Download Q point

Introduction to concepts in dc bias
circuit design of the amplifier
(BJT CE amplifier design-Voltage
divider bias )
Outline
• Concept of faithful amplification
•Identification of dc circuits
• What is Q point?
•How to measure and locate Q point?
• Interpret location of Q point
Concept of faithful amplification
• The major goal of amplifier is to amplify
applied input signal faithfully.
• When
is applied to the amplifier
then different types of output waveforms are
possible .
Let us find out what are the possibilities
Faithful amplification-key specification
• The waveforms shown below are possible
outputs of the amplifier
• Can you decide which of the above waveform
represents faithful amplification of applied
input signal(
)?
• (click on the waveform to select answer)
Look at upper part of waveform it is
clipped which shows distortion in
output waveform and output is not
faithful amplification of input signal
Try again
• Look at lower part of
waveform ,it is clipped which
shows distortion in output
waveform and output is not
faithful amplification of input
signal
Try again
Look at both lower and upper
part of waveform ,it is clipped
which shows distortion in
output waveform and output is
not faithful amplification of
input signal
Try again
• You are right! This is perfect sine wave
with no clippings at extremities and known
as faithful amplification
Go back to
question
Next
Essential components to decide
faithful amplification
For BJT amplifier circuit faithful amplification
can be predicted from
a)Amplitude of input signal and dc bias
circuit
b)Dc bias circuit and ac circuit
c)Ac circuit and amplitude of input signal
• This combination of ac and dc circuit helps to
decide only one parameter required to predict
faithful amplification and thus not sufficient.
Try again
• For this combination, amplitude of input signal
is partially useful to decide faithful
amplification but not complete specification
and ac circuit alone is not sufficient ,it needs
dc circuit to decide other parameter required to
predict faithful amplification.
•
Try again
• You are right !
• Faithful amplification of the amplifier can be
predicted from dc bias circuit and input signal
amplitude.
Go back
to
question
Next
Identification of dc circuits
We will study effect of dc bias circuit on amplifier design
Let us find what is dc bias circuit
• For the given CE amplifier circuit identify which is
appropriate dc circuit?
Select option by clicking on the circuit below
Vcc
Vcc
• In the selected circuit capacitors are present
,but for dc condition you need to open circuit
capacitors and only resistors in the circuit will
remain.
Try again
• You are right! . For dc condition all capacitors
open circuited and only resistors in the circuit
will remain. The circuit for dc conditions is
Vcc
Go back to
question
Next
Dc circuit parameter
Let us find what are important parameters of dc circuit to
decide faithful amplification
Q . Which of the following do you think is useful to
predict faithful amplification?
(click the box to select option)
Value of βdc
Resistor values
Q point location
• The values are helpful in calculating current
and voltage values of the dc circuit but are
not useful to predict faithful amplification
Try again
• You are right!
Faithful amplification depends on
location of Q point
Go back to
question
Next
How to locate Q point?
• To locate Q point which of the following
characteristics are important?
Ic= Vcc/Rc+ RE
a)Output characteristics
b)Load Line
Vcc
c)Input characteristics
• (Select options by clicking the buttons)
Only a
only b only c
a&b both a & c both
b&c both
•
You are right partially. One of the important
characteristics to locate Q point is output characteristics
,but not sufficient to locate Q point.
Select other options
Input characteristics are useful to locate Q
point partially (i.e. value of IBQ)but are not
useful to find other values of Q point(VCEQ,ICQ )
Try again
• You are right partially. One of the
important characteristics to locate Q point
is load line ,but not sufficient to locate Q
point.
Select other options
• This combination cannot fix Q point since ,it
is only set of characteristics .
Try again
• Load line is useful and with input
characteristics it can locate only IBQ value
.Other values(VCEQ and ICQ can not be
calculated.
Try again
• You are right! you need both output
characteristics and load line to locate Q point
Go back
to
question
Next
Animation below will show how Q point is located
• 1.To locate Q point we need to draw
Output characteristics
• Output characteristics –This is a set of
IB8 =
Collector Current( IC)
Ic= Vcc/Rc+ RE
Output
characteristics
characteristics . For each fixed value of base
current(IB) the relation between collector to emitter
voltage(VCE) and collector current(IC) is plotted .
IB7=
IB6=
• 2.Superimpose load line on the output
characteristics
• Load Line—The line joining voltage
Vcc and current Ic= Vcc/Rc+ RE
IB5 =
IB4 =
IBQ
Q point
IB3 =
IB2 =
IB1 =
3.Mark IBQ characteristics
IBQ—Base current of the given circuit
Vcc
under no signal condition. This is also
known as dc bias current. IBQ
characteristics are output characteristics
for IBQ
The point of intersection of IBQ characteristics and load line is “Qpoint”
“Q point” is thus point on the load line representing dc bias conditions of the amplifier
circuit
Collector Current( IC)
Find VCEQ and ICQ
IB8 =
Output
characteristics
IB7=
IB6=
IB5 =
IB4 =
Load line
ICQ
IBQ
Q point
Collector current under
no signal condition also known
as dc bias current(Q point
current)
VCEQ
IB3 =
IB2 =
IB1 =
Collector to emitter voltage under
no signal condition also known
as dc bias voltage(Q point voltage)
How to measure Q point
• To measure Q point, we measure value of base current
,collector current and collector voltage under dc conditions(no
Connect
signal condition).
