Transistor Configuration

Quote of the day

“Try not to become a man of success, but rather try to become a man of

value.”

― Albert Einstein

Modes of Operation

3

Modes EBJ CBJ Application

Cutoff Reverse ReverseSwitching application

in digital circuitsSaturation Forward Forward

Active Forward Reverse Amplifier

Reverse active

Reverse ForwardPerformance degradation

• Relationship between the collector current and the base current in a bipolar transistor

– characteristic isapproximately linear

– magnitude of collectorcurrent is generallymany times that of thebase current

– the device providescurrent gain

B

C

I

I

CEV

ac

constant

dc = IC / IB

BJT Amplification(Current)

The output voltage Vo can be calculated as

B

Cv

V

VA

BJT Amplification (Voltage)

RIVV co

RIV cC

When input voltage Vi change the VB changes which cause IB to change. The change in base current produces change in collector current

BdcC IIei .. The change in Collectorvoltage Vc is

i

ov

V

VA

The voltage amplification Av is

Ic Ic

VB VB

= V0 Vc

Example. Determine the dc collector voltage for the circuit shown below if the transistor has the input characteristics shown in side figure and = 80. Calculate the circuit voltage gain when input variation is 50mV

+50mV

-50mV

Ic =?mA=18V

=10K

=?V

Solution: From side fig IB=15 for VBE= 0.7 V From fig2 V=18V & R-10K, Given =80, VB = 50mV

AII Bc 1580 mAIc 2.1

RIVV co 33 1010102.118

oV

VVo 6

From above fig for IB= 3A, Vi= 50mV

AII Bc 380 AIc 240

RIV cC KAVC 10240

i

ov

V

VA

VVC 4.2

mv

VAv

50

4.2

48vA

Transistor Configurations

• Common-Base Configuration

• Common-Emitter Configuration

• Common-Collector Configuration

Common-Base Configuration

Common-Base Configuration• The common-base configuration with pnp and

npn transistors are shown in the figures in the previous slide..

• The term common-base is derived from the fact that the base is common to both the input and output sides of the configuration.

• The arrow in the symbol defines the direction of emitter current through the device.

• The applied biasing are such as to establish current in the direction indicated for each branch.

• That is, direction of IE is the same as the polarity of VEE and IC to VCC .

• Also, the equation IE = IC + IB still holds.

Input characteristics• The driving point or input

parameters are shown in the figure.

• An input current (IE) is a function of an input voltage (VBE) for various of output voltage (VCB ).

• This closely resembles the characteristics of a diode.

• In the dc mode, the levels of IC and IE at the operation point are related by:

αdc = IC / IE

• Normally, α 1.• For practical devices, α is

typically from 0.9 to 0.998.

Figure: Input characteristics for common-basetransistor

Input characteristics contd..

• As an approximation, the change due to changes in VCB can be ignored.

• The characteristics can be shown in orange curve.• If piecewise-linear approach is applied, the

magenta green curve is obtain.• Furthermore, ignoring the slop of the curve and the

resistance results the magenta curve.• It is this magenta curve that is used in the dc

analysis of transistors.• Once a transistor is in “on” state, the B-E voltage is

assumed to be 0.7V.• And the emitter current may be at any level as

controlled by the external network.

Figure: Output characteristics for common-base transistor

CutoffRegion

SaturationRegion

Active Region

Output characteristics

Figure: Output characteristics for common-base transistor

Output characteristics

Output characteristics• The output set relates an output current (IC ) to an output voltage (VCB)

for various of level of input current (IE ).There are three regions of interest:

Active region• In the active region, the b-e junction is forward-biased, whereas the c-b

junction is reverse-biased.• The active region is the region normally employed for linear amplifier.

Also, in this region,I C IE

Cutoff region• The cutoff region is defined as that region where the collector current

is 0A.• In the cutoff region, the B-E and C-B junctions of a transistor are both

reverse-biased.Saturation region:

• It is defined as that region of the characteristics to the left of VCB= 0 V.• In saturation region, the B-E and C-B junctions of a transistor are both

forward biased.

