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advantages of super source follower
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PRESENTATION ON
DESIGN OF SUPER SOURCE FOLLOWER
CONTENTS
1. Introduction2. Conventional Voltage Follower3. Flipped Voltage Follower4. Super Source Follower5.Quasi Floating Gate Structure6.Class AB Super Source Follower7. Simulation Results8.Conclusion
1. INTRODUCTION
Power consumption in digital blocks is highly dependent on the supply voltage (P ̴ VDD2). This forces to decrease the supply voltage in mixed-signals ASIC’s.
Downscaling of technology forces VDD to decrease but VT does not scale down in the same way. This limits the output voltage swing.
New design techniques and new circuit structures are required for low voltage, low power design .
1. continued……….Parameters to be considered-
2. CONVENTIONAL VOLTAGE FOLLOWER
Current through transistor M1 depends on output current. Therefore, VGS of M1 is signal dependent and the produced output is distorted.
Output resistance ,
• gm is not very high.• Rout is not very low. • Not effective in driving low resistive loads.
2. continued…..HOW TO REDUCE THE OUTPUT RESISTANCE-
• Increase the bias current or W/L ratio to increase gm . This increase area and power
dissipation .
• Introduce shunt negative feedback in a loop
3. FLIPPED VOLTAGE FOLLOWER
• M2 is arranged in a negative feedback loop, thus reducing output resistance.
• Current in transistor M1 now is constant, improving linearity.
• Can source large amount of current but its sinking capabilities is very poor because of the biasing current source.
• Input and output voltage ranges are small.
3. continued…..• O/p Resistance:
-lower than conventional source follower.
• Valid input range :-
decreases with threshold voltage.
Not suitable for deep submicron technologies.
4. SUPER SOURCE FOLLOWER
• To overcome the limitation in flipped voltage follower of low input and output voltage swings.
• Input voltage range is VDD−VIB−|VDS1sat|−VGS2−VSS
Trade off is between VDD and maximum output voltage swing.
• Output Resistance :
• lower than conventional source follower.
1. Continued …..LIMITATIONS OF CLASS A SSF
• Limited current driving capability as the maximum load current is limited by the bias current.
• Because of limited current sourcing capability , slew rate is limited.
HOW TO INCREASE THE SLEW RATE
• Increase the Bias current.It increases the power consumption and supply
voltage requirements.• Use Class AB version of SSF.
5.QUASI FLOATING GATE STRUCTURE
6.CLASS AB SUPER SOURCE FOLLOWER
DEVELOPMENT
(a) (b) (c)
6. continued…….
• A floating capacitor Cbat is placed between the gates of M4 and M2,
• A large-valued resistive device Rlarge between the gate of M4 and the node that sets the dc bias to VB.
• QFG technique has no impact on static performance.
• Improves dynamic performance .
7. SIMULATION RESULTS Simulator : Mentor Graphic’s Simulation Tool.CMOS process 180nm was used for the MOS
transistors. Vtn = 0.37 VVtp = -0.49 V= =
High swing CASCODE Current sources were employed
VDD = 1.5 VVss = -1.5 VA resistive load of 50K and = 50pF (for Slew Rate
calculations )was taken in all the simulations so as to obtain a fair comparison between the Source Follower, Flipped Voltage Follower ,Class A and Class AB Super Source Follower.
7.1 Comparison of Linear Range
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
(), Vin (V)
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
Vout
(V)
v(N_4)
v(N_1)
vout
vin
Conventional source follower
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
Vin (V)
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Volta
ge (V
)
v(N_2)
v(N_3)
Vout
Vin
Flipped Voltage Follower
• Linear range : That range of input in which slope of the transfer characteristics (i.e. the incremental gain) is independent of the signal level.
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
Vin (V)
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
Vout
(V)
v(N_3)
v(N_4)
Vin
Vout
super source follower
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
Vin (V)
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Vout
(V)
v(N_2)
v(N_11)
Class AB super source follower
7.1 Continued …..
S.No
Follower Configuration
Output Voltage(in Volts)
1. Conventional Source Follower
0.22
2. Flipped voltage follower
0.71
3. Class A Super Source Follower
0.81
4. Class AB Super Source Follower
0.84
7.1 THD Comparison• Total Harmonic Distortion : ratio of the sum of
powers of all harmonic components (excluding fundamental freq.) to the power of fundamental frequency .
• Shows distortion produced by the system relative to the fundamental component .
S.No
Follower Configuration
Harmonic distortion(in %)
1. Conventional Source Follower
14.16
2. Flipped voltage follower
13.71
3. Class A Super Source Follower
7.04
4. Class AB Super Source Follower
0.18
7.2 Slew Rate Comparison • Slew Rate : Maximum rate of change of output
voltage .• Cause : Limited current driving capability .
