THS4031 Datasheet, PDF (25/38 Pages) Texas Instruments – 100-MHz LOW-NOISE HIGH-SPEED AMPLIFIERS

THS4031, THS4032
100-MHz LOW-NOISE HIGH-SPEED AMPLIFIERS
APPLICATION INFORMATION
SLOS224C – JULY 1999 – REVISED APRIL 2000
unity-gain concerns (continued)
Original Open
Loop Response
Gain (dB)
See Note A
RF
RC
CC
VI
_
+
VO
Effective
RL
CL
Compensated
O.L. Response
(without CC)
0
Unstable
Intersection
f
Stable
Intersection
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.
360 Ω
Input
360 Ω
_
THS403x
+
20 Ω
Output
CLOAD
Figure 49. Driving a Capacitive Load
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