• common-emitter configurations– Most common configuration

of transistor is as shown

– emitter terminal is common to input and output circuitsthis is a common-emitterconfiguration

– we will look at the characteristics of the device in this configuration

– The current relations are still applicable, i.e.,

– IE = IC + IB and IC =α IE

The common-emitter configuration with npn and pnp transistors are shown in the figures.

Figure: Common-emitter configuration of pnp transistor

Figure: Common-emitter configuration of npn transistor

• Input characteristics– the input takes the

form of a forward-biased pn junction

– the input characteristics are therefore similar to those of a semiconductor diode

An input current (IB) is a function of aninput voltage (VBE) for various of outputvoltage (VCE ).

Figure: Output characteristics for common-emitter transistor

CutoffRegion

SaturationRegion

Active RegionOutput characteristics

• Output characteristics– The magnitude of IB is in μA and not as horizontal as IE

in common-base circuit.

– The output set relates an output current (IC) to an output voltage (VCE) for various of level of input current (IB ).

• There are three portions as shown:Active region The active region, located at upper-right quadrant, has

the greatest linearity. The curve for IB are nearly straight and equally

spaced. In active region, the B-E junction is forward-biased,

whereas the C-B junction is reverse-biased. The active region can be employed for voltage,

current or power amplification.

Cutoff region

• The region below IB = 0μA is defined as cutoff region.

• For linear amplification, cutoff region should be avoided.

Saturation region:

• The small portion near the ordinate, is the saturation region, which should be avoided for linear application.

• In the dc mode, the levels of IC and IB at the operation point are related by: Normally, ranges from 50 to 400.

dc = IC / IB

For ac situations, is defined as B

C

I

I

tconsV

ac

CE

tan

Biasing•The proper biasing is essential to place the device in the active region.•A common-emitter amplifier of a pnp transistor is shown in the figure.•The first step is to indicate the direction of IE as established by the arrow in the transistor symbol.

Figure: Biasing for common-emitter pnp transistor

•The other current , IB and IC , are introduced, satisfyingIC + IB = IE .

•The supplies are introduced with polarities that will support the resulting directions of IB and IC .•If the transistor is a npn transistor, all the current and polarities would be reversed.

Base Width Modulation: “Early” Effect

• When bias voltages change, depletion widths change and

the effective base width will be a function of the bias

voltages

• Most of the effect comes from the C-B junction since the

bias on the collector is usually larger than that on the E-

B junction

Base width gets smaller as applied voltages get larger

The Early Effect

Converge ~ at single point called "Early Voltage" (after James Early)

Large "Early Voltage" = Absence of "Base Width Modulation"

= Transistor ~ immune to operating voltage changes

BUT requires wide base => lower gain

Range is -100V to -200 V

Common-Collector Configuration• The common-collector configuration with npn and

pnp transistors are shown in the figures.

Figure: Common-collector configuration of npn transistor

Figure: Common- collector configuration of pnp transistor

• It is used primarily for impedance-matching purpose since it has a high input impedance and low output impedance.

•The load resistor can be connected from emitter to ground.

•The collector is tied to ground and the circuit resembles common-emitter circuit.

•The output set relates an output current (IE) to an output voltage (VCE) for various of level of input current (IB ).

Common-Collector Configuration

Input characteristics• It is a curve which shows the relationship between

the base current, IB and the collector base voltage VCB

at constant VCE This method of determining the characteristic is as follows.

• First, a suitable voltage is applied between the emitter and the collector.

• Next the input voltage VCB is increased in a number

of steps and corresponding values of IE are noted.

• The base current is taken on the y-axis, and the input voltage is taken on the x-axis. Fig. shows the family of the input characteristic at different collector- emitter voltages.

Input characteristics

Figure: Common-collector circuit used for impedance-

matching purpose

• The following points may be noted from the family of characteristic curves.

• Its characteristic is quite different from those of common base and common emitter circuits.

• When VCB increases, IB is decreased.

• This is almost the same as the output characteristics of common-emitter circuit, which are the relations between IC and VCE for various of level of input current IB.

Since that: IE IC .

Output characteristics

Figure: Output characteristics for common-collector transistor