JaegerBlalock

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Chapter 9Bipolar Logic Circuits

• Microelectronic Circuit Design
• Richard C. JaegerTravis N. Blalock

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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

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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

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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

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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

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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

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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

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ECL Gate Summary
For vI -0.7V
For vI -1.3V
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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

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Noise Margins for the ECL Gate
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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.
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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
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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

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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

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ECL Gate Design Example

• Finally, R can be calculated by using the mean
  output voltage and current levels

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The ECL OR-NOR Gate
Three variations of a 3-input ECL OR-NOR Gate
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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

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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

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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

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The Emitter Follower with a Resistor Load

• The addition of a resistive load will alter the
  minimum voltage of an ECL gate (when iE0)

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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

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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

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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

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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

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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

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ECL Power Dissipation

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

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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

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Power Reduction
Changing the power supply
Repartitioned ECL gate
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Gate Delay
ECL inverter with all capacitors shown
Simplified ECL gate model for dynamic response
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Gate Delay

• The gate delays and voltages can be calculated
  with following expressions

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Power-Delay Product

• The below figures illustrate the tradeoff of
  power and speed for ECL gates

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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

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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

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Saturating Bipolar Inverter Example

• First find the minimum VCE
• Next find ?

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Saturating Bipolar Inverter Example

• Finally, solving for IB

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Load Line Visualization

• The following is a typical load line
  characteristic for a saturating bipolar inverter

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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

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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

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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

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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

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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

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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

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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.

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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

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Fanout Limitations of the TTL Prototype Example

• First find N for vO VL
• Next find the max iC

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Fanout Limitations of the TTL Prototype Example

• Continuing
• The collector current can be no greater than
• Which give the following

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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

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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

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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)

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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

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Output Analysis of the 7404 Inverter
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Power Consumption of the 7404
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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

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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

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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

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Logic Functions in TTL

• The basic structure for the TTL NAND gate

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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

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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

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Other TTL Gates
TTL AND-OR-Invert
Low-power TTL NAND gate
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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

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Input Clamping Diodes for TTL

• To suppress these transient effects, diodes can
  be placed at the input to clamp the signal to
  ground

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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

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Schottky-Clamped TTL Inverter Prototype

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

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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

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Low-Power Schottky TTL
Advanced low-power Schottky TTL
Low-power Schottky TTL
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ECL and TTL PDP Comparison
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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

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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

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BiCMOS Buffers

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

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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

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Other BiNMOS Circuits
Full-swing BiNMOS inverting buffers
BiNMOS buffer using a single npn bipolar
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BiCMOS Logic Gate

• More complex logic gates can also be implemented
  using BiCMOS design

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