h. chan; mohawk college 1 electronic circuits ee451
TRANSCRIPT
H. Chan; Mohawk College1
ELECTRONIC CIRCUITSELECTRONIC CIRCUITSEE451
H. Chan; Mohawk College 2
MAIN TOPICS (2nd half)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.
H. Chan; Mohawk College 3
Power Supply Block DiagramPower Supply Block Diagram
H. Chan; Mohawk College 4
Half-Wave RectifierHalf-Wave Rectifier
7.0V2VSP
FL
Pdc CR
00833.01VV
FL
P
r CR
V0048.0V
V
t
H. Chan; Mohawk College 5
Full-Wave RectifierFull-Wave Rectifier
7.0V707.0VsP
FL
Pdc CR
00417.01VV
FL
P
r CR
V0024.0V
V
t
H. Chan; Mohawk College 6
Bridge-Type RectifierBridge-Type Rectifier
4.1V2VsP
FL
Pdc CR
00417.01VV
FL
P
r CR
V0024.0V
V
t
H. Chan; Mohawk College 7
More Equations . . . More Equations . . .
Rearranging the previous equations: VP = Vdc + 1.736 Vr
The ripple voltage as a percentage of the dc voltage is:
100% xV
Vripple
dc
r
The diode(s) must be rated to withstand the surge current:
W
Psurge R
VI
where RW is the transformer winding’sresistance given by:
FL
FLNLW I
VVR
H. Chan; Mohawk College 8
Comparison of Different Types of RectifiersComparison 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.
H. Chan; Mohawk College 9
Line RegulationLine Regulation
i
o
V
VVmVregulationLine
)/(
oi
o
Vx
V
Vregulationline
100%
is a measure of the effectiveness of a voltage regulatorto maintain the output dc voltage constant despitechanges in the supply voltage.
H. Chan; Mohawk College 10
Load RegulationLoad Regulation
is a measure of the ability of a regulator to maintain aconstant dc output despite changes in the load current.
L
o
I
VAmVregulationLoad
)/(
oL
o
Vx
I
Vregulationload
100%
H. Chan; Mohawk College 11
Other SpecificationsOther Specifications
A common definition for voltage regulation is:
100(%) xV
VVregulationVoltage
FL
FLNL
The ability to reduce the output ripple voltage is:
)(
)(log20)(inr
outr
V
VdBrejectionRipple
Source resistance of regulator is:
morI
VR
L
os
H. Chan; Mohawk College 12
Zener Diode Voltage RegulatorZener Diode Voltage Regulator
Circuit
I-V Characteristic
IZM
H. Chan; Mohawk College 13
Notes on Zener Diode RegulatorNotes on Zener Diode Regulator
VZ depends on I and temperature. Zener diodes with rated voltage < 6 V have negative
temperature coefficient; those rated > 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.
H. Chan; Mohawk College 14
Formulae for Zener Regulator CircuitFormulae for Zener Regulator Circuit
Rs establishes the zener bias current, IZT:
LZT
Zi
Rs
Zis II
VV
I
VVR
For fixed Vi, but variable RL:
ZMRs
Z
L
ZL
Zi
Zs
Rs
ZL
II
V
I
VR
VV
VR
I
VR
(min)
.max
.min
H. Chan; Mohawk College 15
Formulae (cont’d)Formulae (cont’d)
For fixed RL, but variable Vi:
LZMR
ZsRi
ZL
sLi
IIIwhere
VRIV
VR
RRV
(max)
(max).max
.min
The output ripple voltage of the zener regulator is:
)()( //
//inr
sZL
ZLoutr V
RRR
RRV
where RZ = ac resistance
of zener diode.
H. Chan; Mohawk College 16
Transistor Series Voltage RegulatorTransistor Series Voltage Regulator
The simple zener regulatorcan be markedly improvedby adding a transistor.Since VBE = VZ - VL anytendency for VL to decrease or increase will be negatedby an increase or decrease in IE. The dc currents for thecircuit are:
R
VVI
R
VV
R
VI Zi
RL
BEZ
L
LL
;
IL = hFEIB; IZT = IR - IB
H. Chan; Mohawk College 17
Transistor Shunt Voltage RegulatorTransistor Shunt Voltage Regulator
Since VBE = VL - VZ,any tendency for VL
to increase or decreasewill result in a corresponding increase or decrease in IRs. This willoppose any changes in VL because VL = Vi - IRsRs.
