LT1800 Linear Technology, LT1800 Datasheet - Page 11

• Manufacturer Part Number: LT1800
• Description: 80MHz/ 25V/ms Low Power Rail-to-Rail Input and Output Precision Op Amp
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
TYPICAL PERFOR A CE CHARACTERISTICS
2V/DIV
Circuit Description
The LT1800 has an input and output signal range that
covers from the negative power supply to the positive
power supply. Figure 1 depicts a simplified schematic of
the amplifier. The input stage is comprised of two differ-
ential amplifiers, a PNP stage Q1/Q2 and an NPN stage Q3/
Q4 that are active over the different ranges of common
mode input voltage. The PNP differential pair is active
between the negative supply to approximately 1.2V below
V
V
+
0V
–
Q16
+
V
A
R
S
V
L
5V Large-Signal Response
= 5V
= 1
= 1k
I
2
+IN
–IN
Q17
200ns/DIV
U
Q18
ESDD1
ESDD4
U
V
V
+
–
W
V
V
–
+
1800 G35
ESDD2
ESDD3
U
W
D6
D5
Figure 1. LT1800 Simplified Schematic Diagram
50mV/DIV
D8
D7
Q19
0V
U
Q4
V
A
R
Q7
S
V
L
5V Small-Signal Response
D1
D2
= 5V
= 1
= 1k
Q3
Q5
Q6
50ns/DIV
the positive supply. As the input voltage moves closer
toward the positive supply, the transistor Q5 will steer the
tail current I
NPN differential pair and the PNP pair becomes inactive
for the rest of the input common mode range up to the
positive supply. Also at the input stage, devices Q17 to
Q19 act to cancel the bias current of the PNP input pair.
When Q1-Q2 are active, the current in Q16 is controlled to
be the same as the current in Q1-Q2, thus the base current
V
BIAS
Q1
+
D3
D4
Q2
I
1
1
1800 G36
Q10
to the current mirror Q6/Q7, activating the
Q11
Q9
1V/DIV
2V/DIV
R3
R1
V
OUT
Q12
V
0V
0V
IN
Output Overdriven Recovery
V
A
R
Q8
S
V
L
R4
R2
= 5V, 0V
= 2
= 1k
C
C
+
V
OUTPUT BIAS
Q13
–
R5
BUFFER
I
3
AND
100ns/DIV
C2
C1
LT1800
1800 F01
Q15
11
Q14
OUT
1800f
1800 G37