<< Chapter < Page Chapter >> Page >
Lecture 4 deals with low frequency and high frequency model of CB and CE BJT. In part A, I present the EberMoll Model. From Eber Moll Model I derive the T-Model of CB BJT at low as well as high frequency model.

AnalogElectronics_Lecture4_PartA_dc,low&high frequency model of CB BJT.

The DC model of BJT.

PNP is a face to face diode.

NPN is a back to back diode.

DC Biasing of CB BJT (Common Base BJT) In the figure base is grounded.

The DC model of CB BJT is called Eber Moll Model. It is a large signal model.

Figure 4. Eber Moll Model of PNP Transistor under no bias.

As can be seen from the Eber Moll Model:

α F I E is the controlled current source controlled by the current injected from E to B where α F is short circuit forward current transfer ratio.

α I I C is the controlled current source controlled by the current injected from C to B where α I is short circuit inverse current transfer ratio.

Because of asymmetrical doping density, [Heavy Emitter doping(10 19 /cc), Intermediate Base doping(10 17 /cc) and light Collector doping(10 16 /cc)]

emitter to base injection efficiency is 99%

whereas collector to base injection efficiency is less than 10% therefore (α I = 0.1)<<(α F =0.99).

Under Forward Active Mode :

EB diode is forward biased and CB diode is reversed biased. Hence current is being injected from emitter to base but no current is being injected from collector to base. Therefore EB diode is shown and CB diode has been omitted but there is reverse saturation current/reverse leakage current I CBO flowing through reverse biased CB junction. In fact there are two currents flowing across the CB junction. One is the leakage current I CBO and the other is the useful transistor current α F I E due to transistor action at EB junction.

Figure 5. Eber Moll Model under Forward Active Mode.


A capacitively coupled CB Amplifier is shown in Figure 6. It amplifies sinusoidal signal but blocks dc signal.

In Figure 6 we have capacitively coupled source and capacitively coupled load in CB BJT.

In Figure 6 we show a CB BJT Amplifier. Because of near unilaterality of CB BJT (h rb negligible), it is very suitable for RF tuned amplifiers. RF tuned amplifiers have a serious problem of parasitic oscillation because of output - input interaction and this interaction is due to h r (reverse transmission factor).

CE BJT has h re = 10 -4 hence CE BJT Amplifier is very prone to parasitic oscillations. But CB BJT has h rb = 10 -5 hence CB BJT is the configuration of choice for implementing RF tuned amplifiers.

Also capacitive coupling is used because once a Quiescent Point or DC operating point is set up we do not want it to be disturbed by load or source. That is load and source must be a.c. coupled but d.c. decoupled. Capacitive coupling serves this end.

Capacitively coupled Amplifiers (CE, CB&CC) are always ac amplifiers. But inverting amplifiers made of Op Amp with dual power supply is a direct coupled amplifier. There are no coupling capacitors. Hence they are dc amplifiers. DC amplifiers can amplify from dc to high frequencies.

In Figure 7 we have the instantaneous model of CB Amplifier with signal coupled to it.

Therefore i E (instantaneous emitter current) = I E (emitter quiescent current) + i e (emitter incremental current);

Similarly we have i C = I C + i c and i B = I B +i b ;

Also I C = β F ×I B and i c = β f ×i b ;

Here β F = dc short circuit current gain and β f = incremental short circuit current gain.

In BJT : β F = β f ;

The instantaneous model also contains r x = r bb’ = base spreading resistance.

This is an ohmic resistance offered by the thin layer of P type silicon sand-witched between Emitter layer and Collector layer as shown in the cross sectional view of IC vertical npn BJT (Figure 8)

Figure 8. Cross sectional View of Integrated Circuit Vertical NPN BJT.

Figure 9. Incremental Model of CB BJT Amplifier.(the controlled current source is α f i e ).

This incremental model is known as the T-model of CB BJT.

As in incremental diode circuit, here also we short the DC Voltage Sources(V CC and V EE ), we open circuit DC Current sources such as (I CBO + α F I E ) but we retain the incremental current source α f i e .

The coupling capacitances are shorted.

EB Diode is replaced by incremental resistance r e =(V T /I EQ ).

The ohmic resistances are retained as they offer resistance to DC as well as incremental component of the current.


Under high frequency condition the Junction Capacitances associated with the depletion layers EB and BC and Diffusion Capacitance associated with minority carriers stored in base region start effecting the performance of the BJT. These parasitic capacitances effect the Frequency Response as well as the short circuit current gain.

Figure 10. Incremental T-Model of the CB Amplifier Circuit at high frequency. (the controlled current source is α f i e )

At high frequency diffusion and junction capacitances come into play.

= C BO This is generally 5pF.

C D = more than 70pF.


is the transit time across the base.

= Depletion layer width at EB Junction

=Depletion layer width at BC Junction

=Cross sectional area of EB junction

=Cross sectional area of BC junction


is the useful transistor current which controls the output current sources (α fo i e ')and
is the parasitic current.
has no role to play in the transistor action

α fo =short circuit incremental forward current transfer ratio at low frequency.

This is true only when we consider r c = infinity and C C to be open.

This means this model is valid only till

/3. Because only till
/3 the above assumption is valid. Above
/3 this model gives incorrect result.

Here α f = short circuit current transfer ratio at all frequencies for CB BJT.

alpha cut-off frequency


is the transit frequency and
is the transit time.

The Bode Plot of the above expression of

is given in Figure 12.

Questions & Answers

where we get a research paper on Nano chemistry....?
Maira Reply
nanopartical of organic/inorganic / physical chemistry , pdf / thesis / review
what are the products of Nano chemistry?
Maira Reply
There are lots of products of nano chemistry... Like nano coatings.....carbon fiber.. And lots of others..
Even nanotechnology is pretty much all about chemistry... Its the chemistry on quantum or atomic level
no nanotechnology is also a part of physics and maths it requires angle formulas and some pressure regarding concepts
Preparation and Applications of Nanomaterial for Drug Delivery
Hafiz Reply
Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
Jyoti Reply
ya I also want to know the raman spectra
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
yes that's correct
I think
Nasa has use it in the 60's, copper as water purification in the moon travel.
nanocopper obvius
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
scanning tunneling microscope
how nano science is used for hydrophobicity
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
what is differents between GO and RGO?
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
analytical skills graphene is prepared to kill any type viruses .
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
Privacy Information Security Software Version 1.1a
Got questions? Join the online conversation and get instant answers!
Jobilize.com Reply

Get the best Algebra and trigonometry course in your pocket!

Source:  OpenStax, Solid state physics and devices-the harbinger of third wave of civilization. OpenStax CNX. Sep 15, 2014 Download for free at http://legacy.cnx.org/content/col11170/1.89
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Solid state physics and devices-the harbinger of third wave of civilization' conversation and receive update notifications?