S
BEZi
Rs
L
BEZ
L
L
L R
)VV(VI;
R
VV
R
VI
IE = IRs - IL = hFEIZT
H. Chan; Mohawk College 18
Op-Amp Voltage RegulatorsOp-Amp Voltage Regulators
Zo VR
RV
3
21
Series Shunt
H. Chan; Mohawk College 19
Notes on Op-Amp Voltage RegulatorNotes 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.
H. Chan; Mohawk College 20
Constant Current LimitingConstant Current Limitingcan be used for short-circuit or overload protection ofthe series voltage regulator.
4(max)
7.0
RIL
Output currentis limited to:
H. Chan; Mohawk College 21
Fold-back Current LimitingFold-back Current Limitingis a better method of short-circuit protection.
oLooBBE VRIVRR
RVVV
)( 4
65
622
H. Chan; Mohawk College 22
Design Equations for Fold-back Current LimitingDesign Equations for Fold-back Current Limiting
Maximum load current without fold-back limiting:
64
655(max)
)(7.0
RR
RRVRI o
L
Output voltage under current limiting condition:
L
Lo RRRR
RRRV
564
65 )(7.0'
The short circuit current (i.e. when Vo = 0) is:
64
65 )(7.0
RR
RRI short
H. Chan; Mohawk College 23
Characteristics of Fold-back LimitingCharacteristics of Fold-back Limiting
Notice that Ishort < IL(max) and that Vo is regulated (i.e. constant) only after RL > a certain critical value.
For designing purpose, R5 + R6 = 1 k and if Ishort and IL(max) are specified then
(max)4 7.0)7.0(
07
Loshort
o
IVI
VR
Vo
IL
H. Chan; Mohawk College 24
Transistor Current RegulatorsTransistor Current Regulatorsare designed to maintain a fixed current through aload for variations in either Vi or RL.
For the BJT circuit, VEB = VZ - VRE.Any tendency for IL to change willcause an opposing change in VEB,thus nullifying the perturbation.
For the JFET circuit, IL = ID = IDSS aslong as VL < VSS - VP.
H. Chan; Mohawk College 25
IC Voltage RegulatorsIC 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
H. Chan; Mohawk College 26
Multipin IC Voltage RegulatorMultipin IC Voltage Regulator
LM 723C Schematic
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.
H. Chan; Mohawk College 27
Notes on LM723 Voltage RegulatorNotes 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 > Vo(max) + 3)
H. Chan; Mohawk College 28
LM723 in High-Voltage ConfigurationLM723 in High-Voltage Configuration
External pass transistor andcurrent sensing added.
Design equations:
2
21 )(
R
RRVV ref
o
21
213 RR
RRR
max
7.0
IRsens
Choose R1 + R2 = 10 k,and Cc = 100 pF.To make Vo variable,replace R1 with a pot.
H. Chan; Mohawk College 29
LM723 in Low-Voltage ConfigurationLM723 in Low-Voltage Configuration
With external pass transistorand foldback current limiting
sens5
54o4
(max)L RR
)RR(7.0VRI
sens5
54
short RR
)RR(7.0I
(max)Loshort
o
sens I7.0)7.0V(I
V7.0R
L4sens5
54L
o RRRR
)RR(R7.0'V
Under foldback condition:
21
ref2
o RR
VRV
H. Chan; Mohawk College 30
Three-Terminal Fixed Voltage RegulatorsThree-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.
H. Chan; Mohawk College 31
Basic Circuits With 78/79XX RegulatorsBasic 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 F tantalum type or 0.1 F mica type capacitors.
H. Chan; Mohawk College 32
Dual-Polarity Output with 78/79XX RegulatorsDual-Polarity Output with 78/79XX Regulators
H. Chan; Mohawk College 33
78XX Regulator with Pass Transistor78XX 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.
max1
7.0
IR
22
7.0
RIR
H. Chan; Mohawk College 34
78XX Floating Regulator78XX Floating Regulator
It is used to obtain an output > 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.
21
RIR
VVV Q
regrego
1
12
)(
RIV
VVRR
Qreg
rego
or
H. Chan; Mohawk College 35
3-Terminal Variable Regulator3-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).
H. Chan; Mohawk College 36
Basic LM317 Variable Regulator CircuitsBasic LM317 Variable Regulator Circuits
Circuit with capacitorsto improve performance
Circuit with protectivediodes
(a) (b)
H. Chan; Mohawk College 37
Notes on Basic LM317 CircuitsNotes 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.
21
RIR
VVV adj
refrefo
where Vref = 1.25 V, and Iadj isthe current flowing into the adj.terminal (typically 50 A).
1
12
)(
RIV
VVRR
adjref
refo
R1 = Vref /IL(min), where IL(min)
is typically 10 mA.
