LMH6657_16 Datasheet, PDF (17/35 Pages) Texas Instruments – 270-MHz Single Supply, Single and Dual Amplifiers

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7 Detailed Description
LMH6657, LMH6658
SNOSA35G – AUGUST 2002 – REVISED JULY 2015
7.1 Overview
7.1.1 Large Signal Behavior
The LMH6657 and LMH6658 are large-bandwidth, fast slew rate, voltage feedback operational ampplifers ideal
for high-speed, large signal applications. The low input referred voltage noise in conjunction with an input voltage
range, which extends below V–, eases the adoption of this part in applications having a tiny signal at or near
system ground, as well as other high-speed, low-distortion, and low-noise systems. Also, the large Gain
Bandwidth Product allows high gain operation that does not compromise speed.
7.2 Feature Description
The LMH6657 and LMH6658 input stage is designed to provide excess overdrive when needed. This occurs
when fast input signal excursions cannot be followed by the output stage. In these situations, the device
encounters larger input signals than would be encountered under normal closed loop conditions. The LMH6657
and LMH6658 input stage is designed to take advantage of this "input overdrive" condition. The larger the
amount of this overdrive, the greater is the speed with which the output voltage can change. Here is a plot of
how the output slew rate limitation varies with respect to the amount of overdrive imposed on the input:
800
V_S = ±5V
700
600
500
400
300
200
100
0
0.00
1.00
2.00
3.00
INPUT OVERDRIVE (V)
Figure 48. Plot Showing the Relationship Between Slew Rate and Input Overdrive
To relate the explanation above to a practical example, consider the following application example. Consider the
case of a closed loop amplifier with a gain of −1 amplifying a sinusoidal waveform. From the plot of Output vs.
Input (Figure 8), with a 30-MHz signal and 7V_PP input signal, it can be seen that the output will be limited to a
swing of 6.9 V_PP. From the frequency Response plot it can be seen that the inverting gain of −1 has a −32°
output phase shift at this frequency.
It can be shown that this setup will result in about 1.9 V_PP differential input voltage corresponding to 650 V/μs of
slew rate from Figure 48, above (SR = V_O(pp) × π × f = 650V/μs)
Note that the amount of overdrive appearing on the input for a given sinusoidal test waveform is affected by the
following:
• Output swing
• Gain setting
• Input/output phase relationship for the given test frequency
• Amplifier configuration (inverting or noninverting)
Due to the higher frequency phase shift between input and output, there is no closed form solution to input
overdrive for a given input. Therefore, Figure 48 is not very useful by itself in determining the output swing.
The following plots aid in predicting the output transition time based on the amount of swing required for a given
gain setting.
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