Input & Output Limitations – 2 TIPL 1131 TI Precision Labs – Op Amps

Presented by Ian Williams

Prepared by Art Kay and Ian Williams

Prerequisite: Input & Output Limitations

(TIPL 1130)

Real World VCM Range

2

V+ V+

V+V+

Simple MOSFET Input – VCM to Negative Rail

3

+

-V+

V-

5V

OPA336

-0.2V < VIN- < 4V

-0.2V < VIN+ < 4V

+

-V+

V-

+2.5V

OPA336

-2.7V < VIN- < 1.5V

-2.5V

-2.7V < VIN+ < 1.5V

Input Pair Biasing

4

Q4Q3

Q1Q2

+VS

VIN+

VIN-

Vgs=0.9V

-VS

Vgs=0.9V

100µA 100µAVgs = -0.8V

Vgs = -0.9V

Vgs = -1.0V

Vgs = -1.1V

100µA

200µA

ID

Vds0.1V

Q5

Vsat

0.1V

Vsat

0.1V

Vsat

0.1V

OPA336 Input Stage – Maximum VCM

5

Q4Q3

IS1

Q1Q2

+VS

VIN+

VIN-

To second stage

Vgs=0.9V

Vsat=0.1V

VCM swing to positive rail:

Vsat + Vgs = 0.1V + 0.9V = 1.0V

Therefore,

Vcm_max < +VS – 1.0V

-VS

For Vcm > Vcm_max,

Q4 will cutoff and

IS1 will saturate.

OPA336 Input Stage – Minimum VCM

6

Q4Q3

IS1

Q1Q2

-VS

+VS

VIN+

VIN-

Vbe=0.6V

Vsat=0.1V

Vgs=0.9V

VCM swing to negative rail:

Vbe + Vsat – Vgs

= .6V + 0.1V – 0.9V = -0.2V

Therefore,

Vcm_min > 0.6V + 0.1V – 0.9V

= -VS – 0.2VTo second stage

For Vcm < Vcm_min,

Q4 will saturate.

Typical Bipolar or JFET Input – Not Rail-to-Rail

7

+

-V+

V-

+12V

-5V

OPA827

-2V < VIN- < 9V

-2V < VIN+ < 9V

OPA827 Input Stage – Minimum VCM

8

IS1

IN-VIN+

R1 R2IS2

Cc

D1

D2

R3

to output stage

+VS

-VS

VR2Vbe

Vbe

Vsat

Vgs

VCM swing to negative rail:

VCM > -VS + VR2 + 2Vbe + Vsat – Vgs

Therefore,

-VS + 3V < VCM < +VS - 3V

MOSFET Complementary N-P-FET – Rail-to-Rail

9

+

-V+

V-

5V

OPA703

-0.3V < VIN- < 5.3V

-0.3V < VIN+ < 5.3V

+

-V+

V-

+2.5V

OPA703

-2.8V < VIN- < 2.8V

-2.5V

-2.8V < VIN+ < 2.8V

OPA703 Input Stage – Simplified Schematic

10

NCH3

PCH2PCH1

VCM_Iswitch

IS2

IS1

T8

NCH4

T2

PCH input pair active for:

-VS – 0.3V < VCM < +VS – 2V

-VS

+VS

VIN+

VIN-

2V

NCH input pair active for:

+VS – 2V < VCM < +VS + 0.3V

Turns on when

+VS – 2V < VCM < +VS + 0.3V

Steals current from PCH

OPA703 Complementary CMOS – Rail-to-Rail

11

VIN-

VIN+

-VS

Q1 Q2

Q3 Q4

+VS

Common Mode Voltage (V)

200

100

0

-100

-200

-3000.0 1.0 3.0 4.0 5.02.0

Inp

ut O

ffse

t V

olta

ge

(µ

V)

Crossover Distortion

12

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 0.2 0.4 0.6 0.8 1 1.2

Vo

ut

(Vo

lts)

time (ms)

Vout vs. Time (Crossover Distortion)

3.8

3.85

3.9

3.95

4

4.05

4.1

4.15

4.2

0.11 0.12 0.13 0.14 0.15 0.16

Vo

ut

(Vo

lts)

time (ms)

Zoom in on Crossover Distortion

Vout Ideal

Vout Crossover

+5V

-

+

+

Vin

Vout

RL

1k

Common Mode Voltage (V)

5

2.5

0

-2.5

-5

-7.50.0 1.0 3.0 4.0 5.02.0

Inp

ut O

ffse

t V

olta

ge

(m

V)

Vout vs. Time

Vout vs. Time (Zoomed In)

MOSFET Charge Pump – Rail-to-Rail

13

+

-V+

V-

5V

OPA365

-0.1V < VIN- < 5.1V

-0.1V < VIN+ < 5.1V

+

-V+

V-

+2.5V

OPA365

-2.6V < VIN- < 2.6V

-2.5V

-2.6V < VIN+ < 2.6V

Remember from Earlier in the Presentation…

14

Q4Q3

IS1

Q1Q2

+VS

VIN+

VIN-

To second stage

Vgs=0.9V

Vsat=0.1V

VCM swing to positive rail:

Vsat + Vgs = 0.1V + 0.9V = 1.0V

Therefore,

Vcm_max < +VS – 1.0V

-VS

For Vcm > Vcm_max,

Q4 will cutoff and

IS1 will saturate.

