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E346 Implementation of Digital Circuit Design - Coggle Diagram
E346 Implementation of Digital Circuit Design
L1
Micro-electronics is the study of electronics components that are in the (mm) now (nm) range.
Integrated circuits are systems of circuit that's manufactured on the surface of a thin substrate of semiconductor to provide a required electrical function
Why discrete components aren't use anymore:
An audio amp made with discrete components are too bulky
Why Integrated Circuits are preferred:
Can compact everything and more onto just 1 component.
Examples of such progress are ICs' that are 1cm by 1cm
Manufacturing Flow:
1) Design and Layout
2) Evaluation with customers until needs are met ( will loop with design
and layout step if needed)
3) Mass Production in huge quantities
4) Testing
Design and layout
Circuit design and mask layout determines viability and success of a product
Bring together different available devices to achieve a specific function
Many basic circuits for reference and reapplication in all aspects e.g size of each device
Mask is drawn based on circuit design after design is completed
IC performance depends on how well layout is drawn, as it represents real physical connection and sizes of devices.
Linking all together
Customer (e.g. apple, intel, LG)
IC Design House (e.g. Infineon, Marvell, Broadcom
Foundry (e.g. Global Foundries, UMC, SSMC)
Packaging (e.g. Statschip Pac)
Resistors
Characteristics:
Passive device, functionality to provide resistance to current flow, causes voltage drop
One of the most common devices found in analog circuits
R = P(L/A) = P(L/Wt) = (P/t)(L/W) = Rs(L/W)
Terms:
P => Resistivity, L => Length of resistor, A => Area of cross-section, t => Thickness, Rs => Sheet Resistance, W => Width
Resistivities determined via material it is formed
Thickness of resistor
Determined through only the the Width and Length (W/L)
Examples of types of resistors: NWell, N-Diff and Poly
Capacitors
features: Passive device, stores charges, provides filtering effect, takes time to change and discharge
Also used as filters in ICs'
As capacitors are like open circuits, capacitors can block DC voltage
Made with 2 parallel conductors separated by a dielectric e.g. silicon dioxide
C = εA / d
Terms: ε => Permittivity of dielectric, A => area of plates and d => distance between plates
Permittivity is determined via material of dielectric
Capacitance is proportional to area of conductive plates and distance between the plates
To design capacitors: oni area of conductive plates can be set through varying the distance between plates are fixed via fabrication process
Examples of capacitors: Metal-Metal capacitor, MOS capacitor and Polysilicon capacitors (Typically range of capacitance is <100pF)
Inductors
Characteristics: Passive device, formed when wire is coiled, produces electromagnetic field when current flows
Not usually used in designs as it requires a large chip area, another reason is effects of inductors can be replaced by using resistors and capacitors
Can be created as an parasitic effect, may effect performance of IC
Diodes
Characteristics: Typically works in forward-bias and requires minimum voltage to turn on (Threshold Voltage)
Requires minimum voltage to be turned on and the voltage across the diode usually remains constant (Usually used as a reference voltage or a trigger voltage)
Bipolar Junction Transistors (BJT)
Characteristics: active device, requires biasing, current controlled device, 3 states of operation
Depends on biasing conditions resulting in different modes of operations.
BJT is a 3 terminal semiconductor devices consisting of 2 P-N junctions connected in series
Modes of Operation
Cutoff mode: Base-Emitter Junction: Reverse-Bias, Base-Collector Junction: Reverse-Bias, Operation: OFF
Saturation Mode: Base-Emitter Junction: Reverse-Bias, Base-Collector Junction: Forward-Bias, Operation: Partially On
Active Mode: Base-Emitter Junction: Forward-Bias, Base-Collector Junction: Reverse-Bias, Operation: Partially On
To forward bias any junction, voltage across must be biased higher than threshold typically 0.7V
L2
Collector Current
Vce > Vce(SAT) (Active mode), the collector current, Ic can be expressed as: Ic = Ise^(Vbe/Vt)
Ic is dependent on Is and Vbe as Vt is constant at fixed temp
Rate of change of Ic when Vbe > 0.7 is very large
Saturation Current
Is refers to saturation current and can be expressed as: Is = (q
Dn
n
po
A / Wb)
q = electron charge, Dn = diffusion constant for electron, npo = equilibrium concentration of electrons in the base, A is the area of the emitter, Wb = width of the base
Most of the parameters are process-dependent except area of the emitter. Meaning that circuit designers can oni change set area
Is is proportional to area of the BJT and is typically in the range of 10^-12 to 10^-15
Ic is also proportional to the area of BJT
normal operations, BJT's are typically used in active mode with BJT is used as a amplifier
In active region, Vbe and Ib remains constant, collector current Ic will remain constant for Vce > Vce(SAT)
To achieve required Ic, designers need to vary size of BT
BJT comes in different sizes due to different specifications
Increasing current comes from BJT being connected in a parallel. as area of the emitter is multiplied.