H. Chan; Mohawk College 38
Other LM317 Regulator CircuitsOther LM317 Regulator Circuits
Circuit with pass transistorand current limiting
Circuit to give 0V min.output voltage
H. Chan; Mohawk College 39
Block Diagram of Switch-Mode RegulatorBlock Diagram of Switch-Mode Regulator
It converts an unregulated dc input to a regulated dcoutput. Switching regulators are often referred to asdc to dc converters.
H. Chan; Mohawk College 40
Comparing Switch-Mode to Linear RegulatorsComparing Switch-Mode to Linear Regulators
Advantages: 70-90% efficiency (about double that of linear ones) can make output voltage > 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
H. Chan; Mohawk College 41
General Notes on Switch-Mode RegulatorGeneral Notes on Switch-Mode Regulator
The duty cycle of the series transistor (power switch) determinesthe average dc output of the regulator. A circuit to control theduty cycle is the pulse-width modulator shown below:
H. Chan; Mohawk College 42
General Notes cont’d . . . General Notes cont’d . . .
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.
H. Chan; Mohawk College 43
Step-Down or Buck ConverterStep-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.
H. Chan; Mohawk College 44
V & I Waveforms for Buck RegulatorV & I Waveforms for Buck RegulatorPWMoutput
VL
IL
Vo
Normal Low Vo High Vo
H. Chan; Mohawk College 45
Equations for Buck RegulatorEquations for Buck Regulator
T
t
tt
t
V
V on
offon
on
i
o
Selecting IL = 0.4Io where Io
is the max. dc output current:
oscio
oio
fVI
VVVL
)(5.2
oscrms
o
oscpp
o
fV
Ior
fV
IC
01768.005.0
where V is the ripple voltage
H. Chan; Mohawk College 46
Notes on Operation of Buck RegulatorNotes 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.
H. Chan; Mohawk College 47
Step-Up, Flyback, or Boost RegulatorStep-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 > Vin because VL adds on to Vin.
H. Chan; Mohawk College 48
Equations for Boost RegulatorEquations for Boost Regulator
T
t
V
VV on
o
io
oscoo
ioi
fVI
VVVL
2
2 )(5.2
Assuming IL = 0.4Io:
rmsoosc
oio
ppoosc
oio
VVf
IVVor
VVf
IVVC
)(3536.0)(
H. Chan; Mohawk College 49
Voltage-Inverting or Buck-Boost RegulatorVoltage-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.
H. Chan; Mohawk College 50
Equations for Buck-Boost RegulatorEquations for Buck-Boost Regulator
T
t
VV
V on
oi
o
For IL = 0.4Io:
oscioo
oi
fVVI
VVL
)(
5.2
oscoirms
oo
oscoipp
oo
fVVV
VIor
fVVV
VIC
)(
3536.0
)(
H. Chan; Mohawk College 51
Basic Push-Pull Power ConverterBasic Push-Pull Power Converter
Operates as a class D power amplifier. Output rectifier converts the square-wave to dc. Each transistor must withstand 2xVin plusvoltage spikes.
H. Chan; Mohawk College 52
Basic Half-Bridge Power ConverterBasic Half-Bridge Power Converter
Each transistor “sees” approx. Vin. Full flux reversal in the transformer and capacitors across DS prevent voltage spikes.
H. Chan; Mohawk College 53
Basic Full-Bridge Power ConverterBasic Full-Bridge Power Converter
Either Q1 & Q3 or Q2 & Q4 are turned ON simultaneously.Ideal for high power applications.
H. Chan; Mohawk College 54
Single-Package Switch-Mode RegulatorSingle-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. .
H. Chan; Mohawk College 55
Equations for LH1605 Switching RegulatorEquations for LH1605 Switching Regulator
2000800
00125.05.2
1
1
o
o
VR
orRV
oscT f
C40000
1
oscio
oio
fVI
VVVL
)(5.2 With IL = 0.4Io:
oscpp
o
oscrms
o
fV
Ior
fV
IC
05.001768.0
max
7.0
IRsens
Typically, CF = CC = 10 F; RB = 10 k
H. Chan; Mohawk College 56
BiFET IC Operational AmplifierBiFET IC Operational Amplifier
Advantages of TL081 vs bipolar op-amp (LM741): higher input impedance (typically 1012 ) wider unity-gain bandwidth (3 MHz) higher slew rate (13 V/s 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.
H. Chan; Mohawk College 57
Frequency CompensationFrequency 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.
