Datasheet

R = 663 NGxRW -
F I
V
O
V
I
=
=
R R+
F I
1+
Z
(S)
1+
R
F
R
G
a
R R NG+
F I
Z
(S)
1+
aNG
1+
R
F
R
G
NG
=
1+
R
F
R
G
= LoopGain
Z
(S)
R +R
F I
NG
600
550
500
450
400
350
300
250
200
150
100
NoiseGain
0 2010 155
FeedbackResistor( )W
OPA4872
SBOS346C –JUNE 2007–REVISED MARCH 2011 www.ti.com
R
I
, the buffer output impedance, is a critical portion of The OPA4872 is internally compensated to give a
the bandwidth control equation. R
I
for the OPA4872 is maximally flat frequency response for R
F
= 523Ω at
typically about 30Ω. A current-feedback op amp NG = 2 on ±5V supplies. Evaluating the denominator
senses an error current in the inverting node (as of Equation 5 (which is the feedback transimpedance)
opposed to a differential input error voltage for a gives an optimal target of 663Ω. As the signal gain
voltage-feedback op amp) and passes this on to the changes, the contribution of the
output through an internal frequency dependent NG × R
I
term in the feedback transimpedance will
transimpedance gain. The Typical Characteristics change, but the total can be held constant by
show this open-loop transimpedance response. This adjusting R
F
. Equation 6 gives an approximate
open-loop response is analogous to the open-loop equation for optimum R
F
over signal gain:
voltage gain curve for a voltage-feedback op amp.
(6)
Developing the transfer function for the circuit of
Figure 32 gives Equation 4:
As the desired signal gain increases, this equation
will eventually predict a negative R
F
. A somewhat
subjective limit to this adjustment can also be set by
holding R
G
to a minimum value of 20Ω. Lower values
load both the buffer stage at the input and the output
stage, if R
F
gets too low, actually decreasing the
bandwidth. Figure 33 shows the recommended R
F
versus NG for ±5V operation. The values for R
F
versus gain shown here are approximately equal to
where:
the values used to generate the Typical
Characteristics. They differ in that the optimized
values used in the Typical Characteristics are also
(4)
correcting for board parasitics not considered in the
simplified analysis leading to Equation 5. The values
This formula is written in a loop-gain analysis format,
shown in Figure 33 give a good starting point for
where the errors arising from a noninfinite open-loop
design where bandwidth optimization is desired.
gain are shown in the denominator. If Z
(S)
were
infinite over all frequencies, the denominator of
Equation 4 would reduce to 1 and the ideal desired
signal gain shown in the numerator would be
achieved. The fraction in the denominator of
Equation 4 determines the frequency response.
Equation 5 shows this as the loop-gain equation:
(5)
If 20 × log(R
F
+ NG × R
I
) were drawn on top of the
open-loop transimpedance plot, the difference
between the two calculations would be the loop gain
at a given frequency. Eventually, Z
(S)
rolls off to equal
the denominator of Equation 5, at which point the
loop gain reduces to 1 (and the curves intersect).
This point of equality is where the amplifier
Figure 33. Feedback Resistor vs Noise Gain
closed-loop frequency response given by Equation 4
starts to roll off, and is exactly analogous to the
frequency at which the noise gain equals the
The total impedance going into the inverting input
open-loop voltage gain for a voltage-feedback op
may be used to adjust the closed-loop signal
amp. The difference here is that the total impedance
bandwidth. Inserting a series resistor between the
in the denominator of Equation 5 may be controlled
inverting input and the summing junction increases
somewhat separately from the desired signal gain (or
the feedback impedance (denominator of Equation 4),
NG).
decreasing the bandwidth.
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