Ch 11 Bipolar Transistors and Digital Circuits

1
Ch 11 Bipolar Transistors and Digital
Circuits

• Examine bipolar junction transistor (BJT) use
  in inverters for logic circuits.
• Basic Inverter (RTL)
• One npn transistor and a load resistor
• BJT Inverters
• Transistor-Transistor Logic (TTL)
• Emitter-Coupled Logic (ECL)
• Analyze to understand inverter performance
• voltage transfer characteristic,
• noise margins,
• fan-in and fan-out limits,
• power dissipation and
• switching speed.

2-input ECL OR / NOR Gate

Y AB
A
B
VR
Y AB
2
Bipolar Transistor Operation

• Transistor regions of operation
• Forward Active
• VBE gt 0 E jnc forward
• VBClt 0 C jnc reverse
• Cutoff
• VBE lt 0 E jnc reverse
• VBC lt 0 C jnc reverse
• Saturation
• VBE gt 0 E jnc forward
• VBC gt 0 C jnc forward

C
_
VBC
n
Collector


p
B
VCE
Base

_
_
n
VBE
Emitter
E
IC
Active Ic ßIb
Saturation IClt ß IB
Note VCE VBEVCB VBE - VBC
VCE
Cutoff IC 0
3
Bipolar Transistor Operation
Transistor regions of operation

• Forward Active
• Electron injection at emitter and collection
  at collector
• VBE gt 0 E jnc forward
• VBClt 0 C jnc reverse
• Ic ß Ib
• Cutoff
• No electron injection at Emitter
• VBE lt 0 E jnc reverse
• VBC lt 0 C jnc reverse
• IC 0
• Saturation
• Electron injection from both E C
• VBE gt 0 E jnc forward
• VBC gt 0 C jnc forward
• IC lt ß IB

IC
Saturation IClt ß IB
Active Ic ßIb
VCE
Cutoff IC0
4
Bipolar Transistor Operation - DC

• Base bias VBE determines IB
• VCC with RC determine output load line.
• Base IB with output load line determines
  Quiescent Point (VCE , IC )

IC
Output Load Line
VCE
IB
IC
VBE
IB
DC Base Current
Quiescent Point
Base Load Line
VCE
VBE
5
Bipolar Transistor Operation Small Signal AC
iB
iC
vBE
vCE
VBBvi

• In small signal amplifiers,
• AC signal at input is small gt base current
  variation is small.
• Device moves around DC quiescent point
• Device stays in the active region
• Amplifier produces current and voltage gain
  depending upon the configuration, e.g. common
  emitter (above).

6
Bipolar Transistor Operation Digital Circuits
IC
Saturation IClt ß IB
Active IcßIb
VCE

• Input signal vi is large.
• VBE and IB changes are large.
• Important applications ? digital circuits
  and power amplifiers
• Device can be in active, saturation or
  cutoff depending on VBE and IB.
• Transistor still operates on the load line.
  
• Moves from cutoff thru active to saturation
  or vice versa as the input signal changes.

Cutoff IC 0
7
Bipolar Transistor Operation - Characteristics

• Each region of device operation has its own
  unique characteristics
• Active
• Current gain Ic ß Ib
• VBE VBE,active 0.7 V (typical value)
• VCE,active ?, NO typical value!
• Saturation
• Reduced current gain IClt ß IB
• VBE VBE, sat 0.8 V (typical value)
• VCE VCE, sat 0.2V (typical value)
• Cutoff
• No current gain IC 0, IB 0
• VBE lt O
• VCE,cutoff ?, NO typical value!

IC
Active Ic ßIb
Saturation IClt ß IB
0.2 V
VCE
IB
Cutoff IC? 0
0.7 V 0.8V
VBE
8
Resistor Transistor Logic (RTL)

• RTL Logic
• Earliest and simplest logic
• 0 low voltage
• 1 high voltage
• Inverter is the basic building bock
• Combine two inputs in parallel to implement
  NOR
• Combine two inputs in series to implement
  NAND
• Transistors operate in cutoff for low 0
  input (base) voltage, so IC 0 and output
  is high 1.
• Transistors operate in saturation for high
  1 input (base) voltage, so IC mAs and
  the output is low 0 due to IR drop
  across RC.

vo
vo
vi
vi
9
Basic Bipolar Transistor Inverter (RTL)

• Resistor Transistor Logic (RTL)
• For low vi input, output vo is high
• Transistor is off (cutoff) since iB
  0 because base-emitter junction is not
  biased sufficiently (VBE is too small).
• Since iB 0, then iC 0 because iC ß
  iB.
• So vo VCC - iC RC VCC
• For high vi input, output vo is low
• Transistor is on since iB gt 0 because
  base-emitter junction is biased sufficiently
  (VBE is large).
• Since VBE is large (0.8 V), iB gtgt 0, then
  iC gtgt 0 since iC ß iB.
• So vo VCC - iC RC is very small.
• Transistor driven into saturation region so
  . vo VCE,sat 0.2V

IB
IC
n
p

n
VBE
0.7 V 0.8V
VBE
IC
Active IcßIb
Saturation IClt ß IB
VCE
Cutoff IC 0
0.2 V
10
Basic Bipolar Transistor Inverter (RTL)

• Transistor operates along load line.
• Transistor operates in cutoff when input is
  low since iB 0.
• As input vi increases, iB increases and
  transistor moves into active region.
• As input vi increases further, transistor
  moves into saturation region and VCE goes
  towards zero.

