13

LT1498/LT1499

without oscillation at unity gain. When driving a heavy

capacitive load, the bandwidth is reduced to maintain

stability. Figures 2a and 2b illustrate the stability of the

device for small-signal and large-signal conditions with

capacitive loads. Both the small-signal and large-signal

transient response with a 10nF capacitive load are well

behaved.

Feedback Components

To minimize the loading effect of feedback, it is possible to

use the high value feedback resistors to set the gain.

However, care must be taken to insure that the pole formed

by the feedback resistors and the total input capacitance at

the inverting input does not degrade the stability of the

amplifier. For instance, the LT1498/LT1499 in a noninvert-

ing gain of 2, set with two 30k resistors, will probably

oscillate with 10pF total input capacitance (5pF input

capacitance + 5pF board capacitance). The amplifier has a

2.5MHz crossing frequency and a 60

°phasemarginat6dB

of gain. The feedback resistors and the total input capaci-

tance create a pole at 1.06MHz that induces 67

° of phase

shift at 2.5MHz! The solution is simple, either lower the

value of the resistors or add a feedback capacitor of 10pF

of more.

1A Voltage Controlled Current Source

1A Voltage Controlled Current Sink

–

+

1/2 LT1498

1k

500pF

1498/99 TA03

Si9430DY

100

Ω

0.5

Ω

1k

0.5

Ω

1k

–

+

1/2 LT1498

500pF

1498/99 TA04

Si9410DY

100

Ω

0.5

Ω

1k

0.5

Ω

Figure 2b. LT1498 Large-Signal Response

1498/99 F02a

Figure 2a. LT1498 Small-Signal Response

1498/99 F02b