MAT04

–7–

REV. D

Ω

NONLINEAR FUNCTIONS

An application where precision matched-transistors are a power-

ful tool is in the generation of nonlinear functions. These circuits

are based on the transistor’s logarithmic property, which takes

the following idealized form:

V

kT

q

In

l

l

BE

C

S

=

The MAT04, with its excellent logarithmic conformance, main-

tains this idealized function over many decades of collector

current. This, in addition to the stringent parametric matching

of the MAT04, enables the implementation of extremely accu-

rate log/antilog circuits.

The circuit of Figure 5 is a vector summer that adds and sub-

tracts logged inputs to generate the following transfer function:

VV

V

OUT

A

B

=+

1

2

22

This circuit uses two MAT04 and maintains an accuracy of

better than 0.5% over an input range of 10 mV to 10 V. The

layout of the MAT04s reduces errors due to matching and

temperature differences between the two precision quad matched

transistors.

Op amps A1 and A2 translate the input voltages into logarithmic

sistor Q3 and Q5. These currents are summed by transistor Q4

(

1

2

2

2

+

),

which feeds the current-to-voltage converter consisting of op amp

A3. To maintain accuracy, 1% metal-film resistors should be used.