MOSFET (CA1 slide 26)
Description: Active device, complementary MOS, requires biasing, no gate current, voltage controlled device
Many types of MOSFETs like Depletion-Mode or Enhancement-Mode
Complementary MOS (CMOS), are actually Enhancement-Mode MOSFET which have MOS transistors of the both polarity, n-channel and p-channel on the same process
Operating Region
Modes: Cutoff, Triode, Saturation
To operate in the saturation/active mode, Vgs - Vth >= Vds(sat). Thus Vds(sat) is the minimum Vds for the MOSFET to operate in Saturation/Active mode
Typically, Vth range from 0.5V to 1V
Typically, minimum Vds(sat) is approximately 0.2V
Saturation / Active Mode
Id = 0.5k * W/L( Vgs - Vth)^2
Due to gate being separated from source and drain by silicon oxide, there's no gate current
Small Vds and Vgs > Vth, a conductive channel will form between Source and Drain
when (Vgs-Vth) increases, drain current Id increases. Due to channel depth increasing as Vgs-Vth increases. Channel depth increases and hence current increases
Design MOSFET through width/length (W/L) being varied to get Id
Transistor size
As long as the minimum dimensions is fulfilled using suitable W and L value, ratio of W/L can be used to increase/decrease current of Id
Minimum dimensions varies from process
usually minimum length of MOSFET channel can result in proper function
Digital Control
Consists of many functions
Not all functions are use all the time
Will waste power due to being kept on
Need to include means in IC's to turn on/off functional blocks
Digital oni have 1 and 0
Inverter (NOT Gate)
(refer to CA1 slide 36 plus)
Vout = Vcc - IcRc
if Vin < Vth (Vin is LOW), Q1 is OFF, Ic = 0A
Vout = Vcc (high)
If Vin > Vth (Vin is HIGH) , Q1 is ON, Ic is large
Vout = 0V
Vout = Vcc - IcRc = Ic*Ron OR Vout = Vcc( Ron / (Ron + Rc) )
For Vout = 0V
Rc must be very large & Ron must be very small
Means that BJT and Rc has to be huge, but this is not ideal
Requires small base current to function, Ib needs to come from Vin. Leading Vin to be lower
Buffer
Made by cascading 2 inverters in series
Takes double the time needed for inverter.
Used as timing delay circuits
Transistor level: NAND gate
Simpler to design NAND than AND gate
Add inverter to output of NAND gate to get AND gate
For Vout to be high, Yout must be shorted to Vcc and vise versa
Transistor level: NOR gate
Simpler to design
For Vout to be high, Vout must be shorted to Vcc and vise versa
Add inverter to output of NOR gate to get OR gate
Verilog Module
(refer to slides 44 to 55)
Used to simplify the process by replacing the gates with verilog coding
Process
Write verilog code
Simulate verilog code (if doesnt meet requirements rewrite verilog code)
Synthesize to Gate
Simulate Gate level (if result does not meet requirements re-synthesize to gates
Can be split into smaller blocks and coded modules using verilog. Each module describes the function of the particular block
Usually name will give clues to the function of the block
Input / Output needs to be declared as ports. This will indicate the number of port and their directions
Operators
~A (NOT A)
A&B (A AND B)
A|B (A OR B)
Practical Gates
Input voltage to the logic gates are important. Logic gates have 2 states HIGH or LOW
What happens when input voltage is 1/2 Vcc
When Vin = 1/2Vcc, both M1 and M2 will be turned on
There's direct current flow Vcc to GND.
If R1 = R2, Vout will be 1/2Vcc
In that case, Vout is unknown as Vout can oni be high (Vcc) or low (GND)
L3
Circuit Biasing
BJT transistors require a VBE voltage and a Base current to operate. MOSFET needs VGS > VT to be turned on
IC's, besides power supply, all active devices need to be biased with current and voltage in order to operate
Used to bias active devices called biasing currents
Needed in most analog circuit
Requirements:
Easy to reproduce
Economical
Insensitive to power supply
Insensitive to temperature
Using V = IR, if we see all components as resistive devices, a Power Supply connecting to the devices will result in current flows
Resistors
Simplest way to generate a current is to connect a voltage across resistors
Integrated circuits usually requires biasing current lower than 100uA
Example:
For a 10V power supply voltage across a 100K ohm resistor to generate 100uA using N-well resistors which have large sheet resistance (1K ohm/ square), we need a resistor with L/W of 100um / 1um, in order to have a 100K ohm resistor
Solution:
R = Rs (L/W)
100Kohm = 1Kohm/square * L/W
L/W = 100um / 1um
Disadvantages:
Size of 100K ohm resistor is same size of 10 BJT's
Resistors are connected directly to Power supply. When Power Supply changes, the current generated will also change. This makes the biasing current created directly from resistors unreliable
Current mirrors (refer to CA2 slide 10 for formulas)
Current mirrors are widely used in IC's as biasing circuits. Uses similar transistors in ratio, either BJT or MOSFET, to create biasing currents
Current mirror or duplicate the input current into multiple branches. Current mirrors can also generate output currents as a multiple of the input current
For BJT, given that Ic = Is*e^(VBE/VT), if 2 transistors are identical, the same VBE for both BJT will generate the same Ic current.
