THS4031, THS4032

100-MHz LOW-NOISE HIGH-SPEED AMPLIFIERS

SLOS224C – JULY 1999 – REVISED APRIL 2000

25

POST OFFICE BOX 655303

APPLICATION INFORMATION

unity-gain concerns (continued)

_

+

RC

CC

VI

RF

CL

VO

RL

Gain (dB)

Original Open

Loop Response

Unstable

Intersection

Stable

Intersection

Effective

Compensated

O.L. Response

(without CC)

0

See Note A

f

NOTE A: The difference is due to 1+RF/RC noise gain.

Figure 48. Unity Gain Compensation

driving a capacitive load

Driving capacitive loads with high-performance amplifiers is not a problem as long as certain precautions are

taken. The first is to realize that the THS403x has been internally compensated to maximize its bandwidth and

slew-rate performance. When the amplifier is compensated in this manner, capacitive loading directly on the

output will decrease the phase margin of the device leading to high-frequency ringing or oscillations. Therefore,

for capacitive loads of greater than 10 pF, it is recommended that a resistor be placed in series with the output

of the amplifier, as shown in Figure 49. A minimum value of 20

Ω should work well for most applications. For

example, in 75-

Ω transmission systems, setting the series resistor value to 75 Ω both isolates any capacitance

loading and provides the proper line impedance matching at the source end.

+

_

THS403x

CLOAD

360

Ω

Input

Output

360

Ω

20

Ω

Figure 49. Driving a Capacitive Load