LMH6505 Datasheet, PDF (11/18 Pages) National Semiconductor (TI) – Wideband, Low Power, Linear-in-dB, Variable Gain Amplifier

Gain vs. V_G Step
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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 re-
sistor, 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
(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:
(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 dis-
tortion 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 band-
width 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 CFAs, 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 Volt-
age Range specification in the Electrical Characteristics
table. This can be accomplished by either DC biasing the
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