Datasheet

OPA684
16
SBOS219D
www.ti.com
the open-loop voltage gain curve for a voltage-feedback
op amp. Developing the transfer function for the circuit of
Figure 10 gives Equation 1:
(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
()
()
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.
(2)
Z
RRNG
Loop Gain
S
FI
()
+
=
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 sepa-
rately from the desired signal gain (or NG).
The OPA684 is internally compensated to give a maximally
flat frequency response for R
F
= 1kΩ at NG = 2 on ±5V
supplies. That optimum value goes to 1.3kΩ on a single +5V
supply. Normally, with a current-feedback amplifier, it is
possible to adjust the feedback resistor to hold this band-
width up as the gain is increased. The CFB
plus
architecture
has reduced the contribution of the inverting input impedance
to provide exceptional bandwidth to higher gains without
adjusting the feedback resistor value. The Typical Character-
istics show the small-signal bandwidth over gain with a fixed
feedback resistor.
At very high gains, 2nd-order effects in the inverting output
impedance cause the overall response to peak up. If desired,
it is possible to retain a flat frequency response at higher
gains by adjusting the feedback resistor to higher values as
the gain is increased. See Figure 11 for the empirically
determined feedback resistor and resulting –3dB bandwidth
from gains of +2 to +100 to hold a < 0.5dB peaked response.
See Figure 12 for the measured frequency response curves
with the adjusted feedback resistor value. While the band-
width for this low-power part does reduce at higher gains,
OPERATING SUGGESTIONS
SETTING RESISTOR VALUES TO OPTIMIZE BANDWIDTH
Any current-feedback op amp like the OPA684 can hold high
bandwidth over signal-gain settings with the proper adjust-
ment of the external resistor values. A low-power part like the
OPA684 typically shows a larger change in bandwidth due to
the significant contribution of the inverting input impedance
to loop-gain changes as the signal gain is changed. Figure
10 shows a simplified analysis circuit for any current-feed-
back amplifier.
R
F
V
O
R
G
R
I
Z
(S)
i
ERR
i
ERR
α
V
I
FIGURE 10. Current Feedback Transfer Function Analysis
Circuit.
The key elements of this current-feedback op amp model
are:
α ⇒ Buffer gain from the non-inverting 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 the buffer gain α < 1.0, the CMRR = –20 • log(1 – α).
The closed-loop input stage buffer used in the OPA684 gives
a buffer gain more closely approaching 1.00 and this shows up
in a slightly higher CMRR than any previous current feedback
op amp.
R
I
, the buffer output impedance, is a critical portion of the
bandwidth control equation. The OPA684 reduces this ele-
ment to approximately 2.5Ω, using the loop gain of the local
input buffer stage. This significant reduction in output imped-
ance, on very low power, contributes significantly to extend-
ing the bandwidth at higher gains.
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
transimpedance gain. The Typical Characteristics show this
open-loop transimpedance response. This is analogous to