OPA3691 Datasheet, PDF (17/32 Pages) Burr-Brown (TI) – Triple Wideband, Current-Feedback OPERATIONAL AMPLIFIER With Disable

of the circuit for a current-feedback op amp can be treated as
frequency response compensation elements while their ra-
tios set the signal gain. Figure 9 shows the small-signal
frequency response analysis circuit for the OPA3691.
V_I
i_ERR
α
R_I
R_G
V_O
Z_(S) i_ERR
R_F
FIGURE 9. 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 transimpedance
gain from i_ERR to V_O
The buffer gain is typically very close to 1.00 and is normally
neglected from signal gain considerations. It will, however, set
the CMRR for a single op amp differential amplifier configura-
tion. For a buffer gain 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 OPA3691 is typically 37Ω.
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 transimped-
ance gain. The Typical Characteristics show this open-loop
transimpedance response. This is analogous to the open-
loop voltage gain curve for a voltage-feedback op amp.
Developing the transfer function for the circuit of Figure 9
gives Equation 1:
V_O
V_I
=
α1 +
R_F
R_G


R_F
+

R_{I }1+
R_F
R_G


=
1+
αNG
R_F + R_ING
Z_(S)
Z_(S)
(1)

NG

≡

^{1 +}
R_F
R_G





This is written in a loop-gain analysis format where the errors
arising from a non-infinite open-loop gain are shown in the
denominator. If Z_(S) were infinite over all frequencies, the
denominator of Equation 1 would reduce to 1 and the ideal
desired signal gain shown in the numerator would be achieved.
The fraction in the denominator of Equation 1 determines the
frequency response. Equation 2 shows this as the loop-gain
equation:
Z_{(S) = Loop Gain}
R_F + R_ING
(2)
If 20 • log(R_F + NG • R_I) were 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 2 at which point the
loop gain has reduced to 1 (and the curves have intersected).
This point of equality is where the amplifier’s closed-loop
frequency response, given by Equation 1, will start to roll off
and is exactly analogous to the frequency at which the noise
gain equals the open-loop voltage gain for a voltage-feed-
back op amp. The difference here is that the total impedance
in the denominator of Equation 2 may be controlled some-
what separately from the desired signal gain (or NG).
The OPA3691 is internally compensated to give a maximally
flat frequency response for R_F = 402Ω at NG = 2 on ±5V
supplies. Evaluating the denominator of Equation 2 (which is
the feedback transimpedance) gives an optimal target of 476Ω.
As the signal gain changes, the contribution of the NG • R_I term
in the feedback transimpedance will change, but the total can
be held constant by adjusting R_F. Equation 3 gives an approxi-
mate equation for optimum R_F over signal gain:
R_F = 476Ω − NG R_I
(3)
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 will load both the buffer
stage at the input and the output stage if R_F gets too low—
actually decreasing the bandwidth. Figure 10 shows the
recommended R_F versus NG for both ±5V and a single +5V
operation. The values shown in Figure 10 give a good
starting point for design where bandwidth optimization is
desired.
600
500
400
+5V
300
±5V
200
100
0
0
5
10
15
20
Noise Gain
FIGURE 10. Recommended Feedback Resistor vs Noise Gain.
OPA3691
17
SBOS227E
www.ti.com