electronics part ii
DESCRIPTION
ELECTRONICS Part II. EE 213. Course Content. Part I Introduction to Oscilloscope Measurements Introduction to Semiconductors Diode Applications Special-Purpose Diodes Bipolar Junction Transistors (BJTs) Bipolar Transistor Biasing. Course Content (cont’d). Part II - PowerPoint PPT PresentationTRANSCRIPT
H. Chan; Mohawk College 1
ELECTRONICSPart II
EE 213
H. Chan; Mohawk College 2
Course Content
Part I– Introduction to Oscilloscope Measurements– Introduction to Semiconductors– Diode Applications– Special-Purpose Diodes– Bipolar Junction Transistors (BJTs)– Bipolar Transistor Biasing
H. Chan; Mohawk College 3
Course Content (cont’d)
Part II– Small-Signal BJT Amplifiers– Power Amplifiers– Field-Effect Transistor Biasing– Small-Signal FET Amplifiers– Amplifier Frequency Response– Transistor Voltage Regulators– Introduction to Thyristors
H. Chan; Mohawk College 4
Intro to Small-Signal Amplifiers
The Q-point of a transistor is set by dc biasing Small-signal amplifiers are designed to handle small ac
signals that cause relatively small variations about the Q-point.
Convention used for dc and ac values:
– dc values, e.g. IE , RE
– ac values, e.g. Ie (rms is assumed unless otherwise stated), Re , r’e (internal r of t’sistor)
– instantaneous values, e.g. ie
H. Chan; Mohawk College 5
Basic Small-Signal Amplifier+VCC
Vs
Rs C1
R1
R2
RC
RE
C2
RL
VbVBQ
IBQ
Ib
ICQ
Ic
VCEQ
Vce
C1 and C2 block dc voltage but pass ac signal.
H. Chan; Mohawk College 6
Small-Signal Amplifier Operation
Coupling capacitors C1 and C2 prevent Rs and RL from changing the dc bias voltages
Vs causes Vb and Ib to vary slightly which in turn produces large variations in Ic due to
As Ic increases, Vce decreases and vice versa
Thus, Vc (output to RL) is 180o out of phase with Vb
H. Chan; Mohawk College 7
Graphical Picture
IC
VCE
Ic
Vce
Ib
QICQ
IBQ
VCEQ
IB1
IB2
IB3
IB4
IB5
H. Chan; Mohawk College 8
h Parameters
h (hybrid) parameters are typically specified on a manufacturer’s data sheet.– hi: input resistance; output shorted
– hr : voltage feedback ratio, input open
– hf : forward current gain; output shorted
– ho : output conductance; input open
Each h parameter has a 2nd subscript letter to designate configuration, e.g. hfe, hfc, hfb
H. Chan; Mohawk College 9
Amplifier Configurations
Common-emitter amplifier: emitter is connected to ground, input is applied to base, and output is on collector
Common-collector: collector is grounded, input is at base, and output is on emitter
Common-base: base is grounded, input is at emitter, and output is on collector
H. Chan; Mohawk College 10
h-parameter Ratios
hie = Vb/Ib hib = Ve/Ib hic = Vb/Ib
hre = Vb/Vc hrb = Ve/Vc hrc = Vb/Ve
hfe = Ic/Ib hfb = Ic/Ie hfc = Ie/Ib
hoe = Ic/Vc hob = Ic/Vc hoc = Ie/Ve
Common-Emitter Common-Base Common-Collector
H. Chan; Mohawk College 11
h-parameter Equivalent Circuit
hfIin hohrVout
hiVin Vout
Iin
The above diagram is the general form of the h-parameter equivalent circuit for a BJT.
For the three different amplifier configurations, just add the appropriate second subscript letter.
H. Chan; Mohawk College 12
r Parameters
The resistance, r, parameters are perhaps easier to work with ac : ac alpha (Ic/Ie)
ac : ac beta (Ic/Ib)
– re’ : ac emitter resistance
– rb’ : ac base resistance
– rc’ : ac collector resistance
Relationships of h and rparameters:
ac = hfb ; ac = hfe
oe
rec
oe
ree h
hr
h
hr
1';'
)1(' feoe
reieb h
h
hhr
H. Chan; Mohawk College 13
r-Parameter Equivalent Circuits
rb’B
Ib
C
acIe
acIb
acIe rc’
re’ 25 mV/IEIe
EGeneralized r-parameterequivalent circuit for BJT
Simplified r-parameter equivalentcircuit and symbol of BJT
B
E
C C
acIb
E
BIb
re’
re’
H. Chan; Mohawk College 14
Difference between ac and DC
IC
IB0
Q
IC
IB0
Q
IB
IC {ICQ
IBQ
{
DC = ICQ/IBQ ac = IC/IB
DC and ac values will generally not be identical and they also vary with the Q-point chosen.
