Examcrazy Logo
HOME  SITEMAP CONTACT US LOGIN
HOME Engineering AIEEE GATE IES DRDO-SET BSNL-JTO
MBA in India CAT How to Prepare for Exams Technical Freshers Jobs
  Follow us|  twitter  Orkut  facebook
BJT Tutorials
   Bipolar Junction Transistor- NPN
   Bipolar Junction Transistor- PNP
   DC Analysis of BJT Circuits
   The BJT as a Signal Amplifier
   BJT Small-Signal Equivalent Circuit Models
   BJT Small-Signal Amplifier
   Graphical Analysis of a BJT Small-Signal Amplifier
   BJT Biasing - Current Mirror
   Common Emitter Amplifier
   Common Base Amplifier
   Common Collector (Emitter Follower) Amplifier
   BJT Internal Capacitances. High Frequency Circuit Model
Free Electronics Tutorials
   Diode Tutorials
   BJT Tutorials
   MOSFET Tutorials
   Electronics II Tutorials
   Applied Electromagnetics Tutorials
   Microwave Tutorials
GATE preparation tips
   GATE Books & How to prepare
   Objective Solving Tricks
   Other GATE links
   IES exam preparation
   All about DRDO-SET
More Engineering Links
   Directory of coaching Institutes
   Govt engg college rankings
   Private engg college rankings
   Admission notifications for Mtech/PhD
   All Engineering Colleges in India
The BJT as a Signal Amplifier

One very useful application of the transistor is an amplifier of time varying signals. Consider the “conceptual BJT amplifier” circuit shown below:

The DC voltages provide the biasing. The input signal is vbe and the output signal is vc. We will assume the transistor is biased so that VC is greater than VB by an amount that llows for sufficient “signal swing” at the collector, but the transistor remains in the active mode at all times. That is, the transistor does not become saturated or cutoff during the cycle. From the circuit above, the total base-to-emitter voltage is

Correspondingly, the collector current is

or using (5.53) ic=Ice
For small vbe such that Vbe<<2VVr (i.e., the small-signal approximation), then (3) can be approximated by

This is a familiar result: We saw something very similar with small signals and diodes back in Lecture 4.The time varying current in (4)

Can be written as ic=gwVbe

is defined as the transistor small-signal transconductance. Its units are Siemens. Note that gmIc.
Significance of the BJT Small-Signal Transconductance
What is the physical significance of gm? First, gm is the slope of the iC-vBE characteristic curve at the Q point:

Consider the plot shown in Fig.

with ic= Ise from (2), the right-hand side of (8) becomes

therefore
as we defined in (6). Observe that:
• The small-signal vbe assumption restricts the operation of the BJT to nearly linear portions of the iC-vBE characteristic curve.
• From (6), the BJT behaves as a voltage controlled current source for small signals: The small-signal be v controls the small-signal ic
Signal Voltage Gain
Second, gm has an important relationship to the signal voltage gain in this circuit. Using KVL in Fig. 5.48a, the total collector voltage is

where VC is the DC voltage at the collector. So from (11), the AC signal at the collector is Vc=-IcRc This result is negative, which means this circuit operates as an inverting amplifier for small, time varying signals. From (6), c mbe ic= gm vbe = . Using this result in (12) gives Vc=-gmVbeRc=-(gmRc)vbe
Consequently, the small-signal AC voltage gain Av is

In a broad sense, we can see that this transistor circuit can act an amplifier of the time varying input signal, provided this input voltage remains small enough.

gm is a very important amplifier parameter since the voltage gain in (14) is directly proportional to gm. BJTs have a relatively large gm compared to field effect transistors, which we will consider in the next chapter. Consequently, BJTs have better voltage gain in such circuits.


Discuss about BJT here
   START NEW THREADS
Discussion Board for BJT
Discuss all your issues or difficulties on BJT
Thread / Thread Starter Last Post Replies Views
example of BJT small signal
I want do to this


Posted By :-
 socheat
Jul 1, 7:35:08 AM 0 14809
drdo previous papar
preparation for drdo


Posted By :-
 rinkumathur.1988
Oct 16, 2:00:04 PM 1 16450
FET
Its operation and manufacturing.


Posted By :-
 dharmendra608@yahoo.in
Apr 16, 6:54:20 PM 0 11167
diode
sssssssssssssssss


Posted By :-
 sunilkumarmishra88
Dec 2, 4:03:32 PM 0 11803

To start your new thread you must login here.
New user signup at ExamCrazy.com Exam Crazy
To reply/post a comment you need to login, Use your user name and password to login if you are already registered else register here

EXISTING USER LOGIN
(Members Login)
Username:
Password:
NEW USER REGISTERATION FORM
Login-Id
Email-ID
Password
Confirm-Password
Full-Name

  About us | Privacy Policy | Terms and Conditions | Contact us | Email: support@Examcrazy.com  
Copyright © 2014 Extreme Testing House, India. All rights reserved.  4661