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 = A+B A B VR
Y = A+B
Ch 11 Bipolar Digital Pt. 1

2. Bipolar Transistor Operation
Transistor regions of operation
Forward Active
VBE > 0 E jnc forward
VBC< 0 C jnc reverse
Cutoff
VBE < 0 E jnc reverse
VBC < 0 C jnc reverse
Saturation
VBC > 0 C jnc forward C _
VBC n +
Collector + B
VCE p + _
Base _
VBE n
Emitter E IC
Saturation
IC< β IB
Active
Ic= βIb
Note: VCE= VBE+VCB
= VBE - VBC
VCE
Cutoff IC ≈ 0
Ch 11 Bipolar Digital Pt. 1

3. Bipolar Transistor Operation
Transistor regions of operation
Forward Active
Electron injection at emitter and collection at collector
VBE > 0 E jnc forward
VBC< 0 C jnc reverse
Ic = β Ib
* Cutoff
* No electron injection at Emitter
VBE < 0 E jnc reverse
VBC < 0 C jnc reverse
IC ≈ 0
* Saturation
* Electron injection from both E & C
VBC > 0 C jnc forward
IC < β IB IC
Saturation
IC< β IB
Active
Ic= βIb
VCE
Cutoff IC≈0
Ch 11 Bipolar Digital Pt. 1

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
VCE
Output Load Line IB
VBE IC IB
DC Base
Current
Quiescent
Point
Base Load Line
VCE
VBE
Ch 11 Bipolar Digital Pt. 1

5. Bipolar Transistor Operation – Small Signal ACiB iC
vBE
vCE
VBB+vi
In small signal amplifiers,
AC signal at input is small –> 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).
Ch 11 Bipolar Digital Pt. 1

6. Bipolar Transistor Operation – Digital CircuitsIC
Saturation
IC< β IB
Active
Ic=βIb
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
Ch 11 Bipolar Digital Pt. 1

7. Bipolar Transistor Operation - Characteristics
Each region of device operation has its own unique characteristics
Active
Current gain Ic= β Ib
VBE = VBE,active ≈ 0.7 V (typical value)
VCE,active ≈ ?, NO typical value!
Saturation
Reduced current gain IC< β 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 < O
VCE,cutoff ≈ ?, NO typical value! IC
Active
Ic= βIb
Saturation
IC< β IB
~ 0.2 V
VCE IB
Cutoff IC 0
0.7 V 0.8V
VBE
Ch 11 Bipolar Digital Pt. 1

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 ~ mA’s and the output is low “0” due to IR drop across RC. vo vo vi vi
Ch 11 Bipolar Digital Pt. 1

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 ≈ β iB.
So vo = VCC - iC RC ≈ VCC
For high vi input, output vo is low
Transistor is on since iB > 0 because base-emitter junction is biased sufficiently (VBE is large).
Since VBE is large (~0.8 V), iB >> 0, then iC >> 0 since iC ≈ β 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
Ic=βIb
Saturation
IC< β IB
VCE
~ 0.2 V
Cutoff IC≈ 0
Ch 11 Bipolar Digital Pt. 1

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 < β
Input low
vi small
cutoff
VCE
VCE
Ch 11 Bipolar Digital Pt. 1

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 = β 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 < β 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 vo A B I II
III C D vi
Ch 11 Bipolar Digital Pt. 1

12. Noise Margins v01 vi1 v02 vi2 v01 v02 vi1 vi2
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
vi1
v02
vi2
For output of driver high (v01=VOH), then input of load inverter is high (vi2=VOH ).
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 .
v01
v02
Drive
Inverter
Load
Inverter
NML
NMH
vi1
VOL
VIL
VIH
VOH
vi2
Ch 11 Bipolar Digital Pt. 1

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 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 V = 4.2 V
Unequal noise margins for high and low states. IB + +
VCE
VBE
VBE vo A B
VOH I II
III C D
NML
NMH
VOL vi
VOL
VIL
VIH
VOH
Ch 11 Bipolar Digital Pt. 1

14. RTL Power Dissipation + + IB VCE VBE Output High State (Low input)
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 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 vo A B
VOH I II
III C D
VOL vi
VIL
VIH
Ch 11 Bipolar Digital Pt. 1

15. RTL Propagation Delay VCE VBE Output going high vo + + vo C + t tPLH
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 I II
III
Long
charge – up
time! C D
VOL vi
VIL
VIH
Ch 11 Bipolar Digital Pt. 1

16. RTL Propagation Delay VCE VBE Output going high vo + + vo C + t tPLH
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)
VCC=5V vo iR
iCap
VCC + + vo C
VCE +
VBE
VCE,sat t
tPLH
Transient Response
M  N  O vo A B
VOH IC I II M
III C D
VOL P vi N
VIL
VIH O
VCE
Ch 11 Bipolar Digital Pt. 1

17. RTL Propagation Delay VCE VBE Output going low vo + + vo C + t tPHL vo
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 A B
VOH IC I II S R
III C D
VOL vi P
VIL
VIH
VCE
Ch 11 Bipolar Digital Pt. 1

18. RTL Propagation Delay VCE VBE Output going low + + vo C + iR iCap IC S
VCC = 5 V iR
iCap + + vo C +
VCE
VBE IC S R
Short
discharge
time! P
VCE
Ch 11 Bipolar Digital Pt. 1

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 V = 0.5 V
Noise Margin for High State
NMH = VOH - VIH
= 5 V 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
Ch 11 Bipolar Digital Pt. 1