ELECTRONIC CIRCUITS

1
ELECTRONIC CIRCUITS

• EE451

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MAIN TOPICS (2nd half)

• Analog Switching Power Supplies
• Review of rectification filtering
• Review of zener diode as a voltage regulator
• Transistor series shunt voltage regulators
• Transistor current regulators
• IC voltage regulators (e.g. 78/79XX, LM317)
• Switching-mode regulators (e.g. LH1605)
• Linear Integrated Circuit Applications
• BiFET Norton op-amps, 555 timer, 8038 function
  generator, active filters, etc.

3
Power Supply Block Diagram
4
Half-Wave Rectifier
V
t
5
Full-Wave Rectifier
V
t
6
Bridge-Type Rectifier
V
t
7
More Equations . . .
Rearranging the previous equations VP Vdc
1.736 Vr
The ripple voltage as a percentage of the dc
voltage is
The diode(s) must be rated to withstand the surge
current
where RW is the transformer windings resistance
given by
8
Comparison of Different Types of Rectifiers

• Half-wave rectifier needs only a single diode but
  ripple is twice those of the other types.
• Full-wave rectifier requires a centre-tapped
  transformer and its output voltage is about half
  those of the other types.
• Bridge-type rectifier is best overall even though
  it requires four diodes because the diode bridge
  is often available in a single package. However,
  if a single diode in the bridge is defective, the
  whole package has to be replaced.

9
Line Regulation
is a measure of the effectiveness of a voltage
regulator to maintain the output dc voltage
constant despite changes in the supply voltage.
10
Load Regulation
is a measure of the ability of a regulator to
maintain a constant dc output despite changes in
the load current.
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Other Specifications
A common definition for voltage regulation is
The ability to reduce the output ripple voltage
is
Source resistance of regulator is
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Zener Diode Voltage Regulator
I-V Characteristic
Circuit
IZM
13
Notes on Zener Diode Regulator

• VZ depends on I and temperature.
• Zener diodes with rated voltage lt 6 V have
  negative temperature coefficient those rated gt 6
  V have positive temperature coefficient.
• In order to maintain a constant Vo, IZT varies in
  response to a change of either IL or Vi. For
  example, when RL increases, IL decreases, then
  IZT has to increase to keep the current through
  Rs constant. Since the voltage drop across Rs is
  constant, Vo stays constant.

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Formulae for Zener Regulator Circuit
Rs establishes the zener bias current, IZT
For fixed Vi, but variable RL
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Formulae (contd)
For fixed RL, but variable Vi
The output ripple voltage of the zener regulator
is
where RZ ac resistance of zener diode.
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Transistor Series Voltage Regulator
The simple zener regulator can be markedly
improved by adding a transistor.
Since VBE VZ - VL any tendency for VL to
decrease or increase will be negated by an
increase or decrease in IE. The dc currents for
the circuit are
IL hFEIB IZT IR - IB
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Transistor Shunt Voltage Regulator
Since VBE VL - VZ, any tendency for VL to
increase or decrease will result in a
corresponding increase or decrease in IRs. This
will oppose any changes in VL because VL Vi -
IRsRs.
IE IRs - IL hFEIZT
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Op-Amp Voltage Regulators
Shunt
Series
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Notes on Op-Amp Voltage Regulator

• More flexibility possible in design of voltage
  output than IC voltage regulator packages.
• The essential circuit elements are a zener
  reference, a pass or shunt transistor, a sensing
  circuit, and an error/amplifier circuit.
• Equation indicates that Vo depends on R2, R3, and
  VZ.
• The shunt configuration is less efficient but R2
  offers short-circuit current limiting.