H. Chan; Mohawk College 15
Common-Emitter Amplifier+VCC
C1
R1
R2
RC
RE
C3
RLC2
Vin
Vout
C1 : for input couplingC3 : for output coupling
C2 : for emitter bypass
H. Chan; Mohawk College 16
DC Analysis of CE Amplifier
+VCC
R1
R2
RC
RE
CCEDC
EDCB V
RRR
RRV
//
//
21
2
VE = VB - VBE ;
If DCRE = RIN(base) >> R2, then
CCB VRR
RV
21
2
E
EEC R
VII
VC = VCC - ICRC
VB
VC
VE
H. Chan; Mohawk College 17
AC Analysis of CE Amplifier
R1 R2re’
RC
acIb
E
B
C
C1, C2, and C3 arereplaced by shortsassuming XC 0
acground
Rin(tot) = R1//R2//Rin(base) where Rin(base) = acre’Rout RC (not including RL)
Input and output resistance:
Voltage gain of CE amplifier:
'' e
C
ee
Cc
b
c
in
outv r
R
rI
RI
V
V
V
VA
If RL is included: Rout = RC//RL ,and
'
//
e
LCv r
RRA
Vout
Vin
H. Chan; Mohawk College 18
Overall Voltage Gain of CE Amplifier
Vs R1//R2
Rs
Rc
= RC//RL
VbVout
vtotins
totinv
s
bv A
RR
RA
V
VA
)(
)('
Voltage gain withoutemitter bypasscapacitor, C2 :
Ee
cv Rr
RA
'
Rule of thumb onemitter bypasscapacitor:XC2 RE/10
H. Chan; Mohawk College 19
Stability of Voltage Gain+VCC
R1
R2
RC
RE1
C1
C3
RE2 C2
Bypassing RE produces max.voltage gain but it is less stablesince re’ is dependent on IE andtemperature.By choosing RE110 re’, thethe effect of re’ is minimized without reducing the voltagegain too much:
Av -Rc/RE1
Rin(base) = ac(re’+RE1)Swampedamplifier
H. Chan; Mohawk College 20
Current Gain and Power Gain
Vs R1//R2
Rs
Rc
IbIs
Ic
Base to collector current gain is ac butoverall current gain is Ai = Ic/Is where
stotin
ss RR
VI
)(Overall power gain is: Ap = Av’Ai
H. Chan; Mohawk College 21
Common-Collector Amplifier
+VCC
C1
R1
R2 RE RL
C2Vin
Vout
DC analysis:
CCEDC
EDCB V
RRR
RRV
//
//
21
2
VE = VB - VBE
IE = VE / RE
VC = VCC
The CC amplifier is alsoknown as an emitter-follower since Vout followsVin in phase and voltage.
H. Chan; Mohawk College 22
AC Analysis of CC Amplifier
R1//R2re’
acIe
Re= RE//RL
Vout
Vin
)'(
)'(
)( eeacbasein
ee
e
in
outv
RrR
Rr
R
V
VA
Vin = Ie(re’ + Re)Vout = IeRe
Ie
If Re >> re’, then, Av 1,and Rin(base) acRe
Rin(tot) = R1//R2//Rin(base)
Rout (Rs/ac)//Re (very low)
Ai = Ie/Iin ac (if R1//R2>> acRe)
Ap = AvAi Ai
Iin
H. Chan; Mohawk College 23
Darlington Pair
Ie2ac2Ie1 ac1ac2Ie1
So, ac(overall) = ac1ac2
Assuming re’ << RE, Rin = ac1ac2RE
Darlington pair has very high current gain, very high Rin, and very low Rout - ideal as a buffer
RE
Ib1
Ie1
Ie2
+VCC
ac1
ac2
H. Chan; Mohawk College 24
Common-Base Amplifier+VCC
C1
R1
R2
RC
RE
C3
RL
C2
Vin
Vout
CB amplifier provideshigh Av with a max. Ai
of 1.There is no phaseinversion betweenVout and Vin.
DC formulas areidentical to thosefor CE amplifier.
H. Chan; Mohawk College 25
AC Analysis of CB Amplifier
re’
Vin
RE
Rc
Vout
B
E
IcEe
c
Eee
cc
in
outv Rr
R
RrI
RI
V
VA
//')//'(
Rin(emitter) = re’//RE
If RE >> re’, then
;'e
cv r
RA Rin(emitter) = re’
Rout Rc
Ai 1; and Ap Av
H. Chan; Mohawk College 26
Comparing CE, CC, & CB Amplifiers
CE CC CB
Av High (-Rc/re’) Low 1 High (Rc/re’)
Ai(max) High (ac) High (ac) Low 1
Ap Very high(AvAi)
High Ai High Av
Rin(max) Low (acre’) High (acRe) Very low(re’)
Rout High (Rc) Very low(Rs/ac)//Re)
High (Rc)
H. Chan; Mohawk College 27
Multistage Amplifiers
Overall voltage gain, AvT = Av1Av2Av3 . . . Avn = Vout/Vin
Overall gain in dB, AvT(dB) = Av1(dB)+Av2(dB) + . . . +Avn(dB) where, Av(dB) = 20 log Av
The purpose of a multistage arrangement is to increase the overall voltage gain.
Av1 Av2 Av3 AvnVinVout
n amplifier stages in cascade
H. Chan; Mohawk College 28
Two-stage CE Amplifier+VCC
C1
R1
R2
R3
R4
C3
C2
Vin
R5
R6
R7
R8
C5
C4
VoutQ1Q2
Capacitive coupling prevents change in dc bias
H. Chan; Mohawk College 29
Analysis of 2-stage CE Amplifier
R1//R2 R3 R5//R6 Rin(base2)
DC Analysis:VB, VE, VC and IC areidentical to thosefor 1-stage CE ampl.
AC Analysis:Rc1 = R3//R5//R6//Rin(base2) ; ''
;'
722
11
ee
cv
e
cv r
R
r
RA
r
RA
AvT = Av1Av2
Vin
H. Chan; Mohawk College 30
2-stage Direct-Coupled Amplifier
R1
R2
R3
R4
Vin
R5
R6
Vout
Q1Q2
+VCC
Note:No coupling orbypass capacitors
H. Chan; Mohawk College 31
Notes on Direct-Coupled Amplifier
DC collector voltage of first stage provides base-bias voltage for second stage
Amplification down to 0 Hz is possible due to absence of capacitive reactances
Disadvantage - small changes in dc bias voltages due to temperature or power-supply variations are amplified by succeeding stages
H. Chan; Mohawk College 32
Transformer-Coupled Multistage Amplifier
R5 C2
Q1
R6 C4
Q2
Vin C1C3
C5
R2
R1 R3
R4
+VCC
T1T2
T3
Vout
H. Chan; Mohawk College 33
Notes On T’former-Coupled Amplifiers
Transformer-coupling is often used in high-frequency amplifiers such as those in RF and IF sections of radio and TV receivers.
