high FET input impedance and low closed loop output
impedance. Remember that a DC path to the input is still
necessary; even with the ultra low FET input bias current
(5pA), open or capacitively coupled sources will cause the
input to saturate. For best frequency response, a direct short
between the output and inverting inputs is suggested. Since
the input bias currents are not necessarily correlated, match-
ing the non-inverting source resistance with a resistor in the
feedback network is not recommended.
R
F
R
S
V
IN
R
1
V
–
2
6
3
V
O
V
IN
=
V
O
–R
F
R
1
V
SOURCE
R
T
2
Z
I
V
IN
3
OPA655
6
Z
O
V
O
R
L
= 100Ω
V
O
V
IN
=1
FIGURE 4. Inverting Op Amp.
β
= V
–
/V
O
Non-Inverting Gain = Noise Gain = NG = 1/β
Taking the inverting amplifier as an example,
β
is found by
setting V
SOURCE
to zero and calculating the voltage divider
ratio from V
O
to V
–
:
R
1
+ R
T
|| R
S
= total resistance to ground on the
inverting input
NOTE: Power supplies and de-coupling not shown.
FIGURE 2. Non-Inverting Unity Gain Buffer.
The non-inverting amplifier configuration (Figure 3) will
again present a very high input impedance to the input signal
and a low output impedance drive with signal gain. The
100Ω shown for R
F
will give the frequency response shown
in the Typical Performance Curves. Higher values for R
F
and R
1
are possible but for high frequency non-inverting op
amp applications, should be limited to less than 1.0kΩ. The
amplifier will be loaded by (R
F
+ R
1
) in parallel with the
load impedance.
R
1
+
R
T
|| R
S
V
–
β=
=
V
O
R
F
+
R
1
+
R
T
|| R
S
R
F
1
=
1
+
R
1
+
R
T
|| R
S
β
NG
=
R
F
= 100Ω
The resulting bandwidth is approximately the amplifier’s
gain bandwidth product divided by the calculated noise gain:
R
1
2
3
OPA655
6
V
O
V
IN
V
O
=1+
R
F
R
1
BW
≈
GBW/NG
In practice, low noise gains (< 5) will produce a wider
bandwidth than predicted due to the peaking effect of second
order poles. For example, at an inverting gain of –1 from a
zero ohm source impedance, this yields a non-inverting gain
of 2 and an approximate signal bandwidth of 185MHz.
V
IN
FIGURE 3. Non-Inverting Op Amp.
The inverting amplifier configuration (Figure 4) offers a
broadband, low DC error amplifier with a controlled input
impedance. The input impedance may be set by adjusting R
1
to the desired value and then adjusting R
F
to the desired
gain, or by setting R
F
and R
1
to the desired values then
controlling the input impedance independently as the paral-
lel combination of R
1
and an optional R
T
resistor to ground.
To estimate the bandwidth in any configuration, first calcu-
late the gain as a non-inverting amplifier. This is often
referred to as “noise gain” or NG, and is simply the inverse
of the feedback factor
β.
TYPICAL APPLICATIONS
WIDEBAND TRANSIMPEDANCE AMPLIFIER
The high gain bandwidth product and low noise of the
OPA655 make it particularly suitable for wideband
transimpedance applications. The front page of the data
sheet shows measured results for a 1MΩ transimpedance
gain from a relatively large diode having 47pF parasitic
capacitance. The key to broadband transimpedance applica-
tions is to set the compensation capacitance across the
feedback resistor to achieve a flat, or bandlimited, frequency
®
9
OPA655
BURR-BROWN [ BURR-BROWN CORPORATION ]
