MAT02

–10–

REV. E

Substituting in the voltage relationships and simplifying leads

to:









m

, where

(20)

m =

+ KR

A

+ 1– K

A

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

Practical values are 125

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

of 100 k

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

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 OP193/OP293 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

nulling may be needed, particularly for applications requiring a

wide dynamic range. Frequency compensating capacitors, on

µF, to

assure stability.

Accuracy is limited at the higher input levels by bulk emitter

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

transistor pairs. Accuracy at the lower signal levels primarily

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

are readily achievable with this circuit configuration and can be

better than

±0.1% over a limited operating range.

FAST LOGARITHMIC AMPLIFIER

The circuit of Figure 7 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:

+ R

2

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.

Figure 7. Fast Logarithmic Amplifier

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 8. In addition to

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

OP184 is used for the second stage to obtain good speed with

minimal power consumption. Small-signal bandwidth is 4.0

MHz, slew rate is 2.4 V/

µs, and total supply current is approxi-

mately 2.25 mA.

Figure 8. Low-Noise, Single-Ended

× 1000 Amplifier