Datasheet

OPA3691
17
www.ti.com
SBOS227E
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.
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
RRNG
Loop Gain
S
FI
()
+
=
(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:
RNGR
FI
=Ω−476
(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.
FIGURE 9. Current-Feedback Transfer Function Analysis Circuit.
R
F
V
O
R
G
R
I
Z
(S)
i
ERR
i
ERR
α
V
I
FIGURE 10. Recommended Feedback Resistor vs Noise Gain.
600
500
400
300
200
100
0
Noise Gain
02010 155
Feedback Resistor (Ω)
+5V
±5V
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
V
R
R
RR
R
R
Z
NG
RRNG
Z
NG
R
R
O
I
F
G
FI
F
G
S
FI
S
F
G
=
+






++






=
+
+
≡+














α
α
1
1
1
1
()
()
(1)