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EL 6033 類比濾波器 ( 一 ). Analog Filter (I). Instructor : Po-Yu Kuo 教師 : 郭柏佑. Lecture4: Voltage References (1). Outline. Introduction Performance Requirements Zener Diode Voltage Reference Bandgap Voltage References Bandgap Voltage References Implemented in CMOS technologies. Introduction. - PowerPoint PPT Presentation
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Instructor : Po-Yu Kuo
教師:郭柏佑
Lecture4: Voltage References (1)
EL 6033類比濾波器 ( 一 )Analog Filter (I)
2
Outline
Introduction Performance Requirements Zener Diode Voltage Reference Bandgap Voltage References Bandgap Voltage References Implemented in
CMOS technologies
3
Introduction Here, we will learn to build a reference voltage to provide a stable and
accurate supply voltage. The voltage reference is an electronic circuit to provide an accurate and stable DC voltage that is very insensitive to the change in supply voltage and temperature
How accurate is a voltage reference? E.g. Weston cell is an electrochemical device which provides a reproducible voltage of 1.018636 V at 20°C with a small temperature coefficient of 40ppm/°C. For integrated circuit implementation, active solid-state devices can achieve a tempco of 1-4 ppm/°C if appropriate compensation technique is employed
Note To minimize error due to self-heating, voltage reference usually
operates with modest current (e.g. < 1mA) Tempco = temperature coefficient, usually expressed in ppm/°C (parts
per million/°C or 10-6/°C
4
Outline
Introduction Performance Requirements Zener Diode Voltage Reference Bandgap Voltage References Bandgap Voltage References Implemented in
CMOS technologies
5
Performance Parameters (1) The primary requirements of a voltage reference are accuracy and stability.
Some important parameters are:
Load Regulation = ΔVo/ΔIo (usually expressed in mV/mA or mV/A) or
Load Regulation = 100(ΔVo/ΔIo) (in %/mA or %/A)
Line Regulation = ΔVo/ΔVin (usually expressed in mV/V) or
Line Regulation = 100(ΔVo/ΔVin) (in %/V)
Power Supply Rejection Ratio (PSRR) is a measure of the ripple in the reference voltage due to the ripples in the supply voltage
)dBin(V
V20logPSRR
ro
ri10
6
Performance Parameters (2) Example of line regulation / supply-voltage dependence at VDD = 3.3, 4.15
and 5V (step size of 0.85V)
Line regulation at T 27°C is
VmVVVVVVV DDrefDDref
/7.143.35
176.1201.1
3.35
)3.3()5(
7
Performance Parameters (3)
The maximum (Vref(max)) and minimum (Vref(min)) reference voltages are1.1761V and 1.1731V, respectively. The reference voltage at
T = 27°C (Vref) is 1.1761V. The tempco in ppm/°C can be found by
CppmTTV
VV
ref
refref
/5.25)0100(
10
1761.1
1731.11761.1
)(
10 6
minmax
6(min)(max)
8
Outline
Introduction Performance Requirements Zener Diode Voltage Reference Bandgap Voltage References Bandgap Voltage References Implemented in
CMOS technologies
9
Review on Zener Diode Voltage Reference The Zener can be considered as a voltage reference. Since the breakdown
voltage due to Zener breakdown mechanism has a negative temperature coefficient, and the breakdown voltage due to the avalanche multiplication has a positive coefficient, the reference voltage is somewhat independent of the change of temperature
Lzs
zsZK
zs
sin
zs
zo I
rR
rRV
rR
RV
rR
rV
zs
zs
L
o
zs
z
in
o
rR
rR
I
V
rR
r
V
V
RegulationLoad
RegulationLine
10
Improved Zener Diode Reference (1) In the case of Zener diode, the output voltage Vo heavily depends on the
load current IL, which in most cases are not good. It would be better if we could shield the Vz from the influence of the load. This can be done with the help of an op amp as shown below. This method refers to self regulation which shifts the burden of line and load regulations from the diode to the op amp.
11
Improved Zener Diode Reference (2) By observation,
The output voltage is also adjustable via R2
The load current IL is supplied from the opamp such that the current flowing through the Zener diode is almost constant at
Since the diode current is independent of the load current, the diode voltage is insensitive to the load
R3 can be raised to avoid unnecessary power wastage and self-heating effects
VK
KV
R
RV
VRR
RV
zo
zo
102.6)39
241()1(
1
2
21
1
mAkR
VVI ZoZ 15.1
3.3
2.60.10
3
12
Load Regulation (1) The load regulation is directly related to the output
impedance. To find Ro, we suppress the input source Vz and apply the test-voltage technique. By voltage divider formula:
Summing currents at the output node
Eliminating vN and solving for the Ro=v/I, we obtain
vRrR
rRv
in
inN
21
1
//
//
02
o
NN
r
vAv
R
vvi
21
1
2121
where
1)]//1/()//[(1
RR
Rb
Ab
r
rRRRrrRrA
rR o
ininoo
oo
13
Load Regulation (2) Typically rin is in the MΩ range or greater, R1
and R2 are in kΩ range and ro is on the order of 102 Ω. The terms ro/R1, ro/rin, and R2/rin can thus be ignored to yield Ro≈ro/(1+Ab)
The load regulation Ro≈ro/(1+Ab) which is much smaller than the Zener diode voltage reference without opamp
Since ro and A are frequency dependent, so are the load regulation. In general, load regulation tends to degrade with frequency
14
Thermal Stability (1) Thermal stability is one of the most demanding performance requirement of voltage references due to
the fact that semiconductor components are strongly influenced by temperature The forward-bias voltage VD and current ID of a silicon pn junction, which forms the basis of the diodes
and BJTs, are related as VD=VTln(ID/IS), where VT is the thermal voltage and IS is the saturation current. Their expressions are
where
k=1.381x10-23 is Boltzmann’s constant
q=1.602x10-23 C is the electron charge
T is the absolute temperature
B is a proportionality constant
VG0 = 1.205V is the bandgap voltage for silicon
)/exp(and/ 03
TGST VVBTIqkTV
15
Thermal Stability (2) The temperature coefficient (TC) of the thermal voltage:
Assume VD=650mV at 25°C, we get TC(VD) ≈ -2.1mV/°C.
