THS4011 Datasheet, PDF (17/30 Pages) Texas Instruments – 290-MHz LOW-DISTORTION HIGH-SPEED AMPLIFIERS

THS4011, THS4012
290-MHz LOW-DISTORTION HIGH-SPEED AMPLIFIERS
APPLICATION INFORMATION
SLOS216B – JUNE 1999 – FEBRUARY 2000
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 THS401x 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 27. 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.
1.3 kΩ
Input
1.3 kΩ
_
THS401x
+
20 Ω
Output
CLOAD
Figure 27. Driving a Capacitive Load
offset nulling
The THS401x 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 THS4011. 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 28.
VCC+
+
THS4011
_
10 kΩ
0.1 µF
0.1 µF
VCC –
Figure 28. Offset Nulling Schematic
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