Transformer size is usually prohibitive at low frequencies such as audio.
Capacitors are usually connected across the primary windings of the transformers to obtain resonance and increase selectivity.
H. Chan; Mohawk College 34
Typical Troubleshooting Process
Identify the symptom(s). Perform a power check. Perform a sensory check. Apply a signal-tracing technique to isolate the
fault to a single circuit. Apply fault-analysis to isolate the fault further to a
single component or group of components. Use replacement or repair to fix the problem.
H. Chan; Mohawk College 35
Power Amplifiers
Power amplifiers are large-signal amplifiers They are normally used as the final stage of
a communications receiver or transmitter to provide signal power to speakers or to a transmitting antenna.
Four classes of large-signal amplifiers will be covered: class A, class B, class AB, and class C.
H. Chan; Mohawk College 36
Class A Amplifier Characteristics
Q-point is centred on ac load line. Operates in linear region (i.e. no cutoff or
saturation) for full 360o of input cycle. Output voltage waveform has same shape
as input waveform except amplified. Can be either inverting or noninverting. Maximum power efficiency is 25%.
H. Chan; Mohawk College 37
Class A Operation: DC Load Line
+VCC
C1
R1
R2
RC
RE
C3
RL
C2
Vin
Vout
IC(sat) occurs when VCE 0, so
EC
CCsatC RR
VI
)(
VCE(cutoff) occurswhen IC 0, soVCE(cutoff) VCC
VCE
IC
VCC
IC(sat)Q
VCEQ
ICQ
H. Chan; Mohawk College 38
Class A Operation: AC Load Line
R1//R2 Rc
Vin
Rc = RC//RL
From Q-point to saturation point:Vce = VCEQ ; Ic’ = VCEQ/Rc
Ic(sat) = ICQ+Ic’ = ICQ + VCEQ/Rc
From Q-point to cutoff point:Ic = ICQ ; Vce’ = ICQRc
Vce(cutof) = VCEQ + ICQRc
Q
IC
VCE
Ic(sat)
ICQ
VCEQ Vce(cutoff)
Ic’
Vce’
H. Chan; Mohawk College 39
Maximum Class A Operation
Ic(sat)
ICQ
0
0 VCEQ Vce(cutoff)
IC
VCE
Q
AC load line Q-point is at centreof ac load line formax. voltage swingwith no clipping.
To centre Q-point:VCEQ = ICQRc
Ic(sat) = 2ICQ
Vce(cutoff) = 2VCEQ
H. Chan; Mohawk College 40
Non-linearity of re’
For large voltage swings, re’25mV/IE is not valid.
Instead, the average value of re’ = VBE/IC should be used for Av formula.
Non-linearity of re’ leads to distortion at output.
Reduce distortion by setting Q-point higher or use swamping resistor in the emitter.
IC
VBE
Q
H. Chan; Mohawk College 41
Power & Efficiency: Class A Amplifier
Power gain, Ap = AiAv DCRc/re’
Quiescent power, PDQ = ICQVCEQ
Output power, Pout = VceIc = Vout(rms)Iout(rms)
– when Q-point is centred, Pout(max) = 0.5VCEQICQ
Efficiency, = Pout/PDC = Pout/(VCCICQ)
When Q-point is centred, max = 0.25
Max. load power, PL(max) = 0.5(VCEQ)2/RL
H. Chan; Mohawk College 42
Class B Amplifier Characteristics
Biased at cutoff - it operates in the linear region for 180o and cutoff for 180o.
Class AB amplifier is biased to conduct slightly more than 180o.
Advantage of class B or class AB amplifier over class A amplifier - more efficient.
Disadvantage - more difficult to implement circuit for linear reproduction of input wave
H. Chan; Mohawk College 43
Push-pull Class B Operation
+VCC
-VCC
VinRL
Q1
Q2
Complementary amplifier
An npn transistor and a matchedpnp transistor form two emitter-followers that turn on alternately.Since there is no dc base bias, Q1
and Q2 will turn on only when |Vin|is greater than VBE. This leads tocrossover distortion.
Vout t
Q1 on
Q2 on
Vout
H. Chan; Mohawk College 44
Class AB Operation+VCC
RL
Q1
Q2
R1
R2
Vin
C1
C2
C3D1
D2
The push-pull circuit is biased slightly above cutoff to eliminate crossover distortion.
D1 and D2 have closely matched transconductance characteristics of the transistors to maintain a stable bias.
C3 eliminates need for dual-polarity supplies.
H. Chan; Mohawk College 45
Class AB Amplifier: DC Analysis
Pick R1 = R2, therefore VA=VCC/2
Assuming transconductance characteristics of diodes and transistors are identical, VCEQ1=VCEQ2=VCC/2
ICQ 0 (cutoff)
+VCC
Q1
Q2
R1
R2
D1
D2
A
VCEQ1
VCEQ2
H. Chan; Mohawk College 46
Class AB Amplifier: AC Analysis
For max. output, Q1 and Q2 are alternately driven from near cutoff to near saturation.
Vce of both Q1 and Q2 swings from VCEQ = VCC/2 to 0.