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Constant Current Limiting
can be used for short-circuit or overload
protection of the series voltage regulator.
Output current is limited to
21
Fold-back Current Limiting
is a better method of short-circuit protection.
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Design Equations for Fold-back Current Limiting
Maximum load current without fold-back limiting
Output voltage under current limiting condition
The short circuit current (i.e. when Vo 0) is
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Characteristics of Fold-back Limiting
Vo

• Notice that Ishort lt IL(max) and that Vo is
  regulated (i.e. constant) only after RL gt a
  certain critical value.
• For designing purpose, R5 R6 1 kW and if
  Ishort and IL(max) are specified then

IL
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Transistor Current Regulators
are designed to maintain a fixed current through
a load for variations in either Vi or RL.
For the BJT circuit, VEB VZ - VRE. Any tendency
for IL to change will cause an opposing change in
VEB, thus nullifying the perturbation.
For the JFET circuit, IL ID IDSS as long as
VL lt VSS - VP.
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IC Voltage Regulators

• There are basically two kinds of IC voltage
  regulators
• Multipin type, e.g. LM723C
• 3-pin type, e.g. 78/79XX
• Multipin regulators are less popular but they
  provide the greatest flexibility and produce the
  highest quality voltage regulation
• 3-pin types make regulator circuit design simple

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Multipin IC Voltage Regulator

• The LM723 has an equivalent circuit that contains
  most of the parts of the op-amp voltage regulator
  discussed earlier.
• It has an internal voltage reference, error
  amplifier, pass transistor, and current limiter
  all in one IC package.

LM 723C Schematic
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Notes on LM723 Voltage Regulator

• Can be either 14-pin DIP or 10-pin TO-100 can
• May be used for either ve or -ve, variable or
  fixed regulated voltage output
• Using the internal reference (7.15 V), it can
  operate as a high-voltage regulator with output
  from 7.15 V to about 37 V, or as a low-voltage
  regulator from 2 V to 7.15 V
• Max. output current with heat sink is 150 mA
• Dropout voltage is 3 V (i.e. VCC gt Vo(max) 3)

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LM723 in High-Voltage Configuration
Design equations
Choose R1 R2 10 kW, and Cc 100 pF.
External pass transistor and current sensing
added.
To make Vo variable, replace R1 with a pot.
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LM723 in Low-Voltage Configuration
With external pass transistor and foldback
current limiting
Under foldback condition
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Three-Terminal Fixed Voltage Regulators

• Less flexible, but simple to use
• Come in standard TO-3 (20 W) or TO-220 (15 W)
  transistor packages
• 78/79XX series regulators are commonly available
  with 5, 6, 8, 12, 15, 18, or 24 V output
• Max. output current with heat sink is 1 A
• Built-in thermal shutdown protection
• 3-V dropout voltage max. input of 37 V
• Regulators with lower dropout, higher in/output,
  and better regulation are available.

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Basic Circuits With 78/79XX Regulators

• Both the 78XX and 79XX regulators can be used to
  provide ve or -ve output voltages
• C1 and C2 are generally optional. C1 is used to
  cancel any inductance present, and C2 improves
  the transient response. If used, they should
  preferably be either 1 mF tantalum type or 0.1 mF
  mica type capacitors.

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Dual-Polarity Output with 78/79XX Regulators
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78XX Regulator with Pass Transistor

• Q1 starts to conduct when VR2 0.7 V.
• R2 is typically chosen so that max. IR2 is 0.1
  A.
• Power dissipation of Q1 is P (Vi - Vo)IL.
• Q2 is for current limiting protection. It
  conducts when VR1 0.7 V.
• Q2 must be able to pass max. 1 A but note that
  max. VCE2 is only 1.4 V.

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78XX Floating Regulator

• It is used to obtain an output gt the Vreg value
  up to a max.of 37 V.
• R1 is chosen so that
• R1 ? 0.1 Vreg/IQ, where IQ is the quiescent
  current of the regulator.

or
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3-Terminal Variable Regulator

• The floating regulator could be made into a
  variable regulator by replacing R2 with a pot.
  However, there are several disadvantages
• Minimum output voltage is Vreg instead of 0 V.
• IQ is relatively large and varies from chip to
  chip.
• Power dissipation in R2 can in some cases be
  quite large resulting in bulky and expensive
  equipment.
• A variety of 3-terminal variable regulators are
  available, e.g. LM317 (for ve output) or LM 337
  (for -ve output).

