Teledyne 238 - Thermal conductivity analyzer - Manual (Page 27)

4-9 TELEDYNE BROWN ENGINEERING
Analytical Instruments
Thermal Conductivity Analyzer Linearizer 4
When the voltage exceeds 0.25 Volts, the third amplifier works, along
with the first two, adding or subtracting its output in proportion, according
to the setting of its trimpot, P3. The gain is now the sum of all three gains.
In this case, the gain of the third amp is negative, so the total gain is about
0.3.
As the input voltage exceeds each breakpoint, another amplifier joins
in. The slope of each line segment is equal to the sum of the gain of all the
amplifiers in operation at that particular time. The gain of each amplifier is
set by its trimpot. The first amplifier has a gain range of 0 to +4, and all the
others about -3 to +3.
The maximum slope obtainable is limited. Setting the gain too high will
result in the amplifier saturating. However, with the dynamic range inherent
in these amplifiers, this is not likely to happen.
The breakpoints are factory-set by the values of resistors R6, R8, R10,
R-12, R14, R16, and R18. See Figure 4-10 for location of these resistors.
The most efficient way to check the operation of the linearizer circuit
is to drive it using a 1 kHz. triangular wave of 2 Volts peak-to-peak ampli-
tude as shown in Figure 4-6. The effect of the breakpoints and trimpots can
then readily be seen; if you alternate the gain of the stages, a jagged step
effect can be produced. This will show the breakpoints clearly. Alterna-
tively, a DVM may be attached to the junction of D2 and R20; this junction
point accesses the output of a line segment amplifier. As the input voltage is
gradually increased, the DVM at some point will indicate a negative volt-
age. At this point, a breakpoint has been passed. Repeat this test for each
line segment amplifier to determine its breakpoint.
4.2 Linearizer Circuit Theory
Refer to Figure 4-10 for the component position in the following
discussion. AlA is a non-inverting buffer and amplifier with a gain of 2.5,
zeroed with P9. Its output is checked for zero at test point 1 (TP1). R1
provides a bias path in case the input is not DC-loaded.
The amplified output is brought to the inverting inputs of line segment
amplifiers AlB, A1C, A1D, A2A, A2B, A2C, A2D, and A3A, through
resistors R5, R7, R9, R11, R13, R15, R17, and R19.
A1D is configured differently from the other line segment amplifiers. It
is simply an inverting amplifier with P1 as its feedback resistor to set its
gain. The gain for this amplifier may be set between 0 and 4.
The other line segment amplifiers work in a similar fashion. Let us
examine A1C, for example. Refer to Figure 4-7 for the following analysis of
a typical line segment amplifier.