Transcript BJT in Saturation Mode Section 4.5 • Review of BJT in the active Region • BJT in Saturation Mode • Digital Integrated Circuits.

BJT in Saturation Mode
Section 4.5
• Review of BJT in the active Region
• BJT in Saturation Mode
• Digital Integrated Circuits
BJT In the Active Region
Electrons are injected
into the B; holes to the E.
Electrons
are injected
into the BC junction
Electrons
are swept across
the reversed biased BC
Useful Equations
β: typically 100-200.
Base current is amplified!
Apply KCL to the transistor.
BJT in Saturation Mode
Key assumption so far:
BE=Forward Biased
BC=Reverse Biased
What happens when these assumptions are not true?
Review: Forward Bias Diode
E
Depletion region shrinks due to charges from the battery.
The electric field is weaker.
Majority carrier can cross the junction via diffusion;
Greater diffusion current.
Current flows from P side to N side
Review: PN Junction under
Reverse Bias
Reverse: Connect
the + terminal to the
n side.
Depletion region widens.
Therefore, stronger E.
E
Minority carrier to cross
the PN junction easily
through diffusion.
Current is composed
mostly of drift current contributed
by minority carriers.
np to the left and pn to the right.
Current from n side to p side,
the current is negative.
Hole Current into the Collector
A reverse biased BCJ keeps
holes in the base.
But as BCJ becomes forward
biased, the strong electric field
which opposes of the movement
of holes into the collector is weakened.
There is now a hole current into the collector.
Net Result: heavy saturation leads to a sharp rise in the base current and a rapid
fall in β.
Energy Band Diagram of a
Reverse Biased PN Junction
n
p
P
N
pn
np
P
Stronger E field in the
depletion region
N
pn
A Large Signal Model of the BJT
The net collector current decreases as the collector
enter into saturation
General Rules
• As a rule of thumb, we permit soft saturation with
VBC <400 mV because the current in the B-C
junction is negligible, provided that various
tolerances in the component values do not drive
the device into deep saturation.
• For a device in soft saturation or active region, we
approximate IC as Isexp(VBE/VT)
• In the deep saturation region, the collector-emitter
voltage approaches a constant value called VCE, SAT
(about 200 mV).
Use Transistor in Simulation
(Error!, put 2n3904 here!)
Include 2n3904 (NPN) model
BJT Inverter
(Define the input voltage as a variable)
Run Parametric Analysis
Parametric Analysis
Select a Wire to Plot
Use Calculator to Plot
Plot with Calculator (Under
Tools)
RTL (Resistor-Transistor Logic)
Vout
VA
VB
First introduced in 1962! (50 years ago!)
What is the logic function?
RTL Based NOR
A
B
Vout
3.6 V
3.6 V
34.05 mV
3.6 V
0V
42.59 mV
0V
3.6 V
42.59 mV
0V
0V
3.6 V
NOR is an universal gate!
If you can build a NOR, you can build any logic.
Diode-Transistor Logic
What is the logic function?
This resistor allow charges
to be drained from the base
Sweep VB
VS: the input voltage at which the output is approximately 2V.
VS~2V
Condition: VA=4V, VC=4V. VB is swept from 0 to 4V
Diode-Transistor Logic
This resistor allow charges
to be drained from the base
Sweep VB
Fixed VA=4V
VCC=4V
Sweep VB from 0 to 4 V
Increase the VS by about one diode drop.
Basic TTL Gate
Diode is replaced by TTL
A “relative “ of 7400LS Gate
Sweep VB
Fixed VA=4V
VCC=4V
Sweep VB from 0 to 4 V
7400 NAND Gate
7400 Schematic
We will revisit this schematic in a couple of weeks!