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Basic LM317 Variable Regulator Circuits
(a)
(b)
Circuit with capacitors to improve performance
Circuit with protective diodes
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Notes on Basic LM317 Circuits

• The function of C1 and C2 is similar to those
  used in the 78/79XX fixed regulators.
• C3 is used to improve ripple rejection.
• Protective diodes in circuit (b) are required for
  high-current/high-voltage applications.

where Vref 1.25 V, and Iadj is the current
flowing into the adj. terminal (typically 50 mA).
R1 Vref /IL(min), where IL(min) is typically 10
mA.
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Other LM317 Regulator Circuits
Circuit with pass transistor and current limiting
Circuit to give 0V min. output voltage
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Block Diagram of Switch-Mode Regulator
It converts an unregulated dc input to a
regulated dc output. Switching regulators are
often referred to as dc to dc converters.
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Comparing Switch-Mode to Linear Regulators

• Advantages
• 70-90 efficiency (about double that of linear
  ones)
• can make output voltage gt input voltage, if
  desired
• can invert the input voltage
• considerable weight and size reductions,
  especially at high output power
• Disadvantages
• More complex circuitry
• Potential EMI problems unless good shielding,
  low-loss ferrite cores and chokes are used

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General Notes on Switch-Mode Regulator
The duty cycle of the series transistor (power
switch) determines the average dc output of the
regulator. A circuit to control the duty cycle
is the pulse-width modulator shown below
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General Notes contd . . .

• The error amplifier compares a sample of the
  regulator Vo to an internal Vref. The difference
  or error voltage is amplified and applied to a
  modulator where it is compared to a triangle
  waveform. The result is an output pulse whose
  width is proportional to the error voltage.
• Darlington transistors and TMOS FETs with fT of
  at least 4 MHz are often used. TMOS FETs are
  more efficient.
• A fast-recovery rectifier, or a Schottky barrier
  diode (sometimes referred to as a catch diode) is
  used to direct current into the inductor.
• For proper switch-mode operation, current must
  always be present in the inductor.

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Step-Down or Buck Converter

• When the transistor is turned ON, VL is initially
  high but falls exponentially while IL increases
  to charge C.
• When the transistor turns OFF, VL reverses in
  polarity to maintain the direction of current
  flow. IL decreases but its path is now through
  the forward-biased diode, D.
• Duty cycle is adjusted according to the level of
  Vo.

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V I Waveforms for Buck Regulator
PWM output
VL
IL
Vo
Normal
Low Vo
High Vo
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Equations for Buck Regulator
Selecting ?IL 0.4Io where Io is the max. dc
output current
where ?V is the ripple voltage
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Notes on Operation of Buck Regulator

• When ?IL 0.4Io was selected, the average
  minimum current, Imin, that must be maintained in
  L for proper regulator operation is 0.2Io.
• If ?IL is chosen to be 4 instead of 40 of Io,
  the 2.5 factor in the equation for L becomes 25
  and Imin becomes 0.02Io.
• L and C are both proportional to 1/fosc hence,
  the higher fosc is the smaller L and C become.
  But for predictable operation and less audible
  noise, fosc is usually between 50kHz to 100 kHz.

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Step-Up, Flyback, or Boost Regulator

• Assuming steady-state conditions, when the
  transistor is turned ON, L reacts against Vin.
  D is reverse-biased and C supplies the load
  current.
• When the transistor is OFF, VL reverses polarity
  causing current to flow through D and charges C.
  Note that Vout is gt Vin because VL adds on to Vin.

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Equations for Boost Regulator
Assuming ?IL 0.4Io
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Voltage-Inverting or Buck-Boost Regulator

• Vo can be either step-up or step-down and its
  polarity is opposite to input.
• During ON period, Vin is across L, and D is
  reverse-biased.
• During OFF period, VL reverses polarity causing
  current to flow through C and D.

