3/1/2003 BAE 5413 1 of 5

Additional Information on Op-Amp Specifications

3/1/2003 BAE 5413 2 of 5

Additional Op Amp specifications

• Input Offset voltage – The apparent voltage offset at the input of the amplifier measured by determining the amp output (and dividing that output by the gain to calculate the input) when the differential input is shorted. (See the data sheet on the amp.)

• Common mode rejection ratio (CMRR)– Common mode is the average voltage of the two inputs on a

differential amplifier.– Differential voltage is the voltage difference between the two

inputs on a differential amplifier– Common mode rejection ratio is the ratio of the output of the

amp subjected to a common voltage to the output of the amp subjected to the same level of differential input voltage. This would be zero in an ideal amplifier.

3/1/2003 BAE 5413 3 of 5

• Bandwidth – normally specified in terms of Gain – Bandwidth product (GBP). The amplifier behaves like a first order filter where the corner frequency is:

• The transfer function for an Op Amp may be written as:

Computation of Gain Bandwidth Product

1)(

0

sAA

sG

Gain

GBPc

3/1/2003 BAE 5413 4 of 5

Frequency Response for an Op Amp

• Substituting j for s and computing the non-transient response:

• Corner frequency occurs where the term under the root becomes significant with respect to the 1/A2 term

20

22

0

00

0

0

1

1

11

1

1)(

A

jAA

Aj

Aj

AjA

jAA

jG

20

2

220

22 1

1

1

)(

AA

AjG

20

2

2

1

c

A GBPAc 00 1

3/1/2003 BAE 5413 5 of 5

Op-Amp Bandwidth Characteristics

-60

-40

-20

0

20

40

60

80

100

120

140

1.00E+00 1.00E+01 1.00E+02 1.00E+03 1.00E+04 1.00E+05 1.00E+06 1.00E+07 1.00E+08 1.00E+09

Frequency

Am

pli

tud

e R

atio

(d

b)

1000000

100000

10000

1000

100

10

1

GBw = 1.00E06

G = 1

G = 100

G = 100

G = 1000

G = 10000

G = 1E06

G = 1E05

Bandwidth Graphically