0.0 0.5 1.0 1.5 2.0
Time (us)
-2.0
-1.5
-1.0
-0.5
0.0
Volta
ge (V
)
v(N_1)
v(N_4)
Vin
Vout
Slew Rate of Source Follower
0.0 0.5 1.0 1.5 2.0
Time (us)
-1.0
-0.5
0.0
0.5
1.0
1.5
Volta
ge (V
)
v(N_2)
v(N_3)
Vin
Vout
Slew Rate Of Flipped Voltage Follower
0.0 0.5 1.0 1.5 2.0
Time (us)
-1.5
-1.4
-1.3
-1.2
-1.1
-1.0
-0.9
-0.8
Volta
ge (V
)
v(N_1)
v(N_2)
Vin
Vout
Slew Rate Of Super Source Follower
0.0 0.5 1.0 1.5 2.0
Time (us)
-1.5
-1.4
-1.3
-1.2
-1.1
-1.0
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
Volta
ge (V
)
v(N_4)
v(N_5)
vin
vout
Slew Rate of Class AB SSF
7.2 Continued …..
S.No
Follower Configuration
SR+ (in V/
SR- (in V/
1. Conventional Source Follower
0.2 -2.5
2. Flipped voltage follower
13.54 -0.23
3. Class A Super Source Follower
0.40 -2.43
4. Class AB Super Source Follower
2.85 -14.6
S.No
Follower Configuration
1. Conventional Source Follower
0.2 -2.5
2. Flipped voltage follower
13.54 -0.23
3. Class A Super Source Follower
0.40 -2.43
4. Class AB Super Source Follower
2.85 -14.6
7.3 Bandwidth Comparison• Bandwidth : Range of frequencies in which
output is within 3-dB of the maximum value.
1 100k 10G
Frequency (Hz)
-10
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
Volta
ge M
agni
tude
(dB)
vdb(N_4)
frequency response of source follower
100m 1 10 100 1k 10k 100k 1M 10M 100M
Frequency (Hz)
-50
-45
-40
-35
-30
-25
-20
-15
Volta
ge M
agni
tude
(dB)
vdb(N_2)
ac analysis of flipped voltage follower
100m 1 10 100 1k 10k 100k 1M 10M 100M
Frequency (Hz)
-14.5
-14.0
-13.5
-13.0
-12.5
-12.0
-11.5
-11.0
-10.5
-10.0
-9.5
-9.0
-8.5
Volta
ge M
agni
tude
(dB)
vdb(N_1)
bandwidth of super source follower
100m 1 10 100 1k 10k 100k 1M 10M 100M
Frequency (Hz)
-20
-15
-10
-5
0
Volta
ge M
agni
tude
(dB)
vdb(N_3)
bandwidth of class AB SSF
7.3 Continued …..
S.No
Follower Configuration
Bandwidth(in MHz)
1. Conventional Source Follower
124
2. Flipped voltage follower
162
3. Class A Super Source Follower
182
4. Class AB Super Source Follower
206
7.4 Power dissipation Comparison• Power dissipation : rate at which energy is
dissipated in the circuit.• It is of 2 types-Static and Dynamic.
S.No
Follower Configuration
Power dissipation(
1. Conventional Source Follower
43
2. Flipped voltage follower
184
3. Class A Super Source Follower
168
4. Class AB Super Source Follower
168
S.No
Follower Configuration
1. Conventional Source Follower
43
2. Flipped voltage follower
184
3. Class A Super Source Follower
168
4. Class AB Super Source Follower
168
7.5 Noise Analysis • Noise : random fluctuation in an electrical
signal .• Mainly thermal noise and flicker noise are
present . S.No
Follower Configuration
Equivalent Input Noise(nV/)
1. Conventional Source Follower
22
2. Flipped voltage follower
18
3. Class A Super Source Follower
15
4. Class AB Super Source Follower
15
S.No
Follower Configuration
1. Conventional Source Follower
22
2. Flipped voltage follower
18
3. Class A Super Source Follower
15
4. Class AB Super Source Follower
15
8. CONCLUSION
Source Followertraditional voltage buffer.high output resistance.small linear range of operation.Flipped Voltage Follower A transistor is introduced in negative shunt
feedback in Source Follower . larger linear range than SF .smaller output impedance than SF .small input and output range .
8. continued…..Class A Super Source Followerlinear range higher than SF and FVF .output resistance lower than SF and
comparable to FVF. higher input range than FVF .Slew rate limitations .
8. continued…..
Class AB Super Source Followerhighest linear range .Lowest output resistance .Lowest Total Harmonic Distortion .Largest bandwidth .Maximum Slew Rate .Only price paid is increase in Silicon area .
Thank You