voltmeter here
to measure
VCEQ
Connect ammeter
here to measure
ICQ
currents
IBQ
VCEQ
Locate Q point
. Once we measure values of currents and voltage we can locate Q point by two
methods
In the first method we fix output characteristics for IBQ(measured value) then
draw load line(by joiningVCC and Ic.) .Fix point as intersection of the two
Collector Current( IC)
IB8 =
IB7=
Ic= Vcc/Rc+
RE(4.08mA
IB6=
)
Q point
IBQ
IB5 =
IB4 =
IB3 =
IB2 =
IB1 =
Vcc=20V
In the second method we use VCEQ and ICQ values and load
line as shown
Ic=
Vcc/Rc+
RE=4.08
mA
Collector Current( IC)
•
ICQ
IB8 =
IB7=
IB6=
IB5 =
Q point IB4 =
IB3 =
IB2
=IB1 =
VCEQ
Vcc=20V
Interpretation of Q point
• Once we locate Q point by any of the two
methods ,we need to interpret the location of
Q point.
• First we will study different regions of load
line which will help us in interpretation of Q
point.
• Remember load line is essential characteristics
to locate and interpret Q point location.
Collector Current( IC)
40 µA
4mA
30 µA
3mA
20 µA
2mA
10 µA
1mA
In the output
characteristics
the region
below(slashed)
this point is cut
off region(here
both transistor
junctions are
reverse biased)
and transistor is
switched off
VBE<0.7V
Cut off region
In the output characteristics the region
beyond this point (slashed) is saturation region
and transistor saturates(both junctions of
transistor are forward biased) .Output current
is constant and there is no relation between
input and output current
VCE=0.1V
This diagram explains
Important regions of load
line
Interpretation of Q point(contd)
• In the process of interpretation of Q point
• Second important aspect is how to draw
output waveforms for applied input voltage for
given location of Q point.
• In the next slide Q point is already located and
we will observe how can we draw output
waveform?
Input voltage
40 µA
Colle
ctor
4mA
Curre
nt( IC)
3mA
FIG.1
30 µA
Q
20 µA
2mA
1mA
output current
waveform
10 µA
Cut off region
output voltage
waveform
Apply input
voltage
Why Q point location is important?
• If we change location of Q point will the shape of
output waveform change?
• Let us vary location of Q point on the load line and
observe the output waveform
Collector Current( IC)
40 µA
4mA
30 µA
3mA
20 µA
2mA
10 µA
Change the
location of Q
point by
selecting
value of IBQ
Click on
the box to
select value
of IBQ
1mA
Cut off region
Go to
design
tip
Collector
Current( IC)
40 µA
4mA
FIG.1
30 µA
3mA
Input voltage
20 µA
2mA
Q
10 µA
1mA
Cut off region
output current
waveform
This is output waveform in
which upper part of waveform
is clipped ,hence does not
represent faithful amplification
Q point is at
lower
extremity and
input goes to
cut off region)
output voltage
Base
waveform
voltage< 0.7V
Vary Q point
Input voltage
40 µA
Colle
ctor
4mA
Curre
nt( IC)
FIG.1
30 µA
3mA
Q
output current
waveform
•
This is perfect sine wave
with no clippings at
extremities and known as
faithful amplification
Q point
at the
centre of
load line
20 µA
2mA
1mA
•
10 µA
Cut off region
output voltage
waveform
Vary Q
point
Input voltage
Q
Q point at upper extremity and upper
portion of input voltage saturates
transistor
output
current
waveform
40 µA
Collector Current( IC)
4mA
FIG.1
30 µA
3mA
20 µA
2mA
10 µA
1mA
Cut off region
VCEQ= 0.1V
output voltage
waveform
This is output waveform in
which only upper portion of
the waveform is present and
lower portion is clipped ,hence
does not represent faithful
amplification
Vary Q
point
Collector
Current( IC)
Input voltage
40 µA
4mA
30 µA
3mA
Q
FIG.1
2mA
output current
waveform 1mA
For this location of
Q point upper
portion of the
input just reaches
to saturation
20 µA
10 µA
Cut off region
output voltage
waveform This is output waveform
in which small part of
lower portion of the
waveform is clipped ,hence
does not represent faithful
amplification
Vary Q
point
Design tip1
If Q point is located at the extremities then
output waveforms are clipped at upper
extremity or lower extremity.
It is important to decide Q point location near
centre to get faithful amplification
Practical value of VCEQ= Supply(VCC)/2(approx.)
You can go back to find
effect of Q point location
Faithful amplification-key specification
Look at upper part of waveform it is
clipped which shows distortion in
output waveform and output is not
faithful amplification of input signal
Explanatory feedback