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Chapter 9 Bipolar Logic Circuits Microelectronic Circuit Design Richard C. Jaeger Travis N. Blalock Chapter Goals Bipolar switch circuits Emitter-coupled logic (ECL ... – PowerPoint PPT presentation

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Title: Jaeger/Blalock


1
Chapter 9Bipolar Logic Circuits
  • Microelectronic Circuit Design
  • Richard C. JaegerTravis N. Blalock

2
Chapter Goals
  • Bipolar switch circuits
  • Emitter-coupled logic (ECL)
  • Behavior of the bipolar transistor as a saturated
    switch
  • Transistor-transistor logic (TTL)
  • Schottky clamping techniques for preventing
    saturation
  • Operation of the transistor in the inverse-active
    region
  • Voltage reference design
  • BiCMOS logic circuits

3
The Current Switch (Emitter-Coupled Pair)
  • The building block of emitter-coupled logic (ECL)
    is the current switch circuit which consists of
    matched components

4
The Current Switch
  • Depending on how much higher or lower the input
    voltage vI is compared to VREF, the reference
    current will switch to one of the legs creating a
    voltage vC1or vC2

5
Mathematical Model for Static Behavior of the
Current Switch
  • The previous figure showed the ideal case for
    switching the currents between the two legs, but
    in real BJTs current will be present in both legs
    depending upon vBE of each BJT in the pair
  • The collector current difference is given by

6
Current Switch Analysis for vI gt VREF
  • Given the circuit shown under the given bias
    conditions (vI is 300mV larger than VREF), the
    majority of current will flow in the left leg

7
Current Switch Analysis for vI lt VREF
  • Given the circuit shown under the given bias
    conditions (vI is 300mV less than VREF), the
    majority of current will flow in the right leg

8
The Emitter-Coupled Logic (ECL) Gate
  • The outputs of the previous current switch have
    the value of either 0V or 0.6V
  • The difference of the input and output of the
    current switch is exactly one base-emitter
    voltage drop
  • For a complete ECL gate, the voltages are shifted
    by a base-emitter drop as shown in the figure

9
ECL Gate Summary
For vI -0.7V
For vI -1.3V
10
ECL Gate Benefits
  • ECL gates produce both true and complemented
    outputs
  • ECL gates are fast since it the BJTs are always
    in forward active mode, and it only takes a few
    tenths of a volt to get the output to change
    states, hence reducing the dynamic power
  • ECL gates provide near constant power supply
    current for all states thereby generating less
    noise from the other circuits connected to the
    supply

11
Noise Margins for the ECL Gate
12
Current Source Implementation
  • Instead of using actual current sources for the
    current biasing in an ECL gate, resistors can be
    used as shown below

Note that the currents in the emitter-follower
legs will not be equal since the output voltages
will be different. This will instead be looked at
as an average value between the two legs.
13
ECL Gate Design Example
  • Design an ECL gate with the circuit configuration
    shown on the previous slide to operate at a power
    supply of 3.3V knowing the following information

And a mean emitter follower current of 0.1mA
14
ECL Gate Design Example
  • For vI 1.3V, Q1 will be off causing the common
    emitter voltage to be 1.7V. REE can now be
    calculated by the following
  • And RC2 is

15
ECL Gate Design Example
  • For vI 0.7V, Q2 will be off causing the common
    emitter voltage to be 1.4V. IE1 can now be
    calculated by the following
  • Now RC1 can be found as

16
ECL Gate Design Example
  • Finally, R can be calculated by using the mean
    output voltage and current levels

17
The ECL OR-NOR Gate
Three variations of a 3-input ECL OR-NOR Gate
18
The Emitter Follower
  • The main purpose of the emitter follower in ECL
    gates is to create a level shift in the output
  • The figure shows both the circuit and its
    transport model for the forward- active region

19
The Emitter Follower
  • The emitter follower is called such since the
    voltage at the emitter follows the votlage at the
    base, but at an offset which can be seen in the
    ideal VTC

20
The Emitter Follower with a Resistor Bias
  • As previously shown, the current source can be
    replaced with a resistor bias scheme
  • This technique will cause a change in vBE due to
    the variation of iE as vO changes, but this
    change is minimal and vO vI 0.7

21
The Emitter Follower with a Resistor Load
  • The addition of a resistive load will alter the
    minimum voltage of an ECL gate (when iE0)

22
Emitter Dotting or Wired-OR Logic
  • The circuit shown in the figure exhibits two
    emitter followers in parallel with a common
    output
  • The result for the shown bias condition implies
    that Q2 is cutoff and Q1 has to handle 2IEE

