THS4021 Datasheet, PDF (12/22 Pages) Texas Instruments – 350-MHz LOW-NOISE HIGH-SPEED AMPLIFIERS

THS4021, THS4022
350-MHz LOW-NOISE HIGH-SPEED AMPLIFIERS
SLOS265B – SEPTEMBER 1999 – REVISED FEBRUARY 2000
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.
1 kΩ
Input
50 Ω
_
THS402x
+
20 Ω
Output
CLOAD
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.
VCC+
+
THS402x
_
10 kΩ
0.1 µF
0.1 µF
VCC –
Figure 34. Offset Nulling Schematic
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