybrid equivalent circuit : Working

In put and out putvoltage are inopposite phase.

vv

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Since hre and hoe has very small values therefore they can beneglected. So, simplified hybrid equivalent circuit becomes as

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In the above circuit, rL=RC.If load resistance RL is connected across output then rL=RL II RC.Similarly In case of potential divider biasing, The input impedance ofinput circuit = hie II RB ; where RB=R1 II R2.


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Numerical

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PPN College, Kanpur

A Lot of Thanksfor kind attention

27 May 2020 37

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