super source follower

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