OPA365 MOSFET Charge Pump – Rail-to-Rail

15

VOUT = +VS + 1.8V

Zero Drift MOSFET – Rail-to-Rail

16

+

-V+

V-

5V

OPA333

-0.1V < VIN- < 5.1V

-0.1V < VIN+ < 5.1V

+

-V+

V-

+2.5V

OPA333

-2.6V < VIN- < 2.6V

-2.5V

-2.6V < VIN+ < 2.6V

0.0 1.0 2.0 3.0 4.0 5.0

Common Mode Voltage (V)

Inp

ut O

ffse

t V

olta

ge

(u

V)

200

100

0

-100

-200

-300

Common Mode Voltage (V)

200

100

0

-100

-200

-3000.0 1.0 3.0 4.0 5.02.0

Inp

ut O

ffse

t V

olta

ge

(µ

V)

VIN-

VIN+

-VSUPPLY

Q1 Q2

Q3 Q4

Zero Drift MOSFET – Rail-to-Rail

17

With Offset

Correction

No Offset

Correction

18

Thanks for your time! Please try the quiz.

Input & Output Limitations – 2 Multiple Choice Quiz TI Precision Labs – Op Amps

2

Quiz: Input & Output Limitations – 2 1. Single supply amplifiers have _____________.

a. Common mode range that extends to the negative supply.

b. Common mode range that extends to the positive supply.

2. (T/F) Single supply amplifiers can be used in a dual supply configuration.

a. True

b. False

3. (T/F) The common mode range of an amplifier is limited by the saturation and cutoff voltages of transistors in the input stage.

a. True

b. False

4. (T/F) Most rail-to-rail amplifiers use bipolar transistors.

a. True

b. False

3

Quiz: Input & Output Limitations – 2 5. What is a potential issue associated with using a charge pump to power the input stage on an op amp?

a. The charge pump can create crossover distortion.

b. The charge pump will cause a significant increase in the device quiescent current.

c. The charge pump requires external components.

d. Charge pump switching noise can introduce errors.

6. Consider the CMRR specification below. Does this device have crossover distortion?

a. Yes.

b. No.

4

Quiz: Input & Output Limitations – 2 7. Which of the following will NOT minimize crossover distortion on rail to rail amplifiers?

a. Use an internal charge pump to boost the supply on the input stage

b. Use external TVS diodes to minimize the distortion.

c. Use a zero drift amplifier to minimize the overall offset so that the offset shift is small.

8. Consider the CMRR specification below. Does this device have crossover distortion?

a. Yes.

b. No.

Input & Output Limitations – 2 Multiple Choice Quiz: Solutions TI Precision Labs – Op Amps

6

Solutions: Input & Output Limitations – 2 1. Single supply amplifiers have _____________.

a. Common mode range that extends to the negative supply.

b. Common mode range that extends to the positive supply.

2. (T/F) Single supply amplifiers can be used in a dual supply configuration.

a. True

b. False

3. (T/F) The common mode range of an amplifier is limited by the saturation and cutoff voltages of transistors in the input stage.

a. True

b. False

4. (T/F) Most rail-to-rail amplifiers use bipolar transistors.

a. True

b. False

7

Solutions: Input & Output Limitations – 2 5. What is a potential issue associated with using a charge pump to power the input stage on an op amp?

a. The charge pump can create crossover distortion.

b. The charge pump will cause a significant increase in the device quiescent current.

c. The charge pump requires external components.

d. Charge pump switching noise can introduce errors.

6. Consider the CMRR specification below. Does this device have crossover distortion?

a. Yes.

b. No.

8

Solutions: Input & Output Limitations – 2 7. Which of the following will NOT minimize crossover distortion on rail to rail amplifiers?

a. Use an internal charge pump to boost the supply on the input stage

b. Use external TVS diodes to minimize the distortion.

c. Use a zero drift amplifier to minimize the overall offset so that the offset shift is small.

8. Consider the CMRR specification below. Does this device have crossover distortion?

a. Yes.

b. No.