Ic swings from 0 to Ic(sat) = VCEQ/RL Iout(pk)
Input resistance, Rin = ac(re’+RL)
VCE
IC
Ic(sat)
Ic
Vce
VCEQ
ac load line
H. Chan; Mohawk College 47
Power & Efficiency: Class B Amplifier
Average output power, Pout = Vout(rms)Iout(rms)
For max. output power, Vout(rms)= 0.707VCEQ and Iout(rms) = 0.707Ic(sat); therefore, Pout(max)=0.5VCEQIc(sat) = 0.25 VCCIc(sat)
Since each transistor draws current for a half-cycle, dc input power, PDC = VCCIc(sat)/
Efficiency, max = Pout/PDC = 0.25 0.79
The efficiency for class AB is slightly less.
H. Chan; Mohawk College 48
Darlington Class AB Amplifier
+VCC
RL
Q1
Q2
R1
R2
Vin
C1
C2
C3D1D2
D3D4
Q3
Q4
In applications wherethe load resistance islow, Darlingtons areused to increase inputresistance presented todriving amplifier andavoid reduction in Av.4 diodes are requiredto match the 4 base-emitter junctions of thedarlington pairs.
H. Chan; Mohawk College 49
Class C Amplifier Characteristics
Biased below cutoff, i.e. it conducts less than 180o.
More efficient than class A, or push-pull class B and class AB.
Due to severe distortion of output waveform, class C amplifiers are limited to applications as tuned amplifiers at radio frequencies (RF).
H. Chan; Mohawk College 50
Basic Class C Operation
RB
RC
Vin
VBB
+VCC Vin
Ic
0
0
VBB+VBE
The transistor turns on whenVin > VBB+VBE
Vo
Duty cycle = tON / T
tON T
H. Chan; Mohawk College 51
Tuned Operation
RB
Vin
VBB
+VCC
C1 L
C2
Vout
Ic
0
The short pulse of Ic initiates and sustainsthe oscillation of the parallel resonantcircuit . The output sinewave has a pk-pkamplitude of approximately 2VCC.
H. Chan; Mohawk College 52
Power & Efficiency: Class C Amplifier
Using simplification where current is IC(sat) and voltage is VCE(sat) at turn on, and assuming the entire load line is used, then
PD(avg) = (tON/T)VCE(sat)IC(sat)
Max. output power for tuned operation,
Pout(max) = (0.707V2CC) / Rc = 0.5 V2
CC / Rc
Efficiency, = Pout / (Pout + PD(avg) )
When Pout >> PD(avg) , approaches 100%.
H. Chan; Mohawk College 53
Frequency Multiplier
Frequency “doubling” is obtained by tuning tank circuit to second harmonic of input frequency.
By tuning tank circuit to higher harmonics, further frequency multiplication factors are achieved.
Amplitude of each alternate peak drops due to energy loss in circuit.
Ic
0
Output of tank circuit tunedto 2nd harmonic frequency
Vout
H. Chan; Mohawk College 54
Junction Field-Effect Transistor
p n p
Drain
Gate
Source
D
S
G
n-channel
n p n
Drain
Gate
Source
D
S
G
p-channel
Symbol Symbol
H. Chan; Mohawk College 55
Basic Operation of JFET
n
D
G
S
VGG
Depletion region in n-channel increases its resistance.
Channel width and channel resistance can be controlled by varying VGG (or VGS), thereby controlling drain current, ID.
VDDp p
ID
JFET is always operatedwith VGG reverse-biased
H. Chan; Mohawk College 56
JFET Characteristics For VGS=0
VDD
RD
VGS
VDS
+
_+_
IDSS
0
B
A
CVGS = 0
VDS
Constant-currentregion
VP (pinch-off voltage)
ID
Bre
akdo
wn
ID
Between points A and B, ID VDS - ohmic region.IDSS - max. ID for a given JFET regardless of external circuit.
H. Chan; Mohawk College 57
Controlling ID With VGS
VDD
RD
VGG
IDSS
0
VGS = 0
VDSVP
ID
IDVGS= -1V
VGS= -2V
VGS= -3VVGS= -4V
VGS= VGS(off)VGS(off) = cutoff voltage (i.e. ID = 0)VGS(off) = -VP (where VP is measured at VGS = 0)
H. Chan; Mohawk College 58
JFET Transfer Characteristics
IDSS
VGS(off) 0
ID
VGS
2
)(
1
offGS
GSDSSD V
VII
Forward transconductance:
)(
0 1offGS
GSm
GS
Dmfs V
Vg
V
Igory
||
2
)(0
offGS
DSSm V
Ig is value of gm at VGS = 0
Note: gm is max. at VGS = 0 and min. at VGS(off)
Shockley’s equation:
H. Chan; Mohawk College 59
Other JFET Parameters
JFET’s input resistance, RIN = |VGS/IGSS| is very high since its gate-source junction is reverse-biased.
Input capacitance, Ciss is typically a few pF.
Output conducutance, gos, or output admittance, yos, is typically 10 mS, and is the inverse of drain-to-source resistance, r’ds = VDS/ID
H. Chan; Mohawk College 60
JFET With Self-Biasing
Large RG is required to prevent shorting of input signal to ground and to prevent loading on the driving stage.
VGS = -IDRS
VD = VDD - IDRD
VDS = VD - VS
= VDD - ID(RD+RS)
+VDD
RD
RSIS
RG
VG = 0
+
_
H. Chan; Mohawk College 61
Setting Q-Point of JFET
First, determine ID for a desired value of VGS either by using transfer characteristic curve or Shockley’s equation.
Then calculate RS = |VGS/ID|
For midpoint bias (i.e. ID = 0.5 IDSS), make VGSVGS(off)/3.4
To set VD = 0.5 VDD , pick RD = VDD/(2ID)
H. Chan; Mohawk College 62
Graphical Analysis of Self-Biased JFET
First obtain transfer characteristic curve from data sheet or plot using Shockley’s equation.
