Basic BJT Amplifiers Circuits

1 Bipolar junction transistors (BJTs)2 Single-Stage BJT Amplifiers3 Frequency Response4 Power Amplifiers

Saturday, April 18, 2020 1Anil K. Singh, Transistor ;ECC, ALD.

The transistor was probably the most important invention of the 20th Century, and the story behind the invention is one of clashing egos and top secret research.

First - BJTs

Reference:Bell Labs Museum

B. G. Streetman & S. Banerjee ‘Solid State Electronic Devices’, Prentice Hall 1999.Saturday, April 18, 2020 2Anil K. Singh, Transistor ;ECC, ALD.

Picture shows the workbench of John Bardeen (Stocker Professor at OU) and Walter Brattain at Bell Laboratories. They were supposed to be doing fundamental research about crystal surfaces.

The experimental results hadn't been very good, though, and there's a rumor that their boss, William Shockley, came near to canceling the project. But in 1947, working alone, they switched to using tremendously pure materials.

It dawned on them that they could build the circuit in the picture. It was a working amplifier! John and Walter submitted a patent for the first working point contact transistor.

Interesting story…

Shockley was furious and took theirwork and invented the junction transistorand submitted a patent for it 9 days later.The three shared a Nobel Prize in 1955.Bardeen and Brattain continued inresearch (and Bardeen later won anotherNobel).

Shockley quit to start a semiconductorcompany in Palo Alto. It folded, but itsstaff went on to invent the integratedcircuit (the "chip") and to found IntelCorporation.

By 1960, all important computers usedtransistors for logic, and ferrite cores formemory.

Interesting story…

Point-Contact Transistor –first transistor ever made

Qualitative basic operation of point-contact transistor

Problems with first transistor…

First Bipolar Junction TransistorsW. Shockley invented the p-n junction transistorThe physically relevant region is moved to the bulk of the material

Understanding of BJT

force – voltage/currentwater flow – current

- amplification

Basic models of BJT

npn transistor

pnp transistor

Nomenclature

• NPN

• Roughly read as

• NOT POINT IN

• It means points are outward i.e.

• Arrows of the two diodes (P-type) are in outward directions.

Qualitative basic operation of BJTs

Basic models of BJT

BJTs – Basic ConfigurationsFluid Flow AnalogyDifference between FET (field effect transistor) and BJTTechnology of BJTs

pnp BJT npn BJT

BJTs – Practical Aspects

Heat sink

BJTs – Testing

Gain medium

Incoherent Light

Coherent Light

Transistor/switch/amplifier – a 3 terminal device

Source

ArteryVein

Emitter Collector

Ion Channel

Dam Laser Heart

Axonal conductionMOSFETBJT

All of these share a feature with…

• Output current can toggle between large and small

(Switching Digital logic; create 0s and 1s)

• Small change in ‘valve’ (3rd terminal) creates Largechange in output between 1st and 2nd terminal

(Amplification Analog applications; Turn 0.5 50)

Basic BJT Amplifiers Circuits1 Bipolar junction transistors (BJTs)

Construction of Bipolar junction transistors

Base region(very narrow)

Emitter region

Collector region

Collector

Emitter

Emitter-base junction

Collector-base junction

Standard bipolar junction transistor symbols

Depending on the biasing across each of the junctions, different modes of operation are obtained – cutoff, active and saturation

There are three basic configurations for single-stage BJT amplifiers:– Common-Emitter – Common-Base– Common-Collector

VBB VCC

E B CV V V E B CV V V E B CV V V

BJT in Active Mode

Two external voltage sources set the bias conditions for active mode

– EBJ is forward biased and-- CBJ is reverse biased

Hybrid Parameters

Condition

hi Input resistance Output shorted

hr Voltage feedback ratio Input open

hf Forward current gain Output shorted

ho Output conductance Input open

Second subscript indicates common base (b), common emitter

(e), or common collector (c)

Hybrid Parameters

= Slope of curve

Hybrid Parameters

hie = VB/IB Ohm’s Law

hie =input impedance

hre = VB/VC

BJT in Active Mode

Forward bias of EBJ injects electrons from emitter into base(small number of holes injected from base into emitter)

IE＝IEN＋IEP IEN

BJT in Active Mode

• Most electrons shoot through the base into the collector across the reverse bias junction

• Some electrons recombine with majority carrier in (P-type) base region

IB ＝IBN＋ IEP

BJT in Active Mode

Electrons that diffuse across the base to the CBJ junction are swept across the CBJ depletion region to the collector.

IC = ICN + ICBO

BJT in Active Mode

IE＝IEN＋IEP IEN IC = ICN + ICBO

IE = IB + IC

Let ICN＝IE

I ---common-base current gain

IC (1－) = IB + ICBO

IB＝IBN＋IEP

BJT in Active Mode

IE＝IEN＋IEP IEN IC=ICN+ICBO IE=IB+IC

I IC (1－)= IB+ICBO

IB＝IBN＋IEP

BCEOBC

CBOBC III )1( bb

Ib ---common-emitter current gain Beta:

＋＋

－－

vBE vCE

BJT Equivalent Circuits

BJT DC model

＋＋

－－

VBE＝Von VCE

•Use a simple constant-VBE

model– Assume VBE = 0.7V

BJT DC Analysis

• Make sure the BJT current equations andregion of operation match

VBE > 0, VBC < 0, VE < VB <VC

• Utilize the relationships (β and α) betweencollector, base and emitter currents to solvefor all currents

C-E CircuitsI-V Characteristics

Base-emitter Characteristic(Input characteristic)

CCEvBEB vfi

C-E Circuits I-V Characteristics

Collector characteristic (output characteristic)

CiVC BCEfi )(

AμiB 40=

Collector characteristic (output characteristic)CiVC BCE

Saturation

Collector characteristic

Saturation occurs whenthe supply voltage, VCC, isacross the total resistanceof the collector circuit, RC.