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Equations for Buck-Boost Regulator
For ?IL 0.4Io
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Basic Push-Pull Power Converter
Operates as a class D power amplifier. Output
rectifier converts the square-wave to dc. Each
transistor must withstand 2xVin plus voltage
spikes.
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Basic Half-Bridge Power Converter
Each transistor sees approx. Vin. Full flux
reversal in the transformer and capacitors
across DS prevent voltage spikes.
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Basic Full-Bridge Power Converter
Either Q1 Q3 or Q2 Q4 are turned ON
simultaneously. Ideal for high power applications.
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Single-Package Switch-Mode Regulator

• The LH1605 is a 5A step-down switching regulator.
• Vo is adjustable from 3 to 30 V by using a pot.
  for R1.
• In the circuit above, Q1 turns ON when voltage
  across Rsens is 0.7 V. Q2 then turns ON shorting
  Vref to ground and driving Vo to zero. .

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Equations for LH1605 Switching Regulator
With ?IL 0.4Io
Typically, CF CC 10 mF RB 10 kW
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BiFET IC Operational Amplifier

• Advantages of TL081 vs bipolar op-amp (LM741)
• higher input impedance (typically 1012 W)
• wider unity-gain bandwidth (3 MHz)
• higher slew rate (13 V/ms typical)
• lower offset current (5 pA)
• lower bias current (30 pA)
• lower power consumption (1.4 mA supply current)
• All other parameters are comparable to bipolar
  op-amps.

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Frequency Compensation

• Most op-amps contain a small internal
  compensating capacitor (15 to 30 pF) for ensuring
  stability at the expense of bandwidth.
• For a specific application requiring a wider
  bandwidth, an uncompensated op-amp, such as the
  TL080, may be chosen with a small external
  compensating capacitor.
• Two commonly used methods are conventional
  compensation and feed-forward compensation. The
  latter method can increase the BW 5 to 10 x.

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Circuits for Frequency Compensation
Conventional
Feed-forward
C1 is typ.10 to 20 pF
C1 is typ. 100 to 150 pF
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Response With Frequency Compensation
Av
With feed-forward compensation
Increase in BW
With normal compensation
f
10M
1M
1k
10k
100k
Hz
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Astable Multivibrator or Relaxation Oscillator
Circuit
Output waveform
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Equations for Astable Multivibrator
Assuming Vsat -Vsat
where ? RfC
If R2 is chosen to be 0.86R1, then T 2RfC and
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Monostable (One-Shot) Multivibrator
Circuit
Waveforms
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Notes on Monostable Multivibrator

• Stable state vo Vsat, VC 0.6 V
• Transition to timing state apply a -ve input
  pulse such that Vip gt VUT vo -Vsat. Best
  to select RiCi ? 0.1RfC.
• Timing state C charges negatively from 0.6 V
  through Rf. Width of timing pulse is

• If we pick R2 R1/5, then tp RfC/5.

• Recovery state vo Vsat circuit is not ready
  for retriggering
• until VC 0.6 V. The recovery time ? tp. To
  speed up the
• recovery time, RD ( 0.1Rf) CD can be added.

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Norton or Current-Mode Op-Amp

• Amplifies ?I ( I- - I) between the inputs.
• Q3 and D1 form a current mirror (ICQ3 ? ID1). In
  practice, two matched transistors are used the
  1st transistor connected as a diode.
• Current into base of Q1 IB1 ?I.
• Note that VB ? 0.7 for both Q1 Q2.

Simplified circuit
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Notes on LM3900 Op-Amp

• Comes in a standard 14-pin DIP quad package.
• Can operate from a single supply (4 to 32 V) or
  dual supplies (2 to 16 V).
• Rin 1 MW, Rout 8 kW
• Aol 2800
• Unity-gain bandwidth 2.5 MHz (much better than
  the LM741)
• Not as widely used as voltage op-amps because
  circuit designers are less familiar with it.