Draw dc load line by starting with VGS = 0 when ID = 0, and then VGS = -IDSSRS when ID = IDSS.
ID
IDSS
0VGS
Q
VGS(off)
+VDD
RD
RS
RG
_
H. Chan; Mohawk College 63
Voltage-Divider Bias
+VDD
RD
RS
R2
R1
VG
VS
ID
IS
To keep the gate-source junctionreverse-biased, VS > VG
VS = IDRS
DDG VRR
RV
21
2
VS = VG - VGS
S
GSG
S
SD R
VV
R
VI
H. Chan; Mohawk College 64
Graphical Analysis of Voltage-Divider Biased JFET
VGS
ID
0
Q VG
RS
IDSS
VGS(off)
For ID = 0, VS = IDRS = 0, andVGS = VG - VS = VG
For VGS = 0,
S
G
S
GSGD R
V
R
VVI
VG
Draw the dc load line byjoining the two points andextend it to intersect thecurve to get the Q-point.
H. Chan; Mohawk College 65
Q-Point Stability
The transfer characteristic of a JFET can differ considerably from one device to another of the same type.
This can cause a great variation of the Q-point, and consequently, ID and VGS.
With voltage-divider bias, the dependency of ID on the range of Q-points is reduced (i.e. more stable) because the slope is less than for self-bias, although VGS varies quite a bit for both circuits.
H. Chan; Mohawk College 66
Metal Oxide Semiconductor FET
The MOSFET differs from the JFET in that it has no pn junction structure.
The gate of the MOSFET is insulated from the channel by a silicon dioxide layer.
The two basic types of MOSFETs are depletion (D) and enhancement (E).
Because of the insulated gate, these devices are sometimes called IGFETs.
H. Chan; Mohawk College 67
Depletion MOSFETDrain
Gate
SourceChannel
SiO2
p
n
n
Basic structure ofn-channel D-MOSFET
n-channel D-MOSFET is usually operated in the depletion mode with VGS < 0 and in the enhancement mode with VGS > 0.
p-channel D-MOSFET uses the opposite voltage polarity
G
D
SSymbol
H. Chan; Mohawk College 68
Depletion/Enhancement MOSFET
Depletion Mode: negative gate voltage applied to n channel depletes channel of electrons, thus increasing its resistivity. At VGS(off), ID = 0, just like n-channel JFET.
Enhancement mode: when VGS > 0, electrons are attracted into channel, thus increasing (enhancing) the channel conductivity.
H. Chan; Mohawk College 69
Enhancement MOSFETDrain
Gate
Source
SiO2
n
p
p
G
D
SSymbol
InducedChannel
n
p
p
E-MOSFET construction and operation ( p-channel)
RD
VDD
VGG
H. Chan; Mohawk College 70
Notes On E-MOSFET
The E-MOSFET operates only in the enhancement mode.
For a p-channel device, a negative gate voltage above a threshold value induces a channel by creating a layer of positive charges in the substrate region adjacent to the SiO2 layer.
Channel conductivity increases with VGS.
H. Chan; Mohawk College 71
VMOS & TMOS Power MOSFETs
S G S
D
n+
n+
n+
n-
Channel Channel D
n+
S G
pp p pn+ n+
Channel
n-
VMOS (V-groove MOSFET) creates a short and wideinduced channel to allow for higher currents and greaterpower dissipation. Frequency response is also improved.
TMOS is similar to VMOS except it is easier tomanufacture
H. Chan; Mohawk College 72
D-MOSFET Transfer Characteristic
VGS
ID
IDSS
VGS(off)0
VGS
0
ID
IDSS
VGS(off)
n channel p channel
D-MOSFET can operate with either +ve or -ve gate voltage.Shockley’s equation for the JFET curve also applies to theD-MOSFET curve.
H. Chan; Mohawk College 73
E-MOSFET Transfer Characteristic
VGS
0
ID
VGS(th)
p channel
VGS
ID
VGS(th)0
n channel
E-MOSFET uses only channel enhancement. Ideally ID = 0until VGS > VGS(th). Transfer equation: ID = K(VGS - VGS(th))2,where K depends on the particular MOSFET.
H. Chan; Mohawk College 74
MOSFET Handling Precautions
All MOS devices are subject to damage from electrostatic discharge (ESD).
To avoid damage from ESD:– ship/store MOS devices in conductive foam– ground all instruments and metal benches– ground assembler’s/handler’s body via resistor– never remove MOS device from live circuit– do not apply signals while dc supply is off
H. Chan; Mohawk College 75
D-MOSFET With Zero-Bias
+VDD
RG
RD
IDVG = 0
Since VGS = 0, ID = IDSS.
VDS = VDD - IDSSRD
The value of RG is chosen arbitrarily large to prevent loading of the previous stage and isolate any ac input signal from ground.
H. Chan; Mohawk College 76
+VDD
E- or D-MOSFET Bias
R1
Rs
RD RG RD
+VDD
DDGS VRR
RV
21
2
Voltage-divider bias
VDS = VDD - IDRD
ID = K(VGS - VGS(th))2
Drain-feedback bias
Since IG 0, VGS = VDS
H. Chan; Mohawk College 77
Small-Signal JFET Amplifier
C1
RG
RD
RS
C3
RLC2
Vin
Vout
+VDD
VGSQ
VDSQ
Common-source amplifier Voltage waveforms
H. Chan; Mohawk College 78
JFET Transfer Characteristic Curve
VGS
VGS(off)
ID
IDSS
Q IDQ
Id
VGSQ
Vgs
As Vgs swings from its Q-point to a more -ve value, Id decreases from its Q-point.
When Vgs swings to a less -ve value, Id increases.
Similar diagrams can be drawn for MOSFETs.
