THS4601_14 Datasheet, PDF (19/32 Pages) Texas Instruments – WIDEBAND, FET-INPUT OPERATIONAL AMPLIFIER

THS4601
SLOS388B − OCTOBER 2001 − REVISED JUNE 2002
APPLICATION INFORMATION
selection of feedback resistors
Feedback resistor selection can have a significant effect on the performance of the THS4601 in a given
application, especially in configurations with low closed-loop gain. If the amplifier is configured for unity gain,
the output should be directly connected to the inverting input. Any resistance between these two points interacts
with the input capacitance of the amplifier and causes an additional pole in the frequency response. For
non-unity gain configurations, low resistances are desirable for flat frequency response. However, care must
be taken not to load the amplifier too heavily with the feedback network if large output signals are expected. In
most cases, a tradeoff will be made between the frequency response characteristics and the loading of the
amplifier. For a gain of 2, a 250 Ω feedback resistor is a suitable operating point from both perspectives.
If resistor values are chosen too large, the THS4601 is subject to oscillation problems. For example, an inverting
amplifier configuration with a 1-kΩ gain resistor and a 1-kΩ feedback resistor develops an oscillation due to the
interaction of the large resistors with the input capacitance. In low gain configurations, avoid feedback resistors
this large or anticipate using an external compensation scheme to stabilize the circuit.
overdrive recovery
The THS4601 has an overdrive recovery period when the output is driven close to one power supply rail or the
other. The overdrive recovery time period is dependent upon the magnitude of the overdrive and whether the
output is driven towards the positive or the negative power supply. The four graphs shown here depict the
overdrive recovery time in two cases, an attempted 28 V_PP signal on the output and an attempted 30 V_PP signal
on the output. Note that in both of these cases, the output does not achieve these levels as the output voltage
swing is limited to less than these values, but these values are representative of the desired signal swing on
the output for the given inputs. As shown in the figures, the recovery period increases as the magnitude of the
overdrive increases, with the worst case recovery occurring with the negative rail. The recovery times are
summarized in Table 2.
Table 2. Overdrive Recovery Characteristics
VOLTAGE RAIL
+VS
−VS
+VS
−VS
IDEAL OUTPUT SWING
(VPP)
28
28
30
30
OVERDRIVE RECOVERY TIME
(ns)
320
340
540
680
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