Datasheet
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SBOS270C − AUGUST 2003 − REVISED AUGUST 2008
www.ti.com
20
TI web site (www.ti.com). This model does a good job of
predicting small-signal AC and transient performance un-
der a wide variety of operating conditions, but does not do
as well in predicting the harmonic distortion or dG/dP char-
acteristics. This model does not attempt to distinguish be-
tween the package types in small-signal AC performance,
nor does it attempt to simulate channel-to- channel cou-
pling.
OPERATING SUGGESTIONS
SETTING RESISTOR VALUES TO OPTIMIZE
BANDWIDTH
A current-feedback op amp such as the OPA2674 can hold
an almost constant bandwidth over signal gain settings
with the proper adjustment of the external resistor values,
which are shown in the Typical Characteristics; the small-
signal bandwidth decreases only slightly with increasing
gain. These characteristic curves also show that the feed-
back resistor is changed for each gain setting. The resistor
values on the inverting side of the circuit for a current-feed-
back op amp can be treated as frequency response com-
pensation elements, whereas the ratios set the signal gain.
Figure 10 shows the small-signal frequency response
analysis circuit for the OPA2674.
V
O
R
G
V
I
R
I
Z
(S)
I
ERR
α
R
F
I
ERR
Figure 10. Current-Feedback Transfer Function
Analysis Circuit
The key elements of this current-feedback op amp model
are:
α = buffer gain from the noninverting input to the
inverting input
R
I
= buffer output impedance
I
ERR
= feedback error current signal
Z(s) = frequency dependent open
-loop transimpe-
dance gain from I
ERR
to V
O
NG + NoiseGain + 1 )
R
F
R
G
The buffer gain is typically very close to 1.00 and is normal-
ly neglected from signal gain considerations. This gain,
however, sets the CMRR for a single op amp differential
amplifier configuration. For a buffer gain of α < 1.0, the
CMRR = −20 • log(1 − α)dB.
R
I
, the buffer output impedance, is a critical portion of the
bandwidth control equation. The OPA2674 inverting out-
put impedance is typically 22Ω.
A current-feedback op amp senses an error current in the
inverting node (as opposed to a differential input error volt-
age for a voltage-feedback op amp) and passes this on to
the output through an internal frequency dependent trans-
impedance gain. The Typical Characteristics show this
open-loop transimpedance response, which is analogous
to the open-loop voltage gain curve for a voltage-feedback
op amp. Developing the transfer function for the circuit of
Figure 10 gives Equation 14:
V
O
V
I
+
a
ǒ
1 )
R
F
R
G
Ǔ
1 )
R
F
)R
I
ǒ1)
R
F
R
G
Ǔ
Z(s)
+
a NG
1 )
R
F
)R
I
NG
Z(s)
This is written in a loop-gain analysis format, where the er-
rors arising from a non-infinite open-loop gain are shown
in the denominator. If Z(s) were infinite over all frequen-
cies, the denominator of Equation 14 reduces to 1 and the
ideal desired signal gain shown in the numerator is
achieved. The fraction in the denominator of Equation 14
determines the frequency response. Equation 15 shows
this as the loop-gain equation:
Z(s)
R
F
) R
I
NG
+ LoopGain
If 20 log(R
F
+ NG × R
I
) is drawn on top of the open-loop
transimpedance plot, the difference between the two
would be the loop gain at a given frequency. Eventually,
Z(s) rolls off to equal the denominator of Equation 15, at
which point the loop gain has reduced to 1 (and the curves
have intersected). This point of equality is where the ampli-
fier closed-loop frequency response given by Equation 14
starts to roll off, and is exactly analogous to the frequency
at which the noise gain equals the open-loop voltage gain
for a voltage-feedback op amp. The difference here is that
the total impedance in the denominator of Equation 15
may be controlled somewhat separately from the desired
signal gain (or NG). The OPA2674 is internally compen-
sated to give a maximally flat frequency response for R
F
= 402Ω at NG = 4 on ±6V supplies. Evaluating the denomi-
(14)
(15)