Signal operation with n-channel JFET
H. Chan; Mohawk College 79
JFET Drain Characteristic Curve
IDQ
ID
VDSQ
0 VDS
QId
Vds
Vgs VGSQ
n-channeloperation
This is an alternativeview of the JFETamplifier operationshowing the varia-tion of Id with thecorrespondingchange in Vgs & Vds.Note that the CSamplifier is equi-valent to the CEamplifier.
H. Chan; Mohawk College 80
FET Simplified Equivalent Circuit
Transconductance is defined as gm = ID/VGS (siemens)
In ac quantities, gm = Id/Vgs
Rearranging the terms, Id = gmVgs
Vgs
S
D
G gmVgs
r’gs and r’ds are assumedto be very large
H. Chan; Mohawk College 81
DC Analysis of Common-Source Amplifier
+VDD
RD
RS
RG
DC Equivalentcircuit
If the circuit is biased at midpoint,ID = IDSS/2
Otherwise, solve for ID eithergraphically or by finding the rootof the quadratic equation:
2
)( ||1
offGS
SDDSSD V
RIII
where VGS has been substituted by IDRS.
Then, VDS = VD-VS = VDD-ID(RD+RS)
H. Chan; Mohawk College 82
AC Analysis of CS Amplifier
Since Rin is very high, Vgs = Vin
Av = Vout/Vin
= -IdRd/Vgs = -gmRd where, Rd = RD//RL
Vout = AvVin
= -gmRdVin
Rd
RG
Vin
VgsgmVgs
G
AC equivalent circuit
Vout
Id
GSS
GSGin I
VRR //
H. Chan; Mohawk College 83
Common-Source D-MOSFET Amplifier
+VDD
RG
RD
Vin
C1C2
RL
DC analysis is easier than for a JFET because ID = IDSS at VGS = 0
Once ID is known, VD= VDD - IDRD
AC analysis is the same as for JFET
Thus, Av = -gmRd
H. Chan; Mohawk College 84
Common-Source E-MOSFET Amplifier
DC analysis: ID = K(VGS -VGS(th))2
VDS = VDD - IDRD
AC analysis is same as JFET and D-MOSFET
i.e., Av = -gmRd
Rin = R1//R2//RINIgate)
RIN(gate) = VGS/IGSS
R1
Rs
RD
+VDD
Vin
C1
C2
RL
DDGS VRR
RV
21
2
H. Chan; Mohawk College 85
Common-Drain Amplifier
Rin = RG//RIN(gate)
Vout = IdRs = gmVgsRs, where Rs= RS//RL
Vin = Vgs + IdRs = Vgs+ gmVgsRs = Vgs(1+gmRs)
+VDD
RG RS
Vin
C1
C2
RL
sm
sm
in
outv Rg
Rg
V
VA
1CD amplifier is comparable to the CC amplifier.
H. Chan; Mohawk College 86
Common-Gate Amplifier
Rin = Rin(source)//RS where Rin(source) = 1/gm
Vin = Vgs
Vout = IdRd = gmVgsRd where Rd = RD//RL
+VDD
RD
RS
Vin
C1
C2
RL
dmgs
dgsm
in
outv Rg
V
RVg
V
VA
CG amplifier is comparableto the CB amplifier
H. Chan; Mohawk College 87
Amplifier Frequency Response
In the previous discussion of amplifiers, XC of the coupling and bypass capacitors was assumed to be 0 at the signal frequency.
Also, the internal transistor capacitances were assumed to be negligible.
These capacitances, however, do affect the gain and phase shift of the amplifier over a specified range of input signal frequencies.
H. Chan; Mohawk College 88
Effect At Low Frequency
Since XC = 1/(2fC), when f is low (e.g. <10 Hz), XC >>0. The voltage drop across the input and output coupling capacitors become significant, leading to a drop in Av. Also, a phase shift is introduced because the coupling capacitor form a lead (or RC) circuit at the input and the output.
At low f, the significant XC across RE (or RS) makes the emitter (or source) no longer at ground potential, again reducing Av.
H. Chan; Mohawk College 89
Effect At High Frequency
At high f, Cbe causes a drop in signal voltage due to the voltage divider effect with RS.
At high f, Cbc allows negative feedback from output to cancel the input partially.
Av drops in each case.
Vs
Rs
RcCbe
Cbc
Cbc and Cbe are internaljunction capacitanceswhich are usually a few pF.
H. Chan; Mohawk College 90
General Frequency Response Curve
Av (dB) = 20 log Av
Cutoff, critical, or corner frequency is the frequency when Av or Ap drops by 3 dB. This corresponds to 0.707Av(max) or 0.5 Ap(max) (half-power point) respectively.
f
Av (dB)
fcl fcu
Midrangegain
3 dB
fcl = lower cutoff frequency fcu = upper cutoff frequencyGain is max. at midrange,often referenced as 0 dB.
H. Chan; Mohawk College 91
Input RC Circuit At Low Frequency
Vin
C1
Rin
Transistorbase
Rin = R1//R2//Rin(base)
Critical frequency for this circuit is:
12
1
CRf
inc
If Rs of input source is included:
1)(2
1
CRRf
insc
VR(in) leads Vin by:
in
C
R
X 11tan
Note: At fc, XC1 = Rin, = 45o.
ffc
90o
45o
0o
H. Chan; Mohawk College 92
Output RC Circuit At Low Frequency+VCC
C3
RL
Critical frequency for the output RC circuit:
3)(2
1
CRRf
LCc
The phase shift at the output:
LC
C
RR
X 31tan
ffc0.1fc
Av (dB)
-3
-20
Drop in Av for each RCcircuit is 20 dB/decadeor 6 dB/octave
The effect of the output RC circuit on Av
is similar to that of the input RC circuit.