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Norton Amplifiers
Design equations for inverting and non-inverting
amplifiers are exactly the same
Zin RI
Inverting
Neglecting RS and Ro
Non-inverting
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Other Design Equations for Norton Amplifier
Since max. Iin 20 mA dc,
The dc output offset voltage
For max. swing, Voffset VCC/2, thus
Also, min. input bias current is 200 nA, ?
Note that if dual polarity supply is
used, Voffset can be made to be 0V and Cout would
not be required for both circuits.
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Functional Block Diagram of LM555
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Notes on 555 Timer/Oscillator IC

• Widely used as a monostable or astable
  multivibrator.
• Can operate between 4.5 and 16 V.
• Output voltage is approximately 2 V lt VCC.
• Output can typically sink or source 200 mA.
• Max. output frequency is about 10 kHz.
• fo varies somewhat with VCC.
• Threshold input (pin 6) and trigger input (pin 2)
  are normally tied together to external timing RC.

70
555 as a Simple Oscillator
Duty cycle is
Given fo and D,
Note that D must always be gt 0.5. To get 50 duty
cycle, R1 0, which would short out VCC.
tch 0.693(R1 R2)C1 tdisch 0.693 R2C1
T 0.693(R1 2R2)C1
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555 Square-Wave Oscillator
For 50 duty cycle,
tch 0.693 R1C1 tdisch 0.693 R2C1
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555 as a Timer / Monostable Multivibrator
t 1.1 R1C1
Time pulses from a few ms to many minutes
are possible. The main limitation for very
long time delays is the leakage in the
large- value capacitor required for C1.
R2 (typically 10 kW) is a pull-up resistor. C2
(typically 0.001 mF) is for bypass. Timing starts
when trigger input is grounded.
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ICL8038 Function Generator IC

• Triangle wave at pin10 is obtained by linear
  charge and discharge of C by two current sources.
• Two comparators trigger the flip-flop which
  provides the square wave and switches the
  current sources.
• Triangle wave becomes sine wave via the sine
  converter .

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Notes on ICL8038 IC

• To obtain a square wave output, a pull-up
  resistor (typically 10 to 15 kW) must be
  connected between pin 9 and VCC.
• Triangle wave has a linearity of 0.1 or better
  and an amplitude of approx. 0.3(VCC-VEE).
• Sine wave can be adjusted to a distortion of lt 1
  with amplitude of 0.2(VCC-VEE). The distortion
  may vary with f (from 0.001 Hz to 200 kHz).
• IC can operate from either single supply of 10 to
  30 V or dual supply of ?5 to ?15 V.

75
ICL8038 Function Generator Circuit
where R RA RB
If pin 7 is tied to pin 8,
For 50 duty cycle,
VCC gt Vsweep gt ?Vtotal VEE 2 where Vtotal
VCC VEE
76
Active Filters

• Active filters use op-amp(s) and RC components.
• Advantages over passive filters
• op-amp(s) provide gain and overcome circuit
  losses
• increase input impedance to minimize circuit
  loading
• higher output power
• sharp cutoff characteristics can be produced
  simply and efficiently without bulky inductors
• Single-chip universal filters (e.g.
  switched-capacitor ones) are available that can
  be configured for any type of filter or response.

77
Review of Filter Types Responses

• 4 major types of filters low-pass, high-pass,
  band pass, and band-reject or band-stop
• 0 dB attenuation in the passband (usually)
• 3 dB attenuation at the critical or cutoff
  frequency, fc (for Butterworth filter)
• Roll-off at 20 dB/dec (or 6 dB/oct) per pole
  outside the passband ( of poles of reactive
  elements). Attenuation at any frequency, f, is

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Review of Filters (contd)

