OPA2673_1 Datasheet, PDF (26/40 Pages) Texas Instruments – Dual, Wideband, High Output Current Operational Amplifier with Active Off-Line Control

OPA2673
SBOS382A – JUNE 2008 – REVISED OCTOBER 2008..................................................................................................................................................... www.ti.com
Figure 82 shows the small-signal frequency response
analysis circuit for the OPA2673.
V_I
I_ERR
a
R_I
V_O
Z(S) I_ERR
R_F
R_G
Figure 82. 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
transimpedance gain from I_ERR to V_O
NG = Noise Gain = 1 + ^RF
R_G
open-loop
(12)
The buffer gain is typically very close to 1.00V/V and
is normally neglected from signal gain considerations.
This gain, however, sets the CMRR for a single op
amp differential amplifier configuration. For a buffer
gain of 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 OPA2673
inverting output impedance is typically 32Ω.
A current-feedback op amp senses an error current in
the inverting node (as opposed to a differential input
error voltage 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, which is analogous to the
open-loop voltage gain curve for a voltage-feedback
op amp.
Developing the transfer function for the circuit of
Figure 82 gives Equation 13:
V_{O =}
V_I
a 1 + ^RF
R_G
R_F + R_I
R_F
1+
1+
R_G
Z(s)
=
a ´ NG
1 + ^{RF + RI ´ NG}
Z(s)
(13)
This formula 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) is infinite
over all frequencies, the denominator of Equation 13
reduces to 1 and the ideal desired signal gain shown
in the numerator is achieved. The fraction in the
denominator of Equation 13 determines the frequency
response. Equation 14 shows this as the loop-gain
equation:
Z(s)
= LoopGain
R_F + R_I ´ NG
(14)
If 20log(R_F + NG × R_I) is 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 14, at which point the loop
gain has reduced to 1 (and the curves have
intersected). This point of equality is where the
amplifier closed-loop frequency response given by
Equation 12 starts to roll off, and is exactly analogous
to the frequency at which the noise gain equals the
open-loop voltage gain for a voltage-feedback op
amp. The difference here is that the total impedance
in the denominator of Equation 14 may be controlled
somewhat separately from the desired signal gain (or
NG). The OPA2673 is internally compensated to give
a maximally flat frequency response for R_F = 402Ω at
NG = 4V/V on ±6V supplies. Evaluating the
denominator of Equation 14 (which is the feedback
transimpedance) gives an optimal target of 530Ω. As
the signal gain changes, the contribution of the NG ×
R_I term in the feedback transimpedance changes, but
the total can be held constant by adjusting R_F.
Equation 15 gives an approximate equation for
optimum RF over signal gain:
R_F = 530 - NG ´ R_I
(15)
As the desired signal gain increases, this equation
eventually suggests 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 83 shows the recommended R_F
versus N_G for ±6V operation. The values for R_F
versus gain shown here are approximately equal to
the values used to generate the Typical
Characteristics. They differ in that the optimized
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