0
RC
H. Chan; Mohawk College 93
Bypass RC Circuit At Low Frequency
C2RE
RC
+VCCAt low frequency, the impedance at theemitter is Ze = RE//XC2, and Av becomes:
ee
cv Zr
RA
'
The critical frequency is:
2]//)/'[(2
1
CRRrf
Eacthec
where Rth = R1//R2//Rs is the equivalentThevenin resistance looking from thebase toward the input source.
H. Chan; Mohawk College 94
Total Low-Frequency Response
fc1, fc2, and fc3 are the critical frequencies of the bypass, output and input RC circuits (not necessarily in that order).
The RC circuit giving fc3 is known as the dominant RC circuit.
0fc1 fc2 fc3 f
-20
-40
-60-80
-100
-120
-20 dB/dec
-40 dB/dec
-60 dB/dec
Bode plot of amplifier’slow-frequency response
H. Chan; Mohawk College 95
Direct-Coupled Amplifiers
Since direct-coupled amplifiers don’t have coupling or bypass capacitors, their frequency response can extend down to dc.
Because of this, they are commonly used in linear ICs.
Although their gain is not as high as amplifiers with emitter bypass, Av stays constant at lower frequencies.
H. Chan; Mohawk College 96
Miller’s Theorem
Miller’s theorem can be used to simplify the analysis of inverting amplifiers at high frequencies.
C in the diagram can represent either Cbc of a BJT or Cgd of a FET.
Av
Av
C
In Out
C(|Av|+1)
||
1||
v
v
A
AC
Equivalentto
H. Chan; Mohawk College 97
Input RC Circuit At High Frequency
Vs
Rs
Cbe
R1//R2
Critical frequency:
tottotc CR
f2
1
where Rtot = Rs//R1//R2//acr’e ; and Ctot = Cbe+Cin(Miller)
Phase shift:
)(
1tantotC
tot
X
R
Cin(Miller)
= Cbc(|Av|+1)
H. Chan; Mohawk College 98
Output RC Circuit At High Frequency
RcCout(Miller)
||
)1||)(
v
vbcMillerout A
ACC
Critical frequency:
)(2
1
Milleroutcc CR
f
Phase shift:
)(
1tanMilleroutC
c
X
R
If |Av| 10, Cout(Miller) Cbc
H. Chan; Mohawk College 99
Total Amplifier Frequency ResponseAv (dB)
0
Av(mid)
ffc1 fc2 fc3 fc4 fc5
fc3 and fc4 are the two dominant critical frequencies whereAv is 3 dB below its midrange value. fc3 is the lower cutofffrequency, fcl, and fc4 is the upper cutoff frequency, fcu.
Bandwidth= fcu - fcl
H. Chan; Mohawk College 100
Gain-Bandwidth Product
For a given amplifier, its gain-bandwidth product is a constant when the roll-off is -20 dB/dec.
If fcu >> fcl, then BW = fcu - fcl fcu.
Unity-gain frequency, fT = Av(mid)BW = Av(mid)fcu .
Av(mid)
fcu
Av
ffT
fT is the frequency at whichAv = 1 (unity gain) or 0 dB.
BW
H. Chan; Mohawk College 101
FET Amplifier At Low Frequency
+VDD
RG
RD
Vin
C1C2
RL
Input RC circuit:12
1
CRf
inc
where Rin = RG // Rin(gaate)
in
C
R
X 11tan
Rin(gate) = |VGS / IDSS|
Output RC circuit:
2)(2
1
CRRf
LDc
LD
C
RR
X 21tan
H. Chan; Mohawk College 102
FET Amplifier At High Frequency
The high frequency analysis of an FET amplifier is very similar to that of a BJT amplifier. The basic differences are the specs of Cgd (= Crss), and the determination of Rin.
Input RC circuit:
totC
s
totsc X
R
CRf 1tan;
2
1
where Ctot = Cgs + Cin(Miller); Cin(Miller) = Cgd(|Av|+ 1)
Output RC circuit:
||
1||;tan;
2
1)(
1
)( )( v
vgdMillerout
C
d
Milleroutdc A
ACC
X
R
CRf
Millerout
H. Chan; Mohawk College 103
Multistage Amplifiers
If all the stages have the same fcl and fcu, then:
12'
/1
n
clcl
ff and 12' /1 n
cucu ff
For an amplifier formed by cascading several stages, theoverall bandwidth is: BWtot = f’cu - f’cl
If each stage has a different fcl and a different fcu, then f’cl
is determined by the stage with the highest fcl , and f’cu isdetermined by the stage with the lowest fcu.
H. Chan; Mohawk College 104
Amplitude vs Frequency Measurement
Set sinewave frequency to mid-range and Vout at a convenient reference value (e.g. 1V). Decrease frequency until Vout = 0.707V to get fcl. Increase frequency until Vout = 0.707V to get fcu. Then amplifier’s BW = fcu - fcl. Make sure Vin is constant throughout the measurements.
Functiongenerator
AvDual-channeloscilloscope
Test setup
Vin Vout
Procedure:
H. Chan; Mohawk College 105
Step Response Measurement
Using previous test setup but replacing sinewave with a pulse input, measure the rise time (tr) and fall time (tf) of the output.
fcu = 0.35/tr
fcl = 0.35/tf
10%
90%
tr
90%
10%tf
Input
Output
Output
Input
H. Chan; Mohawk College 106
Review of Zener Diode Regulator
If the zener was ideal, Vout would remain constant regardless of changes in Vin or RL.
However, in practice, the zener is limited by its operating parameters IZK, IZM, and ZZ.
