THS4041 Datasheet, PDF (16/28 Pages) Texas Instruments – 165-MHz C-STABLE HIGH-SPEED AMPLIFIERS

THS4041, THS4042
165-MHz C-STABLE HIGH-SPEED AMPLIFIERS
SLOS237B– MAY 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 THS404x has been internally compensated to maximize its bandwidth and
slew rate performance. Typically 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.
However, the THS404x has added internal circuitry that senses a capacitive load and adds extra compensation
to the internal dominant pole. As the capacitive load increases, the amplifier remains stable. But, it is not
uncommon to see a small amount of peaking in the frequency response. There are typically two ways to
compensate for this. The first is to simply increase the gain of the amplifier. This helps by increasing the phase
margin to keep peaking minimized. The second is to place an isolation resistor in series with the output of the
amplifier, as shown in Figure 65. 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. For more information about driving
capacitive loads, refer to the Output Resistance and Capacitance section of the Parasitic Capacitance in Op
Amp Circuits Application Report (literature number: SLOA013).
1.3 kΩ
Input
1.3 kΩ
_
THS404x
+
20 Ω
Output
CLOAD
Figure 65. Driving a Capacitive Load for Extra Stability
offset nulling
The THS404x 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 THS4041. 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 66.
VCC+
+
THS4041
_
0.1 µF
10 kΩ
0.1 µF
VCC –
Figure 66. Offset Nulling Schematic
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