MAT02

–8–

REV. E

more troublesome because they vary with signal levels and are

multiplied by absolute temperature. At 25

°C, kT/q is

Ω for the MAT02 and assum-

ing a collector current range of up to 200

µA, then a peak error

the MAT02. Total error is dependent on the specific application

configuration (multiply, divide, square, etc.) and the required

duce the maximum collector current, but then op amp offsets

and leakage currents become a limiting factor at low input lev-

els. A design range of no greater than 10

µA to 1 mA is generally

recommended for most nonlinear function circuits.

tor base. For this circuit:

(10)

Since the MAT02 bulk resistance is approximately 0.39

Ω, an

In more complex circuits, such as the circuit in Figure 3, it may

be inconvenient to apply a compensation voltage to each indi-

vidual base. A better approach is to sum all compensation to the

±0.1% is readily achievable with

this compensation technique.

Operational amplifier offsets are another source of error. In

Figure 4, the input offset voltage and input bias current will

offset op amp, such as the OP07 with less than 75

± 3 nA, is recommended. The OP193,

micropower op amp, should be considered if low power con-

sumption or single-supply operation is needed. The value of

op amp frequency response and peak collector current. Typi-

Figure 4. Compensation of Bulk Resistance Error

FOUR-QUADRANT MULTIPLIER

A simplified schematic for a four-quadrant log-antilog multiplier

is shown in Figure 5. Similar to the previously discussed one-

This positive offset is then subtracted out at the output stage.

Assuming ideal op amps, the currents are:

I

V

R

V

R

I

V

R

V

R

XR

Y

R

1

12

2

12

=+

=

+

,

(11)

I

V

R

V

R

V

R

V

R

I

V

R

O

XY

R

O

O

R

=+

+

+

=

11

2

3

2

,

2

(12)

Figure 5. Four-Quadrant Multiplier