Teledyne 238 - Thermal conductivity analyzer - Manual (Page 29)

4-11 TELEDYNE BROWN ENGINEERING
Analytical Instruments
Thermal Conductivity Analyzer Linearizer 4
Figure 4-7: A Typical Line Segment Amplifier
Diode D1 effectively shorts the output to the inverting input for any
positive-going signals at the output of AlC, while D2 would not allow any
positive output at AlC to reach the circuit’s output. Negative-going signals,
however, do not get through D1, but can go through D2, and R20 then acts
as a feedback resistor to set the gain of the circuit (to 4 in the example for
A1C). Since D2 is inside the feedback loop, its voltage drop does not
appear at the output. R6 sums a fraction of the -15 V supply to the input
voltage; the output of the circuit"is thus:
V
o
= -R20 (V
in
/R7-15/R6) for V0 < O
V
o
= O if -R20 (V
in
/R7-15/R6) > O
From this it can be seen that the circuit amplifies input voltages above
a cutoff voltage, and otherwise has zero output
.
-R20 (V
in
/R7-15/R6) = O or
V
in
= 15 x R7/R6