MAT02 View Datasheet(PDF) - Analog Devices
Part Name
Description
MFG CO.
'MAT02' PDF : 12 Pages View PDF
MAT02
Figure 6. Multifunction Converter
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 using
a 10 V reference, we have a collector-current range for I1 and I2
of:
MULTIFUNCTION CONVERTER
The multifunction converter circuit provides an accurate means
of squaring, square rooting, and raising ratios to arbitrary pow-
ers. The excellent log conformity of the MAT02 allows a wide
range of exponents. The general transfer function is:
–10
R1
+
10
R2
≤
IC
≤
10
R1
+
10
R2
(13)
Practical values for R1 and R2 would range from 50 kΩ to
100 kΩ. Choosing an R1 of 82 kΩ and R2 of 62 kΩ 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 5. Assuming an equal bulk emitter resistance for each
MAT02 transistor, then the error is nulled if:
(I1 + I2 – I3 – IO) rBE + ρVO = 0
The currents are known from the previous discussion, and the
relationship needed is simply:
VO =
r BE
RO
VO
(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 accurate rela-
tionship. Linearity of better than 0.1% is readily achievable with
this circuit if the MAT02 pairs are carefully kept at the same
temperature.
VO
=
VY
V
V
Z
X
m
(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 function
would simplify to:
VO
=
10
V10Z
m
(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 6, the
voltage drops across the base-emitter junctions of Q1 provide:
RB
RB
+ KRA
V
A
=
kT
q
In
IZ
IX
(17)
IZ is VZ/R1 and IX is VX/R1. Similarly, the relationship for Q2 is:
( ) RB +
RB
1– K
RA
V
A
=
kT
q
In IO
IY
(18)
IO is VO/RO and IY is VY/R1. These equations for Q1 and Q2 can
then be combined.
( ) RB + KRA In IZ = In IO
RB + 1 – K RA IX
IY
(19)
REV. E
–9–
Share Link: 
