9

LT1495/LT1496

Figure 3. Simplified Schematic

+

1495 F03

0.5

Q6

Q4

Q16

Q17

Q19

Q18

Q22

C1

D1

D2

D3

Q3

Q11

Q2

Q7

Q10

Q13

Q20

Q21

OUT

Q12

Q14

Q15

Q5

0.5

0.5

0.5

Q8

R1

R2

+

D6

D5

D4

D7

Q9

Q1

The input stage is formed by two diff amps Q1-Q2 and Q3-

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

and Q4 act as followers, forming a buffer between the

amplifier inputs and the emitters of the Q5-Q6. If the

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.