LMH6505 Datasheet, PDF (12/20 Pages) National Semiconductor (TI) – Wideband, Low Power, Linear-in-dB, Variable Gain Amplifier

Application Information
GENERAL DESCRIPTION
The key features of the LMH6505 are:
• Low power
• Broad voltage controlled gain and attenuation range
(From A_VMAX down to complete cutoff)
• Bandwidth independent, resistor programmable gain
range (R_G)
• Broad signal and gain control bandwidths
• Frequency response may be adjusted with R_F
• High impedance signal and gain control inputs
The LMH6505 combines a closed loop input buffer (“X1”
Block in Figure 1), a voltage controlled variable gain cell
(“MULT” Block) and an output amplifier (“CFA” Block). The
input buffer is a transconductance stage whose gain is set by
the gain setting resistor, R_G. The output amplifier is a current
feedback op amp and is configured as a transimpedance
stage whose gain is set by, and is equal to, the feedback
resistor, R_F. The maximum gain, A_VMAX, of the LMH6505 is
defined by the ratio: K · R_F/R_G where “K” is the gain multiplier
with a nominal value of 0.940. As the gain control input (V_G)
changes over its 0 to 2V range, the gain is adjusted over a
range of about 80 dB relative to the maximum set gain.
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FIGURE 1. LMH6505 Typical Application and Block
Diagram
SETTING THE LMH6505 MAXIMUM GAIN
Eq. 1
Although the LMH6505 is specified at A_VMAX = 9.4 V/V, the
recommended A_VMAX varies between 2 and 100. Higher
gains are possible but usually impractical due to output
offsets, noise and distortion. When varying A_VMAX several
tradeoffs are made:
R_G: determines the input voltage range
R_F: determines overall bandwidth
The amount of current which the input buffer can source/sink
into R_G is limited and is given in the I_{RG_MAX} specification.
This sets the maximum input voltage:
Eq. 2
As the I_{RG_MAX} limit is approached with increasing the input
voltage or with the lowering of R_G, the device’s harmonic
distortion will increase. Changes in R_F will have a dramatic
effect on the small signal bandwidth. The output amplifier of
the LMH6505 is a current feedback amplifier (CFA) and its
bandwidth is determined by R_F. As with any CFA, doubling
the feedback resistor will roughly cut the bandwidth of the
device in half. For more about CFA’s, see the basic tutorial,
OA-20, “Current Feedback Myths Debunked,” or a more
rigorous analysis, OA-13, “Current Feedback Amplifier Loop
Gain Analysis and Performance Enhancements.”
OTHER CONFIGURATIONS
1) Single Supply Operation
The LMH6505 can be configured for use in a single supply
environment. Doing so requires the following:
a) Bias pin 4 and R_G to a “virtual half supply” somewhere
close to the middle of V⁺ and V^− range. The other end of
R_G is tied to pin 3. The “virtual half supply” needs to be
capable of sinking and sourcing the expected current flow
through R_G.
b) Ensure that V_G can be adjusted from 0V to 2V above the
“virtual half supply”.
c) Bias the input (pin 2) to make sure that it stays within the
range of 2V above V^− to 2V below V⁺. See the Input
Voltage Range specification in the Electrical Characteris-
tics table. This can be accomplished by either DC biasing
the input and AC coupling the input signal, or alterna-
tively, by direct coupling if the output of the driving stage
is also biased to half supply.
Arranged this way, the LMH6505 will respond to the current
flowing through R_G. The gain control relationship will be
similar to the split supply arrangement with V_G measured
with reference to pin 4. Keep in mind that the circuit de-
scribed above will also center the output voltage to the
“virtual half supply voltage.”
2) Arbitrarily Referenced Input Signal
Having a wide input voltage range on the input (pin 2)
(±3V typical), the LMH6505 can be configured to control the
gain on signals which are not referenced to ground (e.g. Half
Supply biased circuits, etc.). We will call this node the “ref-
erence node”. In such cases, the other end of R_G which is
the side not tied to pin 3 can be tied to this reference node so
that R_G will “look at” the difference between the signal and
this reference only. Keep in mind that the reference node
needs to source and sink the current flowing through R_G.
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