LM363 Datasheet, PDF (8/22 Pages) National Semiconductor (TI) – LM363 Precision Instrumentation Amplifier

Simplified Schematic (pin numbers in parentheses are for 8-pin package)
Theory of Operation
Referring to the Simplified Schematic it can be seen that
the input voltage is applied across the bases of Q1 and Q2
and appears between their emitters If R_E1-2 is the resist-
ance across these emitters a differential current equal to
V_IN R_E1-2 flows from Q1’s emitter to Q2’s The second
stage amplifier shown maintains Q1 and Q2 at equal collec-
tor currents by negative feedback to Q4 The emitter cur-
rents of Q3 and Q4 must therefore be unbalanced by an
amount equal to the current flow across R_E1-2 Defining
R_E3-4eR5aR6 the differential voltage across the emitters
of Q4 to Q3 is equal to
V_IN
R_{E 1-2}
c
R_E3-4
TL H 5609 – 7
This voltage divided by the attenuation factor
R4
R2
e
R3aR4 R1aR2
is equal to the output-to-reference voltage Hence the over-
all gain is given by
Ge^VOUTe^R3aR4c^RE3-4
V_IN
R4 R_E1-2
8