VCE

VBE
IC
IC
Output Load Line
active
Input high vi large
Saturation iC/iB lt ß
Input low vi small
0
cutoff
VCE
VCE
11
RTL Voltage Transfer Characteristic

• Region I (A to B)
• Transistor is in cutoff
• VBE is small, iB 0, vo VCC.
• Region II (B to C)
• Transistor is on in the active mode
  (iC ß iB).
• iB and VBE are larger VBE 0.7V
• iC and iB increase as vi and VBE
  increase.
• vo and VCE falls as icRC increases.
• Region III (C to D)
• Transistor is in the saturation mode
  (iC lt ß iB).
• iB and VBE are larger, VBE 0.8 V
• iC is larger
• VCE is small, VCE,sat 0.2V

VCC 5 V
IB

VCE

VBE
VBE
0.7 0.8
vo
A
B
II
I
III
D
C
vi
12
Noise Margins

• Noise margins are a measure of the reliability of
  the technology.
• Measure of the sensitivity to noise.
• Consider one inverter driving an identical
  inverter.
• How large a noise spike can be tolerated before
  an error occurs?

Drive Inverter
Load Inverter
v01
v02
vi1
vi2

• For output of driver high (v01VOH), then input
  of load inverter is high (vi2VOH ).
• A negative noise spike on input of load inverter
  reduces input signal.
• Trouble when net input signal is less than VIH so
  noise margin is
  NMH VOH - VIH .
• Similarly, for the low state NML VIL - VOL .

v02
v01
Load Inverter
Drive Inverter
NMH
NML
vi1
VOH
VIH
VIL
VOL
vi2
13
RTL Inverter Noise Margins
VCC 5 V

• Noise Margin for Low State
• NML VIL - VOL
• VIL VBE,active 0.7 V
• VOL VCE,sat 0.2 V
• NML VIL - VOL 0.7 V - 0.2 V
  0.5 V
• Noise Margin for High State
• NMH VOH - VIH
• VOH VCC 5 V
• VIH VBE,sat 0.8 V
• NMH VOH - VIH 5 V - 0.8 V 4.2 V
• Unequal noise margins for high and low
  states.

IB

VCE

VBE
VBE
0.7 0.8
vo
A
B
VOH
II
I
III
D
C
NMH
NML
VOL
vi
VOH
VIH
VIL
VOL
14
RTL Power Dissipation
VCC 5 V
IB

• Output High State (Low input)
• Transistor is in cutoff so iC ? 0.
• No static power dissipation for high state,
  PH 0.
• Output Low State (High input)
• Transistor is in saturation so v o
  VCE,sat 0.2 V.
• iC (VCC - VCE,sat )/RC (5V - 0.2 V)/10K
  0.48 mA.
• PL VCC iC (5 V)(0.48 mA)
  2.4 mW
• Average P 1/2(PH PL) 1.2
  mW

RC 10K

VCE

VBE
VBE
0.7 0.8
vo
B
A
VOH
II
I
III
D
C
VOL
vi
VIH
VIL
15
RTL Propagation Delay

• Output going high
• Transistor turned off (cutoff)
• Charging current flows through RC
• tPLH is time it takes the output to rise
  from VOL VCE,sat 0.2 V to 1/2(VOH VOL)
  2.6 V

VCC 5 V
vo
iR
iCap
VCC

vo

C
VCE

VBE
VCE,sat
t
tPLH
vo
A
B
VOH
II
I
III
Long charge up time!
D
C
VOL
vi
VIH
VIL
16
RTL Propagation Delay

• Output going high
• Transistor turned off (cutoff) (M ? N)
• Charging current flows through RC
• tPLH is time it takes the output to rise
  from VOL VCE,sat 0.2 V to 1/2(VOH
  VOL) 2.6 V (N ? O)

VCC5V
vo
iR
iCap
VCC

vo

C
VCE

VCE,sat
VBE
t
Transient Response M ? N ? O
tPLH
vo
B
A
VOH
IC
II
I
M
III
D
C
P
VOL
vi
N
VCE
VIH
VIL
O
17
RTL Propagation Delay

• Output going low
• Transistor turned on (saturation) and
  providing discharge current (P ?R)
• But current also flows through RC
• tPHL is time it takes the output to fall
  from VOH VCC 5 V to 1/2(VOH VOL) 2.6
  V (R ? S)

VCC 5 V
vo
iR
iCap
VCC

vo

C
VCE

VBE
VCE,sat
t
tPHL
vo
Transient Response P ? R ? S
B
A
VOH
IC
II
I
S
R
III
D
C
VOL
P
vi
VCE
VIH
VIL
18
RTL Propagation Delay

• Output going low

VCC 5 V
iR
iCap

vo

C
VCE

VBE
IC
S
R
Short discharge time!
P
VCE
19
Resistor Transistor Logic (RTL)

• RTL provides simple, basic digital
  technology based on bipolar transistors and
  resistors.
• Logic levels and noise margins
• Noise Margin for Low State
• NML VIL VO
• 0.7 V - 0.2 V 0.5 V
• Noise Margin for High State
• NMH VOH - VIH
• 5 V - 0.8 V 4.2 V
• Unequal noise margins for high and low
  states.
• Propagation delays
• Output going low
• Output going high
• Propagation delay
• Power Delay Product

vo
vo
vi
vi