LT1366/LT1367
LT1368/LT1369
U
W U U
APPLICATIONS INFORMATION
cated on Linear Technology’s proprietary complementary
bipolar process, which ensures very similar DC and AC
characteristics for the output devices Q24 and Q26.
When overdriven, the amplifier draws input current that
exceeds the normal input bias current. Figures 2 and 3
show some typical overdrive currents as a function of
input voltage. The input current must be less than 1mA of
positive overdrive or less than 7mA of negative overdrive,
for the phase reversal protection to work properly. When
the amplifier is severely overdriven, an external resistor
should be used to limit the overdrive current. In addition
to overdrive protection, the amplifier is protected against
ESD strokes up to 4kV on all pins.
A simple comparator Q5 steers current from current
source I1 between the two input stages. When the input
common mode voltage VCM is near the negative supply,
Q5 is reverse biased, and I1 becomes the tail current for
the PNP differential pair Q1/Q2. At the other extreme,
when VCM is within about 1.3V from the positive supply,
Q5 diverts I1 to the current mirror D3/Q6, which furnishes
the tail current for the NPN differential pair Q3/Q4.
110
MEASURED AS A
FOLLOWER
100
90
80
70
60
50
40
30
20
10
0
The collector currents of the two input pairs are combined
in the second stage, consisting of Q7 through Q11. Most
of the voltage gain in the amplifier is contained in this
stage. Differential amplifier Q14/Q15 buffers the output of
the second stage, converting the output voltage to differ-
ential currents. The differential currents pass through
current mirrors D4/Q17 and D5/Q16, and are converted to
differential voltages by Q18 and Q19. These voltages are
also buffered and applied to the output Darlington pairs
Q23/Q24 and Q25/Q26. Capacitors C1 and C2 form local
feedback loops around the output devices, lowering the
output impedance at high frequencies.
+
–
T = 25°C
T = 85°C
T = 70°C
T = –55°C
–500
–300
–100 V 100
300
500
S
COMMON MODE VOLTAGE RELATIVE TO
POSITIVE SUPPLY (mV)
LT1366 F02
Input Offset Voltage
Figure 2. Input Bias Current vs Common Mode Voltage
Since the amplifier has two input stages, the input offset
voltage changes depending upon which stage is active.
The input offsets are random, but bounded voltages.
When the amplifier switches between stages, offset volt-
ages may go up, down, or remain flat; but will not exceed
the guaranteed limits. This behavior is illustrated in three
distribution plots of input offset voltage in the Typical
Performance Characteristics section.
0
MEASURED AS A FOLLOWER
–10
+
–20
–
–30
–40
T = –55°C T = 25°C
T = 70°C
–50
–60
T = 85°C
–70
–80
–90
–100
–110
Overdrive Protection
Two circuits prevent the output from reversing polarity
when the input voltage exceeds the common mode range.
When the noninverting input exceeds the positive supply
by approximately 300mV, the clamp transistor Q12 (Fig-
ure 1) turns on, pulling the output of the second stage low,
which forces the output high. For inputs below the nega-
tive supply, diodes D1 and D2 turn on, overcoming the
saturation of the input pair Q1/Q2.
–800
–600
–400
–200
V
S
200
COMMON MODE VOLTAGE RELATIVE TO
NEGATIVE SUPPLY (mV)
LT1366 F03
Figure 3. Input Bias Current vs Common Mode Voltage
12
Linear [ Linear ]