H. Chan; Mohawk College 58
Circuits for Frequency CompensationCircuits for Frequency Compensation
Conventional Feed-forwardC1 is typ.10 to 20 pF C1 is typ. 100 to 150 pF
H. Chan; Mohawk College 59
Response With Frequency CompensationResponse With Frequency Compensation
10k 100k 1M1k 10Mf
Hz
Av
Increase in BW
With feed-forwardcompensation
With normalcompensation
H. Chan; Mohawk College 60
Astable Multivibrator or Relaxation OscillatorAstable Multivibrator or Relaxation Oscillator
Circuit Output waveform
H. Chan; Mohawk College 61
Equations for Astable MultivibratorEquations for Astable Multivibrator
21
2
21
2 ;RR
RVV
RR
RVV sat
LTsat
UT
1
2121
2ln2
R
RRttT Assuming
|+Vsat| = |-Vsat|
If R2 is chosen to be 0.86R1, then T = 2RfC and
where = RfC
CRf
f2
1
H. Chan; Mohawk College 62
Monostable (One-Shot) MultivibratorMonostable (One-Shot) Multivibrator
Circuit Waveforms
H. Chan; Mohawk College 63
Notes on Monostable MultivibratorNotes on Monostable Multivibrator
Stable state: vo = +Vsat, VC = 0.6 V Transition to timing state: apply a -ve input pulse such
that |Vip| > |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:
LTsat
satfp VV
VCRt
||
6.0||ln
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.
If we pick R2 = R1/5, then tp = RfC/5.
H. Chan; Mohawk College 64
Norton or Current-Mode Op-AmpNorton 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
H. Chan; Mohawk College 65
Notes on LM3900 Op-AmpNotes 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 M, Rout = 8 k 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.
H. Chan; Mohawk College 66
Norton AmplifiersNorton Amplifiers
Inverting
Non-inverting
Design equations for invertingand non-inverting amplifiersare exactly the same:
Zin = RI ;I
Fv R
RA
Neglecting RS and Ro:
LcLout
IcLin
RfC
RfC
2
1
2
1
H. Chan; Mohawk College 67
Other Design Equations for Norton AmplifierOther Design Equations for Norton Amplifier
Note that if dual polarity supply is used,Voffset can be made to be 0V and Cout
would not be required for both circuits.
Since max. Iin = 20 mA dc,
04.0
4.102.0
7.0
(min)
(min)
CCF
CCB
VR
VR
Also, min. input biascurrent is 200 nA,
nA
VR
nA
VR
CCF
CCB
400
4.1
200
7.0
(max)
(max)
The dc output offset voltage:
7.0)7.0(
B
CCFoffset R
VRV
For max. swing, Voffset = VCC/2, thus
7.02/
)7,0(
CC
FCCB V
RVR
H. Chan; Mohawk College 68
Functional Block Diagram of LM555Functional Block Diagram of LM555
H. Chan; Mohawk College 69
Notes on 555 Timer/Oscillator ICNotes 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 < 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.
H. Chan; Mohawk College 70
555 as a Simple Oscillator555 as a Simple Oscillator
tch = 0.693(R1 + R2)C1
tdisch = 0.693 R2C1
T = 0.693(R1 + 2R2)C1
21
21
2RR
RR
T
tD ch
Duty cycle is:
Given fo and D,
12
11 693.0
1;
693.0
12
Cf
DR
Cf
DR
oo
Note that D must always be > 0.5.To get 50% duty cycle, R1 = 0,which would short out VCC.
H. Chan; Mohawk College 71
555 Square-Wave Oscillator555 Square-Wave Oscillator
tch = 0.693 R1C1 ; tdisch = 0.693 R2C1
121 )(693.0
1
CRRfo
21
1
RR
RD
12
11 693.0
1;
693.0 Cf
DR
Cf
DR
oo
For 50% duty cycle,
121 386.1
1
CfRR
o
H. Chan; Mohawk College 72
555 as a Timer / Monostable Multivibrator555 as a Timer / Monostable Multivibrator
R2 (typically 10 k) is a pull-up resistor.C2 (typically 0.001 F) is for bypass.Timing starts when trigger input is grounded.
t = 1.1 R1C1
Time pulses from a fews to many minutes arepossible. The mainlimitation for very longtime delays is theleakage in the large-value capacitor requiredfor C1.
H. Chan; Mohawk College 73
ICL8038 Function Generator ICICL8038 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 .
H. Chan; Mohawk College 74
Notes on ICL8038 ICNotes on ICL8038 IC
To obtain a square wave output, a pull-up resistor (typically 10 to 15 k) 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 < 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.
