Datasheet

R =
O
2
75 (n 1)+804´ -
´ 1+
[75 (n 1)+804]´ -
2
241200
- 1
R
F
V
O
R
G
R
I
Z i
(S) ERR
i
ERR
a
V
I
OPA4872
www.ti.com SBOS346C –JUNE 2007–REVISED MARCH 2011
Solving for R
O
, with n devices connected together, OPERATING SUGGESTIONS
results in Equation 3:
SETTING RESISTOR VALUES TO OPTIMIZE
BANDWIDTH
The output stage of the OPA4872 is a
current-feedback op amp, meaning it can hold an
almost constant bandwidth over signal gain settings
(3)
with the proper adjustment of the external resistor
Results for n varying from 2 to 6 are given in Table 3.
values. This performance is shown in the Typical
Characteristic curves; the small-signal bandwidth
Table 3. Series Resistance versus
decreases only slightly with increasing gain. These
Number of Parallel Outputs
curves also show that the feedback resistor has been
changed for each gain setting. The resistor values on
NUMBER OF OPA4872s R
O
(Ω)
the feedback path can be treated as frequency
2 69
response compensation elements while the ratio sets
3 63.94
the signal gain of the feedback resistor divided by the
4 59.49
gain resistor. Figure 32 shows the small-signal
5 55.59
frequency response analysis circuit for a current
feedback amplifier.
6 52.15
The two major limitations of this circuit are the device
requirements for each OPA4872 and the acceptable
return loss resulting from the mismatch between the
load and the matching resistor.
DESIGN-IN TOOLS
DEMONSTRATION FIXTURE
A printed circuit board (PCB) is available to assist in
the initial evaluation of circuit performance using the
OPA4872. The fixture is offered free of charge as an
unpopulated PCB, delivered with a user's guide. The
summary information for this fixture is shown in
Table 4.
Figure 32. Recommended Feedback Resistor
versus Noise Gain
Table 4. OPA4872 Demonstration Fixture
LITERATURE
PRODUCT PACKAGE ORDERING NUMBER NUMBER
The key elements of this current-feedback op amp
model are:
OPA4872 SO-14 DEM-OPA-SO-1E SBOU045
α → Buffer gain from the noninverting input to the
The demonstration fixture can be requested at the
inverting input
Texas Instruments web site at (www.ti.com) through
the OPA4872 product folder.
R
I
→ Buffer output impedance
i
ERR
→ Feedback error current signal
MACROMODELS AND APPLICATIONS SUPPORT
Z
(s)
→ Frequency-dependent open-loop
Computer simulation of circuit performance using
transimpedance gain from i
ERR
to V
O
SPICE is often useful when analyzing the
performance of analog circuits and systems. This
The buffer gain is typically very close to 1.00 and is
practice is particularly true for video and RF amplifier
normally neglected from signal gain considerations. It
circuits, where parasitic capacitance and inductance
will, however, set the CMRR for a single op amp
can have a major effect on circuit performance. A
differential amplifier configuration. For a buffer gain
SPICE model for the OPA4872 is available through
α < 1.0, the CMRR = –20 × log (1 – α) dB.
the Texas Instruments web site at www.ti.com. This
model does a good job of predicting small-signal ac
and transient performance under a wide variety of
operating conditions. It does not do as well in
predicting the harmonic distortion or dG/dP
characteristics.
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Product Folder Link(s): OPA4872