Download 1 Static Characteristics I

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Static Characteristics I
1.1 The Ideal Inverter
Before examining the performance of the bipolar transistor inverter, it
is useful to have a knowledge of what the requirements of an ideal
inverter are. The gate is expected to accept an input logic signal within
defined voltage levels, invert it and provide an output voltage to drive
other gates as shown in Fig. 1.1.
ISNK
ISCE
Fig. 1.1 The Ideal Inverter
(a) Logic Voltages
transfer
characteristic
Vo
Vcc
logic HI VH = VCC
logic LO VL = 0V
If Vi < VCC/2, VO = VH = VCC
Vcc/2
VCC
If Vi > VCC/2, VO = VL = 0V
Vi
(b) Drive Capability
Ideally ISNK, ISCE → ∞
VO should be independent of load conditions
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(c) Noise Immunity
Vi
Vcc
Vcc/2
t
Vo
Vcc
The output should be insensitive to noise superimposed on the input
and should provide defined switching.
(d)
Switching Speed
Vi
Vo
Ideally, there should be no switching delay but an instantaneous
response of the output to a change at the input.
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1.2 The Bipolar Transistor Switch
Essentially, the bipolar transistor switch shown in Fig. 1.2 operates in
one or other of two states.
OFF state – Transistor in Cut-Off mode.
ON state – Transistor in Saturation mode.
V CC
IC
RC
IB
T
RB
V O = V CE
Vi
Figure 1.2
A Simple Bipolar Transistor Inverter
The base resistor, RB, serves to control the level of base current. The
collector resistor, RC, serves to limit the maximum collector current
but acts essentially as an output load for the transistor.
Fig. 1.3 shows a set of output characteristics for the transistor as IC vs
VCE for a range of values of IB. A load line can be superimposed on
these curves which defines the operating path followed by the output
of the transistor for the particular load resistor, RC and supply voltage
VCC. This path is established as follows.
Load Line:
IC =
VCC − VCE
1
VCC
=−
VCE +
the equation of a straight
RC
RC
RC
line
(i) when IC = 0
VCE VCC
=
RC
RC
(ii) when VCE = 0
IC = VCC/RC = IC max
∴VO = VCE = VCC
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These points can be used to plot the load line superimposed on the
characteristic curves of the transistor. Figs. 1.3 and 1.4 show the
contrast between operating the transistor as an amplifying device in
the forward active region and operating it as a logic switch where it
has two states; either ON in saturation mode or OFF in cut-off mode.
(i) Operation as an Amplifier
When operating in the linear active region, the base emitter junction is
forward biased and the base-collector junction is reverse biased.
Under these conditions:
I C = β F IB
and
VO = VCE = VCC - ICR C
It can be seen that a small signal superimposed on the base current is
amplified to give a much greater change on the collector current. The
variation in collector current, passing through the load resistor, RC,
converts this into an output voltage signal. Note that, as IC increases,
VO decreases and vice-versa so that the signal is inverted from input to
output.
(ii) Operation as a Switch
When acting as a switch, the transistor operates outside of the linear
active region under steady-state conditions and only passes through it
when changing state. As a switch, the transistor operates either in the
cut-off region or the saturation region.
Cut-Off
With Vi = 0,
VBE = 0,
IB = 0,
then IC = 0,
Transistor is OFF
Then VO = VCC – ICRC = VCC
Hence, with Vi = 0 = VL input LO, we have VO = VCC = VH output HI
This is a logic inverting action.
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Saturation
In the linear active region, I C = β F I B . When using the transistor as a
switch, the load resistor, RC, is used as a current limiting resistor to
limit the collector current to IC = IC max = VCC/RC. If a high base current
is injected into the transistor such that:
β F IB >> IC MAX
i.e.
IB >>
IC MAX
βF
Then the transistor cannot amplify the base current, IB, because the
potential resulting current cannot flow in the collector, as the collector
current is limited to IC max. In this case, the transistor cannot continue
to operate in the forward active region and is driven into the
saturation region. Here with:
Vi = VCC ,
IB =
VCC - VBE
RB
then
The Transistor is ON and VO = VCE SAT → 0
IC = IC MAX =
VCC - VCE SAT
RC
typically 0.1 – 0.2V
Hence with:
Vi = VCC = VH
input HI
we have VO = VCE SAT = VL
output LO
This is also a logic inverting action.
In saturation, the base of the transistor is said to be overdriven.
That is to say more current is fed into the base than is required to
produce the maximum current that can flow in the collector. Normally,
4 to 5 times the current needed to bring the collector current to IC max
is injected into the base to guarantee that it is overdriven and the
transistor operates in the saturation mode.
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IC
mA
VCC/ RC
IB µA
10
A
90
IB9
9
80
IB8
8
70
IB7
7
Forward Active Region
60
IB6
6
t
50
IB5
5
IC = β F IB
IB
40
IB4
4
30
IB3
3
20
IB2
2
10
IB1
1
VCE
1
2
3
4
5
V
VCC
VO = VCC - ICRC
Fig. 1.3
Operation of the Bipolar Transistor as an Amplifier
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t
IC
mA
VCC / R C
IB
10
90
Saturation
IC = ICMAX
=
VCC - VCE SAT
RC
9
8
7
IB8
70
IB7
60
4
3
2
IB5
40
IB4
30
IB3
20
IB2
0
V O = V CE SAT
1
2
3
4
IB6
50
10
1
IB9
80
6
5
µA
5
VCC
IB1
VCE
IC → 0
V
Cut-off V O = V CC
Fig. 1.4
Operation of the Bipolar Transistor as a Switch
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