For MOSFET, given that ID = 0.5k * (W/L)(VGS - VT)^2, if 2 transistors are identical, the same VGS for both MOSFET will generate the same ID current
Advantages of BJT Current Mirrors
BJT current mirrors are more economical in large-scale circuits as it requires a smaller area than resistors to generate the same current
Insensitive to power supply and temperature variation
Output current is multiplier of input current and multiplier depends on the multiplier depends on the ratio of the transistors
Easy to reproduce (mini need 2 transistors)
Disadvantages if BJT Current Mirrors
Simple BJT current mirror is not an ideal circuit
Generate IOUT, Beta needs to be huge, so that IB is negligible. When IB is negligible, Ic1 is approximately IIn. Thus IOUT is approximately IIN
Beta is process dependent and cannot be controlled by IC designers
MOSFET Current Mirror (refer to CA2 slide 14 for formulas)
Similar theory as BJT
ID current for both transistor will be similar
No dependency on Beta cos MOSFET doesn't have gate current
Requires HIGH VT
Extending
Multiple branches can be added
ID2, ID3, ID4 will be proportional to ID1, depending on ratio of their size (width/length)
IIN is approximately ID2 / ID3 / ID4 if the ratio of the width / length of all MOSFETs are the same
Multiplying
Multiple branches can be added and current in each branch is similar, IOUT is approximately !D2 + ID3 + ID4 or 3( IIN )
Ratio of width / length of output transistor (Q2) is a multiplier of input transistor (Q1), IOUT will be a multiplier of IIN, IOUT = N * IIN
(Ref) MOSFET Channel Length Modulation
ID increases with VDS, until it reaches saturation where it remains constant
When effective channel length (L), is reduced die to channel length modulation, ID current will increase when VDS increases
Length typically designed to be >3X minimum length of process, reduces channel length modulation effects
Current Mirrors as Active Loads
Current Mirrors can be used to replace resistors to provide high resistive load in IC's to achieve large voltage gains and use smaller areas.
Transistors = active devices
Current mirrors called Active Loads
Voltage Reference
Requires voltage biasing and current biasing
should provide a constant voltage that's independent of Power Supply voltage and temperature
Common methods to generate voltage biasing
Voltage divider using resistors in series
VBE of a diode-connected BJT
Thermal voltage, VT
Zener diode
Disadvantages
Voltage divider has HIGH POWER SUPPLY dependency. Output of diver is proportional to POWER SUPPLY e.g.7V power supply voltage
VBE of a diode-connected BJT has lesser power supply dependency but is temperature dependent. VBE is inversely proportional to temperature
Thermal voltage, VT = (kT / q) is dependent on temperature, when temperature increase VT also increases
Bandgap Voltage Reference
Commonly used in IC design
Typical output of 1.25V
Close to theoretical 1.22eV bandgap of silicon at 0K
Designed to be temperature independent and power-supply independent
Advisable to start analysis from BJTs
Q1, Q2 and R2 forms a KVL loop ( Refer to CA2 slide 28 to 30)
VBE1 = VBE2 + IR2 * R2
∆𝑉𝐵𝐸 = VBE1 - VBE2
Assume Beta is large, therefore IB3 is negligible and IC2 is approximately IE2
IE3 is approximately IR2
VOUT = VBE3 + IR3 * R3
VOUT = VBE3 + ∆𝑉𝐵𝐸( R3/R2 )
Temperature Coefficient
Refers to the CHANGE I PROPERTY when temperature changes
+ve temp coefficient means the property increases when temperature increases
-ve temp coefficient means the property decreases when temperature increases
To achieve ZERO TEMPERATURE COEFFICIENT, the bandgap circuits needs the effects of positive temperature coefficient to cancel out the effects of negative temperature coefficient
+TC + -TC = Zero TC
BJT Properties
Formula for BJT's collector Current:
IC = TS* e^(VBE / VT)
VBE = VT * ln ( IC / IS )
( IC / IS ) = e^(VBT / VT)
ln ( IC / IS) = (VBT / VT)
VBE has -ve temp coefficient, while VT has + temp coefficient
Common to use BJT in bandgap circuits
Combination of BJTs can achieve output voltage with 0 temp coefficient
Bandgap Temperature Coefficient (refer to slide 28 to 33)
VOUT = VBE3 + KVT
VBE3 is -ve TC
K (multiplier) = (R3/R2) ln( [R3/R1]*[A2/A1] )
VOUT is dependent on VBE, VT and K
VBE has -ve temp coefficient and VT has +ve temp coefficient. As temp increase, VBE decrease while VT increase. VOUT changes when temp increases
For VOUT to be insensitive to temp, change in -ve coefficient = +ve coefficient. So VOUT achieves 0 temp coefficient.
K has 0 temp coefficient but R1, R2, R3 and the size of Q1 and @ (A2/A1) can be chosen so KVT has a +ve TC which equates to the value of -ve TC of VBE3
Ideal VS Practical Temperature Coefficient
Ideal Bandgap Reference Voltage should not vary when temp changes
Practical circuits, usually has a -ve temp coefficient at temp lower than room temp (25 degrees). A +ve temp coefficient at higher temp than room temp