R
RLVZ
Vin
Vout
H. Chan; Mohawk College 107
Line Regulation
%100xV
VregulationLine
in
out
VV
xV
VregulationLine
outin
out /%100
Line regulation is a measure of the effectiveness ofa voltage regulator to maintain the output dc voltageconstant despite changes in the supply voltage.
OR
H. Chan; Mohawk College 108
Load Regulation
Load regulation is a measure of the ability of aregulator to maintain a constant dc output despite changes in the load current.
%100xV
VVregulationLoad
FL
FLNL
mAI
xV
VVregulationLoad
FLFL
FLNL /%100
OR
H. Chan; Mohawk College 109
Regulator Block Diagram
The essential elements in a series voltage regulator are shownin the block diagram below:
Controlelement
Referencevoltage
Errordetector
Sensingcircuit
VIN VOUT
H. Chan; Mohawk College 110
Transistor 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 forthe circuit are:
R
VVI
R
VV
R
VI Zin
RL
BEZ
L
LL
;
IL = hFEIB; IZT = IR - IB
Q1
VinVo
RRL
VZ
VL
H. Chan; Mohawk College 111
Series Variable Voltage Regulator
Q1
VinVo
R1
R2
VZ
R3
R4Q2
Q2 detects and amplifies the difference between VZ and sample voltages and adjusts the conduction of Q1 so as to oppose any changes at the output.
R5
H. Chan; Mohawk College 112
Short-Circuit Protection
Q1
VinVo
R1
R2
VZ
R4
R5
Q2
R6
Q3
R3
The current limitingcircuit consists of transistor Q3, and resistor R3. When IL
< IL(max), Q3 is off, but ifit exceeds IL(max), Q3 turns on,drawing current.
away from the base of Q1
making Q1 conducts lesswhich in turn limits theload current to IL(max).3
(max)
7.0
RIL
H. Chan; Mohawk College 113
Foldback Limiting
Q1
Vin
Vo
R1
R3
VZ
R4
R2
Q2
Vo
ILISC IL(max)
Vo vs IL graph
Note that the short-circuitcurrent, ISC, is < IL(max) toprevent overheating of Q1.
Foldback action startswhen VR2 - VR3 0.7
H. Chan; Mohawk College 114
Transistor 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 = Vin - IRsRS.
S
BEZinRs
L
BEZ
L
LL R
VVVI
R
VV
R
VI
)(;
IE = IRs - IL = hFEIZT
Vin
RS
VZRLVL
+
_
IL+
_
H. Chan; Mohawk College 115
Improved Shunt Regulator
Q1
Vin
VoRS
VZ
Q2
R1
Vo = VZ + VBE1 + VBE2
If Vo tries to decrease,Q2 and Q1 become lessconductive. Since lessshunt current goesthrough Q1, morecurrent will go to theload, thus raising Vo.Q2 provides a larger IB1
than the previous circuitso this regulator canhandle a larger IL
H. Chan; Mohawk College 116
Silicon-Controlled Rectifier
SCR is a four-layer pnpn device. Has 3 terminals: anode, cathode, and gate. In off state, it has a very high resistance. In on state, there is a small on (forward)
resistance. Applications: motor controls, time-delay
circuits, heater controls, phase controls, etc.
H. Chan; Mohawk College 117
SCR
p
pn
n
Anode (A)
Cathode (K)
BasicConstruction
Gate (G)G
A
K
SchematicSymbol
A
K
Q1
Q2
EquivalentCircuit
G
H. Chan; Mohawk College 118
Turning The SCR On
Q1
Q2IG
+V
RA
IB2
IB1
IA
IK
VBR(F2) VBR(F1) VBR(F0)
VF
IA
IG0=0IG1>IG0IG2>IG1
IH0
IH1
IH2
SCR characteristic curvesfor different IG Values
K
A
H. Chan; Mohawk College 119
Notes on SCR Turn-On
The positive pulse of current at the gate turns on Q2 providing a path for IB1.
Q1 then turns on providing more base current for Q2 even after the trigger is removed.
Thus, the device stays on (latches). The gate current, IG , controls the value of the
forward-breakover voltage: VBR(F) decreases as IG is increased.
H. Chan; Mohawk College 120
Half-Wave Power Control
RL
A
B
Vin
R1
R2
D1
f
IL
)cos1(2)( f
PAVGL
II
where f = firing angleMax. f = 90o
IP
IL
H. Chan; Mohawk College 121
The Diac and The Triac
Both the diac and the triac are types of thyristors that can conduct current in both directions (bilateral). They are four-layer devices.
The diac has two terminals, while the triac has a third terminal (gate).
The diac is similar to having two parallel Shockley diodes turned in opposite directions.
The triac is similar to having two parallel SCRs turned in opposite directions with a common gate.
H. Chan; Mohawk College 122
The Diac
A1
A2
npnpn
A1
A2
IF
IH
VBR(F)
VFVR
-VBR(R)
-IH
IR
BasicConstruction
SymbolCharacteristic Curve
H. Chan; Mohawk College 123
Diac Equivalent Circuit
Q1
Q2
Q3
Q4
A1
A2
R
A1
A2
Vin
Current can flow inboth directions
H. Chan; Mohawk College 124
The Triac
A1
A2
Q1
Q2
Q3
Q4
A1
A2
Gate
A1
A2
G
BasicConstruction
Symbol
Equivalent circuit
G
np
pn
n
n
n
H. Chan; Mohawk College 125
Triac Phase-Control Circuit
A1
A2
GVin
D1
D2
R1
RL
Trigger Point(adjusted by R1)
Trigger Point
Voltage Waveformacross RL
H. Chan; Mohawk College 126
Diac Controlling Triac Triggering
The rated value of the gate trigger voltage for a triac is often too low for many applications.
The above circuit can used to raise the triggering level for the triac.
A1
A2
GA