Datasheet

11
LT1494/LT1495/LT1496
APPLICATIONS INFORMATION
WUU
U
Figure 3. Simplified Schematic
The input stage is formed by two diff amps Q1-Q2 and Q3-
Q6. For signals with a common mode voltage between V
EE
and (V
CC
– 0.8V), Q1 and Q2 are active. When the input
common mode exceeds (V
CC
– 0.8V), Q7 turns on,
diverting the current from diff amp Q1-Q2 to current
mirror Q8-Q9. The current from Q8 biases on the other diff
amp consisting of PNP’s Q5-Q6 and NPN’s Q3-Q4. Though
Q5-Q6 are driven from the emitters rather than the base,
the basic diff amp action is the same. When the common
mode voltage is between (V
CC
– 0.8V) and V
CC
, devices Q3
and Q4 act as followers, forming a buffer between the
amplifier inputs and the emitters of the Q5-Q6. If the
common mode voltage is taken above V
CC
, Schottky
diodes D1 and D2 reverse bias and devices Q3 and Q4 then
act as diodes. The diff amp formed by Q5-Q6 operates
normally, however, the input bias current increases to the
emitter current of Q5-Q6, which is typically 180nA. The
graph, Input Bias Current vs Common Mode Voltage
found in the Typical Performance Characteristics section,
shows these transitions at three temperatures.
The collector currents of the two-input pairs are combined
in the second stage consisting of Q11 to Q16, which
furnishes most of the voltage gain. Capacitor C1 sets the
amplifier bandwidth. The output stage is configured for
maximum swing by the use of common emitter output
devices Q21 and Q22. Diodes D4 to D6 and current source
Q15 set the output quiescent current.
+
+
1495 F03
0.5
Q6
Q4 Q16 Q17(V
+
) – 0.8V Q19
Q18
Q22
C1
D1 D2
V
+
D3
Q3
Q11
Q2
Q7
IN
–
IN
+
Q10 Q13
Q20
Q21
OUT
Q12
Q14 Q15
Q5
0.5
0.5
0.5
Q8
R1 R2 I
2
+
I
1
D6
D5
D4
D7
Q9
Q1