LMH6657 Datasheet, PDF (15/29 Pages) National Semiconductor (TI) – 270MHz Single Supply, Single & Dual Amplifiers

LMH6657, LMH6658
www.ti.com
APPLICATION SECTION
SNOSA35F – AUGUST 2002 – REVISED MARCH 2013
LARGE SIGNAL BEHAVIOR
The LMH6657/6658 is specially designed to handle large output swings, such as those encountered in video
waveforms, without being slew rate limited. With 5V supply, the LMH6657/6658 slew rate limit is larger than that
might be necessary to make full allowable output swing excursions. Therefore, the large signal frequency
response is dominated by the small signal characteristics, rather than the conventional limitation imposed by
slew rate limit.
The LMH6657/6658 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 LMH6657/6658 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 50. 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 (Typical Performance Characteristics section), with a 30MHz signal and 7V_PP input signal, it can be seen
that the output will be limited to a swing of 6.9V_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 1.9V_PP differential input voltage corresponding to 650V/μs of slew rate from Figure 50, 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 non-inverting)
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 50 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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