Datasheet

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RECOMMENDED R
F
(:)
GAIN (V/V)
INVERTING (SOT23-6)
NON-
INVERTING
(
SOT
23
-
6
)
INVERTING (SOIC
)
NON-INVERTING
(SOIC)
LMH6703
www.ti.com SNOSAF2D –FEBRUARY 2005–REVISED MARCH 2013
Figure 29. Recommended R
F
vs. Gain
Since a current feedback amplifier is dependant on the value of R
F
to provide frequency compensation and since
the value of R
F
can be used to optimize the frequency response, different packages use different R
F
values. As
shown in Figure 29, Recommended R
F
vs. Gain, the SOT-23-6 and the SOIC package use different values for
the feedback resistor, R
F
. Since each application is slightly different, it is worth some experimentation to find the
optimal R
F
for a given circuit. In general, a value of R
F
that produces ≈0.1 dB of peaking is the best compromise
between stability and maximum bandwidth. Note that it is not possible to use a current feedback amplifier with
the output shorted directly to the inverting input. The buffer configuration of the LMH6703 requires a 560Ω (390Ω
for SOIC package) feedback resistor for stable operation.
The LMH6703 was optimized for high speed operation. As shown in Figure 29, the suggested value for R
F
decreases for higher gains. Due to the output impedance of the input buffer, there is a practical limit for how
small R
F
can go, based on the lowest practical value of R
G
. This limitation applies to both inverting and non
inverting configurations. For the LMH6703 the input resistance of the inverting input is approximately 30Ω and
20Ω is a practical (but not hard and fast) lower limit for R
G
. The LMH6703 begins to operate in a gain bandwidth
limited fashion in the region when R
G
is nearly equal to the input buffer impedance. Note that the amplifier will
operate with R
G
values well below 20Ω, however results may be substantially different than predicted from ideal
models. In particular the voltage potential between the Inverting and Non-Inverting inputs cannot be expected to
remain small.
Inverting gain applications that require impedance matched inputs may limit gain flexibility somewhat (especially
if maximum bandwidth is required). The impedance seen by the source is R
G
|| R
T
(R
T
is optional). The value of
R
G
is R
F
/Gain. Thus for a SOT-23 in a gain of —5V/V, an R
F
of 460Ω is optimum and R
G
is 92Ω. Without a
termination resistor, R
T
, the input impedance would equal R
G
, 92Ω. Using an R
T
of 109Ω will set the input
resistance to match a 50Ω source. Note that source impedances greater then R
G
cannot be matched in the
inverting configuration.
For more information see Application Note OA-13 (SNOA366) which describes the relationship between R
F
and
closed-loop frequency response for current feedback operational amplifiers. The value for the inverting input
impedance for the LMH6703 is approximately 30Ω. The LMH6703 is designed for optimum performance at gains
of +1 to +10 V/V and −1 to −9 V/V. Higher gain configurations are still useful, however, the bandwidth will fall as
gain is increased, much like a typical voltage feedback amplifier.
The LMH6703 data sheet shows both SOT-23-6 and SOIC data in the Electrical Characteristic section to aid in
selecting the right package. The Typical Performance Characteristics section shows SOT-23-6 package plots
only.
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