Transcript lecture1:ac analysis

AC ANALYSIS BJT
1
Content
BJT SMALL SIGNAL
- BJT small signal operation
- BJT AC equivalent circuits
2
Objectives
Understand the concept of an analog
signal and the principle of linear amplifier.
Investigate the process a single-transistor
circuit can amplify a small, time-varying
input signal.
3
Introduction
 Signals contain some type of information.
 The electrical signals in form of time-varying
current and voltage are analog signal.
 Electronic circuit that process analog signal –
analog circuit, example linear amplifier.
 Linear amplifier – magnify an input signal and
produce an output signal whose magnitude is
larger and directly proportional to input signal
4
Example of linear amplifier
Block diagram of a compact disc player
system.
5
Cont..
 From figure, a dc voltage source connected to
amplifier.
 The amplifier contain transistors that must be
forward biased so that they can act as amplifying
devices.
 We want the o/p signal to be linearly proportional
to input signal o/p of speaker is an exact
reproduction of signal from compact disc.
 So, we need linear amplifier.
6
Cont..
2 type of amplifier analysis:
dc analysis due to applied dc voltage source.
ac analysis due to time-varying signal source.
dc analysis is performed by ac source set
to zero ~ large signal analysis.
ac analysis is performed by dc source set
to zero ~ small signal analysis.
7
Bipolar linear amplifier
 Transistor -- heart of an amplifier.
 Bipolar transistors is used in linear amplifier cct
because of their high gain.
 Figure (a) – cct where input signal vI contain dc and
ac signal. Figure (b) – VBB is dc voltage to bias
transistor at Q-point and vs is ac signal that is to be
amplified.
 DC biased transistor @ Q-point  transistor biased
in forward-active region.
 A time-varying (sinusoidal) signal is superimposed on
dc input voltage, VBB, o/p voltage change along the
curve producing a time-varying o/p voltage.
8
9
Cont bipolar linear amplifier
 If o/p voltage directly proportional to and larger than i/p
voltage  linear amplifier cct.
 If transistor is not biased (in cutoff or saturation), o/p
voltage doesn’t change with a change in i/p  cct is not
an amplifier.
 Summary of notation
Variable
iB, vBE
IB, VBE
ib, vbe
Ib, Vbe
Meaning
Total instantaneous values
DC values
Instantaneous ac values
Phasor values
10
Graphical analysis & ac equivalent circuit
 The graph shows collector current, iC vs c-e voltage, vCE
for different values of iB. Q-point is chosen where
distance between iB curves are even to get linear
amplification.
 Line between VCC / RC and VCC –- dc load line.
 Signal source, vs produce ac base current superimposed
on quiescent base current.
 ac collector current produce a time-varying voltage
across RC, that induces an ac c-e voltage, vCE.
 vCE or vO will be larger than i/p to produce signal
amplification.
11
Graphical analysis cont..
Common-emitter cct with
a time-varying signal source
in series with dc source.
Common-emitter transistor characteristic,
dc load line and sinusoidal variation
in base current, collector current
and collector-emitter voltage.
12
Graphical analysis cont..
 Signal source, vs in base cct
generate time-varying component
in base cct –- iB and vBE.
 Figure shows the exponential
relationship between iB and vBE.
 If magnitude of time-varying signal
superimposed on dc quiescent pt
is small => develop a linear r/ship
between ac vBE and ac iB.
 This r/ship corresponds to the
slope of curve at Q-point.
 Slope at Q-point is inversely
proportional to a small-signal
parameter, rΠ.
13
Cont..
 From figure above, relation between vBE and iB is:
 v BE
iB 
. exp

 VT
IS



 If vBE is composed of dc term with sinusoidal component
superimposed, vBE=VBEQ + vbe, then
 V BEQ  v be
iB 
. exp

VT

IS




 The term [IS/β].exp (VBEQ / VT) is quiescent base current,
we can write
 v BE
i B  I BQ . exp
 VT



14
Cont..
 The base current eq. is not linear and can’t be written as
ac current superimposed on dc quiescent value.
 If vbe << VT, we can expand the exponential term in a
Taylor series, keeping only linear term which lead to
small signal approximation.
 v
i B  I BQ  1  BE
 VT
I BQ

  I BQ 
.v BE  I BQ  i b
VT

 Where ib is the time-varying base current
 I BQ
i b  
 VT

v be


15
AC equivalent circuit
RC
i
c
vO
RB
i
vs
b
+
vbe -
+
vce
-
AC equivalent circuit of C-E with npn transistor
16
Equations
Input loop:
vs  ib RB  vbe
 I BQ 
vbe
ib  
 VT 
Output loop:
ic RC  vce  0
Set all dc current and voltage
to zero – voltage become
short cct & current become
open cct.
0.026 V
ic  ib
17
Rules for ac analysis
Replacing all capacitors by short circuits
Replacing all inductors by open circuits
Replacing dc voltage sources by ground
connections
Replacing dc current sources by open
circuits
18