MAT02 datasheet(10/12 Pages) AD

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MAT02

–10–

REV. C

Collector-current range is the key design decision. The inher-

ently low rBE of the MAT02 allows the use of a relatively high

collector current. For input scaling of

±10 V full-scale and us-

ing a 10 V reference, we have a collector-current range for I1

and I2 of:

–10









≤ I

C ≤









(13)

Practical values for R1 and R2 would range from 50 kΩ to

100 k

Ω. Choosing an R

1 of 82 k

Ω and R

2 of 62 k

Ω provides a

collector-current range of approximately 39

µA to 283 µA. An

RO of 108 kΩ will then make the output scale factor 1/10 and

VO = VXVY/10. The output, as well as both inputs, are scaled

for

±10 V full scale.

Linear error for this circuit is substantially improved by the

small correction voltage applied to the base of Q1 as shown in

Figure 21. Assuming an equal bulk emitter resistance for each

MAT02 transistor, then the error is nulled if:

(I1 + I2 – I3 – IO) rBE +

ρV

O = 0

The currents are known from the previous discussion, and the

relationship needed is simply:

VO =

rBE

(14)

The output voltage is attenuated by a factor of rBE/RO and ap-

plied to the base of Q1 to cancel the summation of voltage

drops due to rBEIC terms. This will make In (I1 I2/I3 IO) more

nearly zero which will thereby make IO = I1 I2/I3 a more accu-

rate relationship. Linearity of better than 0.1% is readily achiev-

able with this circuit if the MAT02 pairs are carefully kept at

the same temperature.

MULTIFUNCTION CONVERTER

The multifunction converter circuit provides an accurate means

of squaring, square rooting, and of raising ratios to arbitrary

powers. The excellent log conformity of the MAT02 allows a

wide range of exponents. The general transfer function is:

VO = VY

V Z

V X









(15)

VX, VY, and VZ are input voltages and the exponent “m” has a

practical range of approximately 0.2 to 5. Inputs VX and VY are

often taken from a fixed reference voltage. With a REF01 pro-

viding a precision +10 V to both VX and VY, the transfer func-

tion would simplify to:

VO = 10

V Z









(16)

As with the multiplier/divider circuits, assume that the transistor

pairs have excellent matching and are at the same temperature.

The In ISA/ISB will then be zero. In the circuit of Figure 22, the

voltage drops across the base-emitter junctions of Q1 provide:

RB + KRA

V A =

(17)

IZ is VZ/R1 and IX is VX/R1. Similarly, the relationship for Q2 is:

RB + 1– K

()R

V A =

(18)

IO is VO/RO and IY is VY/R1. These equations for Q1 and Q2 can

then be combined.

RB + KRA

RB + 1– K

()R

= In

(19)

Figure 21. Four-Quadrant Multiplier

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