Datasheet

OPA695
15
SBOS293G
www.ti.com
The single-supply test circuits of Figures 3 and 4 show +5V
operation. These same circuits can be used over a single-
supply range of +5V to +12V. Operating on a single +12V
supply, with the Absolute Maximum Supply voltage specifica-
tion of +13V, gives adequate design margin for the typical
±5% supply tolerance.
RF SPECIFICATIONS AND APPLICATIONS
The ultra-high, full-power bandwidth and 3rd-order intercept
of the OPA695 may be used to good advantage in IF
amplifier applications. Additional benefits to using a wideband
op amp such as the OPA695 include extremely good (and
independent) I/O impedance matching as well as very high
reverse isolation. A designer more accustomed to using
fixed-gain RF amplifiers will get almost perfect gain accuracy,
much higher I/O return loss, and 3rd-order intercept points
exceeding 30dBm (up to 110MHz) using only a 13mA supply
current for the OPA695. Using the considerable design
freedom achieved by adjusting the external resistors, the
OPA695 can replace a wide range of fixed-gain RF amplifiers
with a single part. To understand (in RF amplifier terms) how
to take advantage of this, consider first the 4-S parameters
(this will be done using the example circuits of Figures 1 and
2 on ±5V supplies, but similar results can be obtained on a
single +5V to +12V supply).
INPUT RETURN LOSS (S
11
)
Input return loss is a measure of how nearly (over frequency)
the input impedance matches the source impedance. This is
relatively independent of gain setting for both the noninverting
and inverting configurations. The Typical Characteristic curves
show the magnitude of S
11
for the circuits of Figures 1 and
2 through 1GHz (noninverting gain of +8 and inverting gain
of –8 operation, respectively). Noninverting operation does
offer much better matching to higher frequencies, with the
only deviation due to the parasitic input capacitance of the
input pin. The noninverting input match is simply set by the
resistor to ground on the noninverting input, since the ampli-
fier itself shows a very high input impedance. Inverting
operation is also very good, but rises more quickly due to
loop gain roll-off effects appearing at the inverting node. The
inverting mode input match is set by the parallel combination
of R
G
and R
T
in Figure 2, since the inverting amplifier node
may be considered a virtual ground. A good, fixed-gain, RF
amplifier would have an input, Voltage Standing Wave Ratio
(VSWR) < 1.2:1. This corresponds to an S
11
of –21dB. The
OPA695 exceeds this performance through 100MHz for the
inverting mode of operation, and through 400MHz for the
noninverting mode.
OUTPUT RETURN LOSS (S
22
)
Output return loss is a measure of how nearly (over fre-
quency) the output impedance matches the load impedance.
This is relatively independent of gain setting for both the
noninverting and inverting configurations. The output match-
ing impedance, to a first order, is, simply set by adding a
series resistor to the low impedance output of the op amp.
Since the op amp itself shows a very low output impedance
that increases with frequency, an improvement in the output
match can therefore be obtained by adding a small equaliz-
ing capacitor across this output resistor. The Typical Charac-
teristic curves show the measured S
22
with and without this
2.5pF capacitor (across the 50Ω output resistor). Again, a
very good match for a fixed-gain RF amplifier would give a
VSWR of 1.2:1 (S
22
< –21dB). The Typical Characteristic
curves show the measured S
22
with and without this 2.5pF
capacitor across the 50Ω output resistor. The Typical Char-
acteristic curves show that a simple 50Ω output resistor holds
better than –21dB to 140MHz, but up to 380MHz with the
tuning capacitor.
FORWARD GAIN (S
21
)
In all high-speed amplifier data sheets, this is referred to as
the small signal gain which is plotted over frequency. The
difference between noninverting and inverting operation is
that the phase of S
21
starts out at 0° for the noninverting and
–180° for the inverting. This initial phase shift for inverting
mode is inconsequential to most IF strip applications. The
phase of S
21
was not shown in the Typical Characteristic
curves, but is very linear with frequency and may be accu-
rately modeled as a constant time delay through the ampli-
fier.
The Typical Characteristic Curves for the OPA695 show S
21
over a range of signal gains where the external resistors
have been adjusted to re-optimize flatness at each gain
setting. Since this is a current feedback op amp, the signal
bandwidth can be held relatively constant as the desired gain
setting is changed. The plot of the noninverting bandwidth
versus gain shows some change in bandwidth versus gain
(due to parasitic capacitive effects on the inverting node) with
very little change showing up for the inverting mode of
operation.
Signal gains are most often referred to as V/V in op amp data
sheets. This is the voltage gain from input to output and is set
by external resistor ratios. Since the output impedance is set
by a physical series resistor, the voltage gain to the matched
load is cut in 1/2 by this resistor divider. The log gain to the
matched load for the noninverting circuit of Figure 1 is:
G
R
R
dB
F
G
+
=+






20
1
2
1log
The log gain to the matched load for the inverting circuit of
Figure 2 is:
G
R
R
dB
F
G
–
log=






20
1
2
The specific resistor values used in Figures 1 and 2 give both
a maximally flat bandwidth and a 12dB gain to the matched
load. The design tables at the end of this section summarize
the required resistor values over a range of desired gains for
the circuits of Figures 1 and 2.
As the desired signal gain increases, the achievable band-
widths will decrease. In the noninverting case, it decreases
relatively quickly as shown in the Typical Characteristic
(1)
(2)