IC(sat) = VCC/RC

Once the base current is high enough to produce saturation, further increases inbase current have no effect on the collector current and the relationship IC = bIB isno longer valid. When VCE reaches its saturation value, VCE(sat), the base-collectorjunction becomes forward-biased.

Cutoff

When IB = 0, the transistor is incutoff and there is essentially nocollector current except for avery tiny amount of collectorleakage current, ICEO, which canusually be neglected. IC 0.

In cutoff both the base-emitterand the base-collector junctionsare reverse-biased.

linearity

Discussion of an amplification effect

CEBEi L

B Ci iWith i ov v

50 ~ 300ov

E.g. for common-base configuration transistor:

DC Load Line and Quiescent Operation Point

DC load line

Q-point

)(40 AR

Base-emitter loop:

kiRiVv CCCCCCE 410 Collector-emitter loop:Saturday, April 18, 2020 43Anil K. Singh, Transistor ;ECC, ALD.

Amplifiers CircuitsSingle-Stage BJT Amplifiers

C-E Amplifiers

To operate as an amplifier, the BJT must be biased to operate in active mode and then superimpose a small voltage signal vbe to the base.

i vviivv CBC 12 b

DC + small signal

OC vi Bi iv CB ii

coupling capacitor(only passes ac signals)

C-E Amplifiers

vBE=vi+VBE

bBB iIi

Apply a small signal input voltage and see ib

C-E Amplifiers

• vi = 0 IB、IC、VCE

ceCECE

iIiv 0

)()( ioiMoM ffVV •

• vo out of phase with vi

iC=ic+IC

vCE=vce+VCE

See how ib translates into vce.

C-E Amplifiers Considering (all the capacitors are replaced by open circuits)

Considering (all the capacitors are replaced by short circuits)

C-E Amplifiers

Considering (all the capacitors are replaced by open circuits)

Considering (all the capacitors are replaced by short circuits)

Graphical Analysis

• Can be useful to understand the operation of BJT circuits.• First, establish DC conditions by finding IB (or VBE)• Second, figure out the DC operating point for IC

Can get a feel for whether the BJT will stay in active region of operation– What happens if RC is larger or smaller?Saturday, April 18, 2020 50Anil K. Singh, Transistor ;ECC, ALD.

Graphical Analysis

')//( LcLCcce RiRRiv

Graphical Analysis

Q-point is centered on the ac load line:

Graphical Analysis

Clipped at cutoff(cutoff distortion)

Q-point closer to cutoff:

Graphical Analysis

Clipped at cutoff(saturation distortion)

Q-point closer to saturation:

Graphical Analysis

Steps for using small-signal models1. Determine the DC operating point of the BJT

－ in particular, the collector current2. Calculate small-signal model parameters: rbe3. Eliminate DC sources

– replace voltage sources with short circuits and current sources with open circuits4. Replace BJT with equivalent small-signal models5. Analysis

IC ≈ βIB, IE = IC + IB = (1+β)IB

eEBEbBCBC RIVRIR)II(V

))(1( eb

)( eECCCCE RRIRIVV

Example 1

Example 2

BEBEC RV

VVII /

)RR(IVV eCCCCCE

Common-Base Amplifier (C-B Configuration)

(a) 共基极电路

eEBEB RIVV

II )( eCCCCeECCCCCE RRIVRIRIVV

Ground the base and drive the input signal into the emitter

Common-Base Amplifier

(a) 共基极电路

)//()//( b

For RL<<RC, CEi IIA

since1)1( b

//)1( b

Ro≈RCSaturday, April 18, 2020 60Anil K. Singh, Transistor ;ECC, ALD.

Common-Base Amplifier

(a) 共基极电路

For RL<<RC, 1)1(

)1( bb

Ro≈RCCB amp characteristics:• current gain has little dependence on β• is non-inverting• most commonly used as a unity-gain current amplifier or current buffer and not as a voltage amplifier: accepts an input signal current with low input resistance and delivers a nearly equal current with high output impedance

• most significant advantage is its excellent frequency responseSaturday, April 18, 2020 61Anil K. Singh, Transistor ;ECC, ALD.

C-C C-E C-B

Output

Functions

Summary for three types of diodes:

BI BIBI

EI CI CIZout < Zin

Vout > Vin

Zout > Zin

Vout > VinVout ≈ Vin

Zout > Zin

Basic Concepts

Lower cut off frequency Upper cut off frequency

)()()()( vvv AAorffAA

The drops of voltage gain (output/input) is mainly due to:1、Increasing reactance of (at low f)2、Parasitic capacitive elements of the network (at high f)3、Dissappearance of changing current (for transformer coupled amp.)

ecs CCC ,,

Frequency Response of the CE Amplifier

At low frequencies, C1, C2 are anopen circuit and the gain is zero.Thus C1 has a high pass effect on thegain, i.e. it affects the lower cutofffrequency of the amplifier.

)////( 2111 bebbs rRRRC

2 is the time constant for C2. 12 ---is neglected

)////( 2111 bebbs rRRRC

12 ---is neglected

Capacitor Ce is an open circuit. Thepole time constant is given by theresistance multiplied by Ce.

eebesb

e CRrRR

11.1 LeLLL ffff e

C'rbe C'

vs )1)(1(HL

frequency-mid0,, —vmv

HL AAf

ffffFor

frequency-low

,0),( —

fffffFor

frequencyHigh

fffffFor

decadedecade

Class-A Amplifiers

Class-B Amplifiers

Class-AB Amplifiers

• THANKS

Basic BJT Amplifiers Circuits

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