OPA4684 Datasheet, PDF (18/26 Pages) Burr-Brown (TI) – Quad, Low-Power, Current-Feedback OPERATIONAL AMPLIFIER

MACROMODELS
Computer simulation of circuit performance using SPICE is
often useful in predicting the performance of analog circuits
and systems. This is particularly true for Video and RF
amplifier circuits where parasitic capacitance and inductance
can have a major effect on circuit performance. Check the TI
web site (www.ti.com) for SPICE macromodels within the
OPA4684 product folder. These models do a good job of
predicting small-signal AC and transient performance under
a wide variety of operating conditions. They do not do as well
in predicting distortion or dG/dP characteristics. These mod-
els do not attempt to distinguish between the package types
in their small-signal AC performance.
OPERATING SUGGESTIONS
SETTING RESISTOR VALUES TO OPTIMIZE BANDWIDTH
Any current-feedback op amp like the OPA4684 can hold
high bandwidth over signal-gain settings with the proper
adjustment of the external resistor values. A low-power part
like the OPA4684 typically shows a larger change in band-
width due to the significant contribution of the inverting input
impedance to loop-gain changes as the signal gain is changed.
Figure 14 shows a simplified analysis circuit for any current-
feedback amplifier.
V_I
i_ERR
α
R_I
R_G
V_O
Z_(S) i_ERR
R_F
FIGURE 14. 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 configuration. For the buffer gain α < 1.0, the
CMRR = –20 • log(1 – α). The closed-loop input stage buffer
used in the OPA4684 gives a buffer gain more closely
approaching 1.00 and this shows up in a slightly higher
CMRR than previous current-feedback op amps.
R_I, the buffer output impedance, is a critical portion of the
bandwidth control equation. The OPA4684 reduces this
element to approximately 4.0Ω using the local loop gain of
the input buffer stage. This significant reduction in output
impedance, on very low power, contributes significantly to
extending 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
the open-loop voltage gain curve for a voltage-feedback op
amp. Developing the transfer function for the circuit of Figure 13
gives Equation 1:
(1)
V_O
V_I
=
α

¹⁺
R_F
R_G


1+
R_F
+ R_I

¹⁺
R_F
R_G


=
1+
α
R_F
NG
+ R_I
NG
Z_(S)
Z_(S)

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
(2)
R_F + R_I NG
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 OPA4684 is internally compensated to give a maximally
flat frequency response for R_F = 800Ω at NG = 2 on ±5V
supplies. That optimum value goes to 1.0kΩ on a single +5V
supply. Normally, with a current-feedback amplifier, it is
18
OPA4684
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