Download 2 Static Characteristics II

2. Static Characteristics II
2.1 Simple Transistor Inverter
VCC
IC
RC
IB
Logic Levels:
HI Vout = VCC
LO Vout = 0V ( VCE SAT)
C
B
RB
T
VO
E
Vi
Fig. 2.1 Simple Bipolar Transistor Inverter
With V  VL  0V,
IB  0,
i
Input LO
Output HI
IC  0
VO  VCC  VH
logic inverter action
Under these conditions, the transistor is cut-off or simply “OFF”. This
is equivalent to it acting like an open switch as shown in Fig. 2.2,
where the state of the switch is controlled by the input to the base.
Since the switch does not allow collector current to flow, then the
output voltage is pulled up to the supply rail by the collector resistor,
RC. In reality, the transistor is not an ideal switch and some leakage
current flows through it making it look like a very large resistance >>
100k but which does not appreciably lower the HI output voltage.
VCC
Rc
Ic = 0
Fig. 2.2 The Transistor
Inverter in OFF Switch Mode
Vi
controls
switch
Vo = Vcc
1
On the other hand with,
Vi  VH  VCC ,
Input HI
IB 
VCC - VBE
,
RB
Output LO :
IC 
VCC - VCE SAT
 IC MAX
RC
and VO  VCE SAT  VL
logic inverter action
Under these conditions, with IB > IC max/βF, the transistor is driven into
saturation and is referred to as “ON”. This is equivalent to it acting like
a closed switch as shown in Fig. 2.3. In this case, the switch allows
maximum collector current to flow so that all of the supply voltage is
dropped across the resistor, RC, and the output voltage falls close to
zero. In practice, there will be some small voltage drop between the
collector and emitter of the transistor. This is usually less than 0.1V
but at worst case is taken as 0.2V and is called VCE sat.
V CC
Rc
IC  IC
MAX
Figure 2.3 The Transistor
Inverter in ON Switch Mode
Vi
controls
switch
V O 0V ( V CE )
SAT
Note that, since VCE sat is close to zero in the saturation mode of
operation then the power dissipated by the transistor, which is given
as P = VCE x IC, is very small. The important thing is to ensure that
sufficient base current is provided with the input voltage HI to
guarantee and maintain the transistor operating in the saturation
region.
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2.2 Base Overdrive Factor
The base overdrive factor is a measure of how far into saturation a
transistor is driven i.e. how much the base current is above that which
is necessary to bring the transistor to the edge of saturation (the
boundary between saturation and the forward active region).
Base Overdrive Factor σ
u

Actual Base Current
Base Current Required to Reach Edge of Saturation
The base current required to reach the edge of saturation is simply the
base current required to initially bring the collector current to its
maximum value. This is ICMAX/βF .
Then:
σu 
IB
I C max β F

(VCC  VBE SAT )/R B
(VCC  VCE SAT )/β FR C
If VCC  VBE SAT , VCE SAT then this approximates to:
σu 
VCC RB
R
 F C
VCC FR C
RB
Normally, a σ u of 5 is used to allow for temperature variations in  F and
also the fact that  F itself is reduced when a transistor is operating in
the saturation region. It also allows for manufacturing variations
in  F which can be as much as 3:1 for a discrete transistor. The degree
of overdrive also becomes reduced when a load is connected to the
circuit.
2.3
Base Charge in Saturation
As the base current of the transistor is increased from zero, the
collector current rises proportionately in the forward active region.
The excess minority charge concentration present in the base region
also increases proportionately. When the transistor enters the
saturation region, however, the collector current remains constant at
its maximum value, IC MAX , but the minority charge concentration
continues to increase proportionately as the base current is further
increased and the transistor is overdriven well into the saturation
region as shown in Fig. 2.4. The excess minority charge stored in the
base rises as the base current is increased in the forward active
region. The slope of this profile reaches a maximum at the edge of
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IC
Forward
active
region
Saturation
region
Q 'B
IC
IC
MAX
Q’ B EOS
Slope

F
Edge of saturation
IB
IB 
IC
IB   u
MAX

F
ICMAX

F
Fig. 2.4 Increase in Base Charge with Transistor Overdriven
Edge of
saturation
Excess
charge
associated
with base
overdrive
Rising
charge
profile as IB
is increased
in F.A. mode
Fig. 2.5
Charge Profile in Base Region with Transistor in Saturation
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saturation. On being overdriven, the charge continues to rise in the
base but the slope of the profile remains constant at the value reached
at the edge of saturation as can be seen in Fig. 2.5. Note that it is the
slope of the minority carrier profile that determines the value of the
diffusion current.
2.4 Physical Mechanism in Saturation
In the forward active mode most of the electrons injected from
the emitter into the base make their way to the collector. With a
reasonable reverse bias on the base-collector junction, there is a
substantial voltage drop across the transistor from collector to base.
As the base current is increased, the collector current also rises and
with the load resistor, RC, in the collector circuit, the voltage drop
across the transistor, VCE, falls. Ultimately, with further increase in
base current, VCE falls to allow the base-collector junction to become
forward biased. This defines the onset of saturation of the transistor
when both junctions become forward biased (see Fig. 2.6).
When the base-collector junction becomes forward biased, it
begins to conduct also. In this case, holes are injected from the base
into the collector and also electrons are injected from the collector into
the base. The holes crossing from the base into the collector region
constitute the primary component of the base overdrive current. On
reaching the collector, these holes can recombine with electrons
reaching the collector region from the emitter. This also offsets any
tendency for the collector current to rise with increasing bias on the
base-emitter junction. This situation leads to a significant rise in the
minority carrier concentration in the base as seen in the charge
profiles in Fig. 2.6. The base can be considered in saturation as having
a forward component of minority carrier charge flow and a reverse
component of minority carrier charge flow due to the base overdrive.
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VBE
VBC
+
+
Base-Emitter
Forward Biased
IB Base-Collector
Forward Biased
IC
IE
h
h
h
e-
e-
e-
ee-
eE
n
B
p
C
n-
E
nb
pc
pe
forward component
reverse component
Fig. 2.6
Transistor Physical Operation in Saturation
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B