THS4021, THS4022

350-MHz LOW-NOISE HIGH-SPEED AMPLIFIERS

SLOS265B – SEPTEMBER 1999 – REVISED FEBRUARY 2000

12

POST OFFICE BOX 655303

APPLICATION INFORMATION

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 THS402x 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 device’s phase margin 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 33. 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.

+

_

THS402x

CLOAD

1 k

Ω

Input

Output

50

Ω

20

Ω

Figure 33. Driving a Capacitive Load

offset nulling

The THS402x has very low input offset voltage for a high-speed amplifier. However, if additional correction is

required, an offset nulling function has been provided on the THS4021. The input offset can be adjusted by

placing a potentiometer between terminals 1 and 8 of the device and tying the wiper to the negative supply. This

is shown in Figure 34.

_

+

THS402x

VCC –

VCC+

0.1

µF

0.1

µF

10 k

Ω

Figure 34. Offset Nulling Schematic