10
LT1497
the normally active emitter of Q2 will turn off as the entire
current available from the current source is used to slew
the base of Q3. The base of Q4 is driven by Q1 without slew
limitation. When the differential input voltage exceeds two
diode drops (about 1.4V) the extra clamp emitter on Q1
turns on and drives the base of Q3 directly. Once the base
of Q3 has been driven within 1.4V of its final value, the
clamp emitter of Q1 turns off and the node must finish
slewing using the current source.
This effect can be seen in Figure 2 which shows the large
signal behavior in a gain of 1 on ±15V supplies. The
clamping action enhances the slew rate beyond the input
limitation, but always leads to slew overshoot after the
clamps turn off. Figure 3 shows that for higher gain
APPLICATIONS INFORMATION
WUU
U
configurations there is much less slew rate enhancement
because the input only moves 2V, barely enough to turn on
the input clamps. In inverting configurations as shown in
Figure 4 the noninverting input does not move so there is
no input slew rate limitation. Slew overshoot is due to
capacitance on the inverting input and can be reduced with
a larger feedback resistor.
The output slew rate is set by the value of the feedback
resistors and the internal capacitance. Larger feedback
resistors will reduce the slew rate as will lower supply
voltages, similar to the way the bandwidth is reduced.
The larger feedback resistors will also cut back on slew
overshoot.
Figure 2. Large-Signal Response
A
V
= 1
V
S
= ±15V
R
F
= 560Ω
R
L
= 100Ω
1497 F02
Figure 3. Large-Signal Response
A
V
= 10
V
S
= ±15V
R
F
= 560Ω
R
L
= 100Ω
1497 F03
R
G
= 62Ω
Figure 4. Large-Signal Response
A
V
= –1
V
S
= ±15V
R
F
= R
G
= 560Ω
R
L
= 100Ω
1497 F04
One Amplifier
SI PLIFIED SCHE ATIC
WW
1497 SS
+IN
–IN
V
OUT
V
+
V
–
Q2
Q1
Q4
Q11
Q12
Q14
Q9
Q8
Q7
Q13
Q6
Q3
Q5
Q10