H. Chan; Mohawk College 75
ICL8038 Function Generator CircuitICL8038 Function Generator Circuit
+VCC > Vsweep > Vtotal + VEE + 2where Vtotal = VCC + |VEE|
total
sweepCCo VRC
VVf
12
)(3
where R = RA = RB
If pin 7 is tied to pin 8,
BA
AA
o
RRR
CR
f
215
3
1
For 50 % duty cycle,
1
3.0
RCfo
H. Chan; Mohawk College 76
Active FiltersActive 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.
H. Chan; Mohawk College 77
Review of Filter Types & ResponsesReview 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:
decc
fatdBattenxf
ffatdBatten )(.log)(.
H. Chan; Mohawk College 78
Review of Filters (cont’d)Review of Filters (cont’d)
Bandwidth of a filter: BW = fcu - fcl
Phase shift: 45o/pole at fc; 90o/pole at >> 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
H. Chan; Mohawk College 79
Frequency Response of FiltersFrequency Response of Filters
f
A(dB)
fc
f
A(dB) HPF
fcl fcu
f
A(dB)BPF
fcl fcu
f
A(dB)
BRF
fc
f
A(dB)LPF
Pass-band
Butterworth
BesselChebyshev
H. Chan; Mohawk College 80
Unity-Gain Low-Pass Filter CircuitsUnity-Gain Low-Pass Filter Circuits
2-pole 3-pole
4-pole
H. Chan; Mohawk College 81
Design Procedure for Unity-Gain LPFDesign Procedure for Unity-Gain LPF
Determine/select number of poles required. Calculate the frequency scaling constant, Kf = 2f 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:
valueCdesired
valueCscaledfrequencyK x
Divide all frequency-scaled C values by Kx
Set R = Kx
H. Chan; Mohawk College 82
An ExampleAn 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. Let’s pick C1 = 0.01 F (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 = 3,444Make all R = 3.6 k (standard value)
H. Chan; Mohawk College 83
Unity-Gain High-Pass Filter CircuitsUnity-Gain High-Pass Filter Circuits
2-pole 3-pole
4-pole
H. Chan; Mohawk College 84
Design Procedure for Unity-Gain HPFDesign 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 F to 0.1 F, 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 F, R1 = 4.49 k, R2 = 11.43 k, R3 = 78.64 k.; pick standard values of 4.3 k, 11 k, and 75 k).
H. Chan; Mohawk College 85
Equal-Component Filter DesignEqual-Component Filter Design
2-pole LPF 2-pole HPF
Select C (e.g. 0.01 F), then:
CfR
o2
1
Av for # of poles is given ina table and is the same forLP and HP filter design.
1I
Fv R
RA
Same value R & same value Care used in filter.
H. Chan; Mohawk College 86
Example Example
Design an equal-component LPF with a critical frequency of 3 kHz and a roll-off of 20 dB/oct.
Minimum # of poles = 4Choose C = 0.01 F; R = 5.3 kFrom table, Av1 = 1.1523, and Av2 = 2.2346.
Choose RI1 = RI2 = 10 k; then RF1 = 1.5 k, and RF2 = 12.3 k .
Select standard values: 5.1 k, 1.5 k, and 12 k.
H. Chan; Mohawk College 87
Bandpass and Band-Rejection FilterBandpass and Band-Rejection Filter
fctr fctrfcu fcufcl fcl
f fAtt
enua
tion
(dB
)
Att
enua
tion
(dB
)The quality factor, Q, of a filter is given by:
BW
fQ ctr
where BW = fcu - fcl and
clcuctr fff
BPF BRF
H. Chan; Mohawk College 88
More On Bandpass FilterMore On Bandpass FilterIf BW and fcentre are given, then:
24;
242
22
2 BWf
BWf
BWf
BWf ctrcuctrcl
A broadband BPF can be obtained by combining a LPF and a HPF:
The Q of this filteris usually> 1.
H. Chan; Mohawk College 89
Broadband Band-Reject FilterBroadband Band-Reject FilterA LPF and a HPF can also be combined to give a broadbandBRF:
2-pole band-reject filter
H. Chan; Mohawk College 90
Narrow-band Bandpass FilterNarrow-band Bandpass Filter
CRQ
fBW ctr
12
1
12 21
3
Q
RR
R2 = 2 R1
3
1
1
122
1
R
R
CRfctr
R3 can be adjusted or trimmedto change fctr without affectingthe BW. Note that Q < 1.
C1 = C2 = C
H. Chan; Mohawk College 91
Narrow-band Band-Reject FilterNarrow-band Band-Reject FilterEasily 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 >> R1.