TC(VT) have a positive tempco and TC(VD) have a negative tempco. Hence, these two equations form the basis of two common approaches to thermal stabilization, namely, thermal compensated Zener diode references and bandgap references
CmV/0862.0)(
q
k
T
VVTC TT
q
k
T
VV
T
V
VT
VT
V
T
I
I
VI
I
T
VVTC DGT
G
TDS
D
TS
DTD
3ln3ln
ln)( 0
0
16
Thermally Compensated Zener Diode Reference Idea of thermally compensated Zener diode is to connect a forward-biased
diode in series with a Zener diode having an equal but opposing tempco as shown below
Since TC(Vz) is a function of Vz and Iz. We can fine tune Iz to drive the tempco of the composite device to zero. In this case, a 7.5mA is used to give a reference voltage of Vz = 5.5+0.7 = 6.2V.
17
Outline
Introduction Performance Requirements Zener Diode Voltage Reference Bandgap Voltage References Bandgap Voltage References Implemented in
CMOS technologies
18
Bandgap Voltage Reference (1)
Since the best breakdown voltages of the Zener diode references range from 6 to 7V, they usually require supply voltages on the order of 10V to operate. This can be a drawback in systems powered from lower power supplies, such as 5V. This limitation is overcome by bandgap voltage references, so called because their output is determined primarily by the bandgap voltage of silicon VG0 = 1.205V
19
Bandgap Voltage Reference (2)
Addition of the voltage drop VBE of a base-emitter junction, which has a negative tempco, to a voltage proportional to the thermal voltage VT, which has a positive tempco, to generate a reference voltage, which is independent of temperature
20
Basic Concept
As TC(VBE) ≈ -2.1mV/°C and TC(VT) = 0.086mV/°C, then zero tempco is achieved at a particular temperature (e.g. T=300K):
If for a particular transistor with certain bias current such that
VBE =650mV, then
Note that VT = kT/q T, i.e. VT is proportional to absolute temperature. We call VT a Proportional To Absolute Temperature voltage, or in short, PTAT voltage
4.24086.0
1.2
)(
)(
0)()()(i.e. 300
T
BE
TBEKBG
TBEBG
VTC
VTCK
VTCKVTCVTC
KVVV
V28.1)0259.0(4.2465.0 TBEBG KVVV
21
Bandgap Voltage Reference Circuit (1)
From the circuit shown in the right hand side, the emitter area of Q1 is n times as large as the emitter area of Q2, hence Is1/Is2 = n
By op amp action with identical collector resistances, the collector currents are also identical, i.e. IC1 = IC2. Ignore the base currents, we have
KVT=R4(IC1+IC2)=2R4I
Combine two equations give
)ln(/
/ln
11
22123 nV
II
IIVVVIR T
SC
SCTBEBE
TBETBEBG
T
T
VnR
RVKVVV
nR
Rn
R
V
V
RK
)ln(2
)ln(2)ln(2
3
422
3
4
3
4
22
Bandgap Voltage Reference Circuit (2)
From the previous discussion, for a zero tempco voltage reference VBG, K≈24, with n = 4, then
Note that I = VTln(n)/R3 VT, I is a PTAT current
8.84ln2
4.24
)ln(23
4 n
K
R
R
23
Brokaw Cell
Brokaw cell is commonly used bandgap-cell realization circuit and is shown in the figure
The function of op amp is replaced by Q3,Q4 and Q5. Q3 and Q4 form a current mirror to enforce the collector currents of Q1 and Q2 are identical
The emitter follower Q5 raises the reference voltage to Vref = (1+R1/R2)VBG
24
Stability of a Bandgap Reference
In a bandgap reference, there exists 2 feedback loops, 1 positive loop and 1negative loop
For the negative loop (the outer loop),
For the positive loop (the inner loop),
For stability, we must have a negative loop gain magnitude > positive loop gain magnitude.
)(/1
/1GainLoopNegative
121
12 sAgRR
gR
m
m
)(/1
/1GainLoopPostive
11
1 sAgR
g
m
m
25
Stability of Simple Brokaw Cell (1)
If we neglect R3, then clearly Q1 and Q2 form a differential pair with positive and negative terminals tied together
Above is the way to break the loop for measuring loop gain. The circuit should have a DC closed loop and AC open loop. The DC closed loop is for biasing and the AC loop is to measure loop gain
26
Stability of Simple Brokaw Cell (2)
With the presence of R3, the positive loop looks like an amplifier with degenerated emitter => the gain is smaller than that with R3.Therefore, negative loop gain magnitude > positive loop gain magnitude, i.e. stability requirement is satisfied
Cc is the compensation capacitor. Here, dominant pole compensation is employed