• Bandwidth of a filter BW fcu - fcl
• Phase shift 45o/pole at fc 90o/pole at gtgt fc
• 4 types of filter responses are commonly used
• Butterworth - maximally flat in passband highly
  non-linear phase response with frequecny
• Bessel - gentle roll-off linear phase shift with
  freq.
• Chebyshev - steep initial roll-off with ripples
  in passband
• Cauer (or elliptic) - steepest roll-off of the
  four types but has ripples in the passband and in
  the stopband

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Frequency Response of Filters
80
Unity-Gain Low-Pass Filter Circuits
2-pole
3-pole
4-pole
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Design Procedure for Unity-Gain LPF

• Determine/select number of poles required.
• Calculate the frequency scaling constant, Kf
  2pf
• Divide normalized C values (from table) by Kf to
  obtain frequency-scaled C values.
• Select a desired value for one of the
  frequency-scaled C values and calculate the
  impedance scaling factor

• Divide all frequency-scaled C values by Kx
• Set R Kx W

82
An Example

• Design a unity-gain LP Butterworth filter with a
  critical frequency of 5 kHz and an attenuation of
  at least 38 dB at 15 kHz.
• The attenuation at 15 kHz is 38 dB
• ? the attenuation at 1 decade (50 kHz) 79.64
  dB.
• We require a filter with a roll-off of at least
  4 poles.
• Kf 31,416 rad/s. Lets pick C1 0.01 mF (or
  10 nF). Then
• C2 8.54 nF, C3 24.15 nF, and C4 3.53 nF.
• Pick standard values of 8.2 nF, 22 nF, and 3.3
  nF.
• Kx 3,444
• Make all R 3.6 kW (standard value)

83
Unity-Gain High-Pass Filter Circuits
2-pole
3-pole
4-pole
84
Design Procedure for Unity-Gain HPF

• The same procedure as for LP filters is used
  except for step 3, the normalized C value of 1 F
  is divided by Kf. Then pick a desired value for
  C, such as 0.001 mF to 0.1 mF, to calculate Kx.
  (Note that all capacitors have the same value).
• For step 6, multiply all normalized R values
  (from table) by Kx.
• E.g. Design a unity-gain Butterworth HPF with a
  critical frequency of 1 kHz, and a roll-off of 55
  dB/dec. (Ans. C 0.01 mF, R1 4.49 kW, R2
  11.43 kW, R3 78.64 kW. pick standard values of
  4.3 kW, 11 kW, and 75 kW).

85
Equal-Component Filter Design
2-pole LPF
2-pole HPF
Av for of poles is given in a table and is the
same for LP and HP filter design.
Same value R same value C are used in filter.
Select C (e.g. 0.01 mF), then
86
Example

• Design an equal-component LPF with a critical
  frequency of 3 kHz and a roll-off of 20 dB/oct.
• Minimum of poles 4
• Choose C 0.01 mF ? R 5.3 kW
• From table, Av1 1.1523, and Av2 2.2346.
• Choose RI1 RI2 10 kW then RF1 1.5 kW, and
  RF2 12.3 kW .
• Select standard values 5.1 kW, 1.5 kW, and 12 kW.

87
Bandpass and Band-Rejection Filter
BPF
BRF
Attenuation (dB)
Attenuation (dB)
f
f
fcu
fctr
fctr
fcu
fcl
fcl
The quality factor, Q, of a filter is given by
where BW fcu - fcl and
88
More On Bandpass Filter
If BW and fcentre are given, then
A broadband BPF can be obtained by combining a
LPF and a HPF
The Q of this filter is usually gt 1.
89
Broadband Band-Reject Filter
A LPF and a HPF can also be combined to give a
broadband BRF
2-pole band-reject filter
90
Narrow-band Bandpass Filter
C1 C2 C
R2 2 R1
R3 can be adjusted or trimmed to change fctr
without affecting the BW. Note that Q lt 1.
91
Narrow-band Band-Reject Filter
Easily obtained by combining the inverting output
of a narrow-band BRF and the original signal
The equations for R1, R2, R3, C1, and C2 are the
same as before. RI RF for unity gain and is
often chosen to be gtgt R1.