23
Wired-OR Logic Function
  • The parallel emitter on the previous slide can be
    used to implement an OR function as shown in the
    figure, also called the Wired-OR
  • This is distinct to ECL logic since in most logic
    families, the outputs cannot be tied together

24
Design of Reference Voltage Circuits
  • So far the implementation of the VREF signal has
    not been discussed, but it can be created with a
    simple resistor voltage divider as seen below
  • The Thévenin equivalent circuit is used to show
    that the voltage at the base of Q2 will not be
    exactly 1V as designed due to the fact that there
    will be a resistive voltage drop across the
    Thévenin resistance induced by iB2

25
Temperature Compensation
  • Since the vBE of the BJT changes by approximately
    1.8mV/K, it is obvious that when REE is used to
    replace the current switch current source, that
    iE2will vary with temperature
  • Two techniques are shown below that temperature
    compensate (track) the variation

26
Diodes in Bipolar Integrated Circuits
  • In ICs it is desired to have a diode match the
    base-emitter characteristics of a BJT
    (temperature compensation circuit)
  • Since a normal diode structure takes about the
    same amount of Silicon area as a BJT, it is just
    as easy to tie base to the collector
    (diode-connected) of a BJT to create a diode

27
ECL Power Dissipation
  • The static average power of an ECL inverter can
    be found by the following (referring to the shown
    circuit)

28
Power Reduction
  • Approximately 40 of the power is dissipated by
    the emitter-follower stages
  • One technique to reduce this current is to make
    the bias the emitter-follower resistors at a less
    negative value thereby reducing the current,
    however this requires an additional power supply
  • Another technique is to share the current in the
    manner shown on the next slide (similar to the
    wired-OR), however any output that is not driving
    another logic gate needs to be terminated with a
    resistor to the negative power rail

29
Power Reduction
Changing the power supply
Repartitioned ECL gate
30
Gate Delay
ECL inverter with all capacitors shown
Simplified ECL gate model for dynamic response
31
Gate Delay
  • The gate delays and voltages can be calculated
    with following expressions

32
Power-Delay Product
  • The below figures illustrate the tradeoff of
    power and speed for ECL gates

33
The Saturating Bipolar Inverter
  • One of the most basic circuits for BJT logic
    gates is the saturating bipolar inverter
  • The resistor pull the output high when vI is low,
    and the output goes to vCE when vI is high

34
Saturating Bipolar Inverter Example
  • Design a saturating bipolar inverter such that
    the collector saturation voltage is 0.1V with a
    collector of 10A. Find the base current required
    to achieve these specs given the following

35
Saturating Bipolar Inverter Example
  • First find the minimum VCE
  • Next find ?

36
Saturating Bipolar Inverter Example
  • Finally, solving for IB

37
Load Line Visualization
  • The following is a typical load line
    characteristic for a saturating bipolar inverter

38
Switching Characteristics of the Saturated BJT
  • An important switching factor is that when excess
    base current required to drive the BJT into
    saturation is stored into the base region. This
    charge needs to be removed before the BJT can be
    turned off.
  • This delay is called the storage time (tS)
  • The figures show typical switching characteristics

39
Switching Characteristics of the Saturated BJT
  • The storage time delays can be calculated using
    the following expressions
  • Where aF and aR are the forward and reverse
    common-base current gains, and tF and tR are the
    forward and reverse transit times

40
A Transistor-Transistor Logic (TTL) Prototype
  • TTL has the workhorse for digital systems such as
    microprocessors for years
  • The basic structure for the TTL inverter is shown
    below

41
TTL Inverter Operation
  • The two figures show the bias points for the two
    standard low and high inputs
  • The output ranges from VOL 0.15V to VOH 5V

42
Power in the Prototype TTL Gate
  • The power the TTL inverter dissipates for a low
    output is
  • The power the TTL inverter dissipates for a low
    output is

43
VIH, VIL, and Noise Margins for the TTL Prototype
  • The figure shows where VIL and VIH occur, and
    they can be approximated by the following
    expressions using standard TTL values

44
Fanout Limitations of the TTL Prototype
  • For NMOS, CMOS, and ECL gates, fanout was not
    investigated in detail since the input current to
    these gates were considered to be zero. However,
    this is not the case for TTL as seen in the
    figure.

