MAT02

–11–

REV. C

Substituting in the voltage relationships and simplifying leads

to:









m

, where

(20)

m =

A

The factor “K” is a potentiometer position and varies from zero

Practical values are 125

Ω for R

Ω for R

ues will provide an adjustment range of 0.2 to 5.0. A value of

100 k

Ω is recommended for the R

scale input range of 10 V. As with the one-quadrant multiplier/

must all be positive.

The op amps should have the lowest possible input offsets. The

OP07 is recommended for most applications, although such

programmable micropower op amps as the OP22 or OP32 offer

advantages in low-power or single-supply circuits. The micro-

power op amps also have very low input bias-current drift, an

important advantage in log/antilog circuits. External offset null-

ing may be needed, particularly for applications requiring a

wide dynamic range. Frequency compensating capacitors, on

µF, to as-

sure stability.

Accuracy is limited at the higher input levels by bulk emitter re-

sistance, but this is much lower for the MAT02 than for other

transistor pairs. Accuracy at the lower signal levels primarily de-

pends on the op amp offsets. Accuracies of better than 1% are

readily achievable with this circuit configuration and can be bet-

ter than

±0.1% over a limited operating range.

FAST LOGARITHMIC AMPLIFIER

The circuit of Figure 23 is a modification of a standard logarith-

mic amplifier configuration. Running the MAT02 at 2.5 mA per

side (full-scale) allows a fast response with wide dynamic range.

The circuit has a 7 decade current range, a 5 decade voltage

range, and is capable of 2.5

µs settling time to 1% with a 1 V to

10 V step.

The output follows the equation:

kT

q

In

(21)

The output is inverted with respect to the input, and is nomi-

nally –1 V/decade using the component values indicated.

LOW-NOISE

1000 AMPLIFIER

The MAT02 noise voltage is exceptionally low, only 1 nV/

√Hz

at 10 Hz when operated over a collector-current range of 1 mA

to 4 mA. A single-ended

×1000 amplifier that takes advantage of

this low MAT02 noise level is shown in Figure 24. In addition

to low noise, the amplifier has very low drift and high CMRR.

An OP32 programmable low-power op amp is used for the sec-

ond stage to obtain good speed with minimal power consump-

tion. Small-signal bandwidth is 1 MHz, slew rate is 2.4 V/

µs,

and total supply current is approximately 2.8 mA.

Figure 22. Multifunction Converter