LMH6517 Datasheet, PDF (17/34 Pages) National Semiconductor (TI) – Multi Standard, IF and Baseband, Dual, DVGA

LMH6517
www.ti.com
SNOSB19K – NOVEMBER 2008 – REVISED MARCH 2013
Excel can also be used for simple circuits; however, the “Analysis ToolPak” add-in must be installed to calculate
complex numbers.
OUTPUT CHARACTERISTICS
The LMH6517 has a low impedance output very similar to a traditional Op-amp output. This means that nearly
any load can be driven with minimal gain loss. Matching load impedance for proper termination of filters is as
easy as inserting the proper value of resistor between the filter and the amplifier. This flexibility makes system
design and gain calculations very easy. The LMH6517 was designed to run from a single 5V supply. In spite of
this low supply voltage the LMH6517 is still able to deliver very high power gains when driving low impedance
loads.
The ability of the LMH6517 to drive low impedance loads creates an opportunity to greatly increase power gain, if
required. One example of using power gain to offset filter loss is shown in Figure 59. A graph showing power
gain over various load conditions is shown below in Figure 44. This graph clearly shows the reduction in power
gain caused by back termination. While many RF amplifiers have internal resistance and deliver maximum power
into a matched load the LMH6517 has an output resistance very near to zero Ohms. The graph shows that
maximum power transfer does indeed occur with a load of nearly zero Ohms. Another useful feature of the graph
is the ability to determine how much gain can be recovered by dropping load resistance when it is necessary to
back terminate either a transmission line or a filter.
40
NON TERMINATED
35
30
25
20
BACK TERMINATED
15
10
5
1
10
100
1000
LOAD RESISTANCE (:)
Figure 44. Power Gain vs Load
Note 6dB power loss when adding load matching resistors.
Here is an example of how to use the chart in Figure 44. In a system it is desired to have at least 20dB of
maximum gain from the amplifier input to output. The system noise and harmonic distortion requirements dictate
a 200 Ohm filter between the amplifier and the ADC. Using the chart we can see that a back terminated 200
Ohm filter will result in a net 16 dB of gain at the filter input. To recover this loss it is possible to use a 1:4 balun
to drop the load condition of the filter to 50 Ohms at the amplifier output. This gives an additional 6dB of power
gain. Since the transformer has a power loss of approximately 1dB we end up with 21dB of gain at the filter
output instead of 16dB. See Figure 59 for an example where the filter performs the impedance transformation
function.
The LMH6517, like most high frequency amplifiers, is sensitive to loading conditions on the output. Load
conditions that include small amounts of capacitance connected directly to the output can cause stability
problems. In order to ensure output stability resistors should be connected directly at the amplifier output
followed by a small capacitor. This circuit sets a dominant pole that will cancel out board parasitics in most
applications. An example of this is shown in figure Figure 45 . In this example the amplifier and ADC are less
than 0.1 wavelength apart and do not require a terminated transmission line. A more sophisticated filter may
require better impedance matching. Some example filters are shown later.
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