45
Fanout Limitations of the TTL Prototype Example
  • For a TTL gate find
  • a) the fanout limit (N) for a VCESAT2 less that
    0.1V
  • b) the input current iIH and fanout limit for vI
    vOH assuming ßR1 2
  • Given the following

46
Fanout Limitations of the TTL Prototype Example
  • First find N for vO VL
  • Next find the max iC

47
Fanout Limitations of the TTL Prototype Example
  • Continuing
  • The collector current can be no greater than
  • Which give the following

48
Fanout Limitations of the TTL Prototype Example
  • But computing for vO VH, it can be found the
    the fanout (N) is 7. Therefore, the max fanout
    for the circuit is 7
  • Part b) analysis - Finding iIH and N with ßR1 2

49
The Standard 7400 Series TTL Inverter
  • One problem of the TTL inverter prototype
    described so far is that the dynamic response is
    asymmetrical due to the use of a resistive load
    to pull the output up and a BJT to pull the
    output down
  • Another problem is that the fanout capability is
    highly sensitive to ßR

50
The Standard 7400 Series TTL Inverter
  • The classic approach to fixing these problems is
    the implementation of the 7404 hex inverters in a
    dual-in-line package (DIP)

51
The Standard 7400 Series TTL Inverter
  • In the 7404 TTL inverter circuit, Q4 replaces the
    passive resistive load pull-up in the prototype
    TTL inverter to make it an active pull-up circuit
  • Q3 and D1 ensure that the Q4 is turned off when
    Q2 is on

52
Output Analysis of the 7404 Inverter
53
Power Consumption of the 7404
54
TTL Propagation Delay and Power Delay Product
  • The analysis of propagation delay for TTL gates
    is difficult due to the number of transistors
    involved, so the results can be approximated
    through simulation as shown

55
TTL VTC and Noise Margins
  • The figure shows the VTC simulation results of
    the TTL inverter
  • Using the results from the simulation the noise
    marigns can be calculates as

56
Fanout Limitations of Standard TTL
  • The active pull drastically improves the fanout
    capabilities of the TTL inverter
  • However, due to process variations, and the
    requirement for the device to operate over a
    range of temperatures, N is specified to be less
    than 10

57
Logic Functions in TTL
  • The basic structure for the TTL NAND gate

58
TTL NAND Gates
  • The parallel input can be applied to create
    multiple input NAND gates as seen in the complete
    circuit schematic for the 7410 three-input NAND
    gate

59
TTL NAND Gates
  • One good thing about the multiple input NAND gate
    is that it can use a merged transistor structure
    to save silicon area since the input BJTs share
    their emitters and collectors

60
Other TTL Gates
TTL AND-OR-Invert
Low-power TTL NAND gate
61
Input Clamping Diodes for TTL
  • From a transient simulation of the TTL inverter,
    a negative-going transient can be observed due to
    the fast input signal transition
  • Another source of the transients is from the
    distributed L-C interconnection network between
    gates causing ringing

62
Input Clamping Diodes for TTL
  • To suppress these transient effects, diodes can
    be placed at the input to clamp the signal to
    ground

63
Schottky-Clamped TTL
  • Since the saturated transistors in TTL gates
    substantially slows down the dynamic response of
    the logic gates, the Schottky-clamped transistor
    can be used to help this problem
  • The Schottky diode keeps the BJT from going into
    deep saturation

64
Schottky-Clamped TTL Inverter Prototype
  • Replacing the two BJTs with Schottky-clamped
    transistors, the Schottky TTL inverter can be
    formed

65
Three-Input Schottky TTL NAND Gate
  • Each saturating transistor is replaced be a
    Schottky-clamped transistor
  • Q6, R2, and R6 replaces RE in the original
    version which eliminates the first knee voltage
    thereby making the transition region narrower
  • Q5 eliminates the need for D1 by providing extra
    drive to Q4

66
Low-Power Schottky TTL
Advanced low-power Schottky TTL
Low-power Schottky TTL
67
ECL and TTL PDP Comparison
68
BiCMOS Logic
  • BiCMOS is a complex processing technology that
    provides both NMOS and PMOS, as well as npn and
    pnp bipolars
  • The high impedance input of logic gates (does not
    require much to drive them) are provided from the
    MOSFETs and high current drive can be provided
    from the BJTs due to their high current gain and
    transconductance

69
BiCMOS Buffers
  • The CMOS inverter only has to supply enough
    current to drive the bases of the BJTs in the
    BiCMOS buffer
  • The BJT stage can then be designed to drive the
    capacitive load at a certain speed

70
BiCMOS Buffers
  • The BiCMOS buffers in the figures present two
    method to restore the full logic swing at the
    output

71
BiNMOS Buffer
  • In some BiCMOS processes, a good npn might be
    provided, but a sub-par pnp is available, which
    could put limitations on your design
  • A buffer can be implemented in the manner shown
    in the figure using only npn bipolars

72
Other BiNMOS Circuits
Full-swing BiNMOS inverting buffers
BiNMOS buffer using a single npn bipolar
73
BiCMOS Logic Gate
  • More complex logic gates can also be implemented
    using BiCMOS design

Two-input BiCMOS NOR gate
Two-input BiNMOS NOR gate
74
  • End of Chapter 9
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