TL441

LOGARITHMIC AMPLIFIER

SLVS328 – OCTOBER 2000

13

POST OFFICE BOX 655303

APPLICATION INFORMATION

Z

Z

A1

A2

Y

Y

B1

B2

TL441

A1

A2

Y

Y

1/2

TL441

Input

A

Input

B

see

Note A

OUTPUT W

(see Note B)

RR

R

R

R

R

R

RR

R

+

–

+

–

+

–

+

–

+

–

NOTES: A. Connections shown are for multiplication. For division, Z and Z connections are reversed.

B. Output W may need to be amplified to give actual product or quotient of A and B.

C. R designates resistors of equal value, typically 2 k

Ω to 10 kΩ.

Multiplication: W = A

• B ⇒ log W = log A + log B, or W = a(logaA + logaB)

Division: W = A/B

⇒ log W = log A – log B, or W = a(logaA + logaB)

Figure 16. Multiplication or Division

Input

A

A1

A2

Y

Y

1/2

TL441

Z

Z

B1

B2

Output

W

1/2

TL441

RR

R

nR

R

nR

R

R

–

+

+

–

+

–

–

+

NOTE: R designates resistors of equal value, typically 2 k

Ω to 10 kΩ. The power to which the input variable is raised is fixed by setting nR.

Output W may need to be amplified to give the correct value.

Exponential: W = An

⇒ log W = n log A, or W = a(n loga A)

Figure 17. Raising a Variable to a Fixed Power