MAT04 Analog Devices, MAT04 Datasheet - Page 7

• Manufacturer Part Number: MAT04
• Description: Matched Monolithic Quad Transistor
• Manufacturer: Analog Devices

NONLINEAR FUNCTIONS
An application where precision matched-transistors are a power-
ful tool is in the generation of nonlinear functions. These circuits
are based on the transistor’s logarithmic property, which takes
the following idealized form:
The MAT04, with its excellent logarithmic conformance, main-
tains this idealized function over many decades of collector
current. This, in addition to the stringent parametric matching
of the MAT04, enables the implementation of extremely accu-
rate log/antilog circuits.
The circuit of Figure 5 is a vector summer that adds and sub-
tracts logged inputs to generate the following transfer function:
V
BE
=
kT
q
In
l
l
C
S
This circuit uses two MAT04 and maintains an accuracy of
better than 0.5% over an input range of 10 mV to 10 V. The
layout of the MAT04s reduces errors due to matching and
temperature differences between the two precision quad matched
transistors.
Op amps A1 and A2 translate the input voltages into logarithmic
valued currents (I
sistor Q3 and Q5. These currents are summed by transistor Q4
which feeds the current-to-voltage converter consisting of op amp
A3. To maintain accuracy, 1% metal-film resistors should be used.
A
(I
and I
V
O
OUT
= I
B
A
=
in Figure 5) that flow through tran-
+ I
1
Ω
2
B
=
V
A
l
1
2
2
+
+
V
l
2
B
2
2
),
MAT04