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

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

1497 F02

Figure 3. Large-Signal Response

1497 F03

Figure 4. Large-Signal Response

1497 F04

One Amplifier

SI

PLIFIED SCHE ATIC

1497 SS

+IN

–IN

Q2

Q1

Q4

Q11

Q12

Q14

Q9

Q8

Q7

Q13

Q6

Q3

Q5

Q10