Datasheet

OPA2677
14
SBOS126I
www.ti.com
APPLICATION INFORMATION
WIDEBAND CURRENT-FEEDBACK OPERATION
The OPA2677 gives the exceptional AC performance of a
wideband current-feedback op amp with a highly linear, high-
power output stage. Requiring only 9mA/ch quiescent cur-
rent, the OPA2677 swings to within 1V of either supply rail
and delivers in excess of 380mA at room temperature. This
low-output headroom requirement, along with supply voltage
independent biasing, gives remarkable single (+5V) supply
operation. The OPA2677 delivers greater than 150MHz band-
width driving a 2V
PP
output into 100Ω on a single +5V supply.
Previous boosted output stage amplifiers typically suffer from
very poor crossover distortion as the output current goes
through zero. The OPA2677 achieves a comparable power
gain with much better linearity. The primary advantage of a
current-feedback op amp over a voltage-feedback op amp is
that AC performance (bandwidth and distortion) is relatively
independent of signal gain. Figure 1 shows the DC-coupled,
gain of +4, dual power-supply circuit configuration used as
the basis of the ±6V Electrical and Typical Characteristics.
For test purposes, the input impedance is set to 50Ω with a
resistor to ground and the output impedance is set to 50Ω
with a series output resistor. Voltage swings reported in the
electrical characteristics are taken directly at the input and
output pins, whereas load powers (dBm) are defined at a
matched 50Ω load. For the circuit of Figure 1, the total
effective load is 100Ω || 535Ω = 84Ω.
Figure 2 shows the DC-coupled, bipolar supply circuit con-
figuration used as the basis for the Inverting Gain ±6V
Typical Characteristics. Key design considerations of the
inverting configuration are developed in the
Inverting Ampli-
fier Operation
section.
Figure 3 shows the AC-coupled, gain of +4, single-supply
circuit configuration used as the basis of the +5V Electrical
and Typical Characteristics. Though not a rail-to-rail design,
the OPA2677 requires minimal input and output voltage
headroom compared to other very wideband current-feed-
back op amps. It will deliver a 3V
PP
output swing on a single
+5V supply with greater than 100MHz bandwidth. The key
requirement of broadband single-supply operation is to main-
tain input and output signal swings within the usable voltage
ranges at both the input and the output. The circuit of Figure 3
establishes an input midpoint bias using a simple resistive
divider from the +5V supply (two 806Ω resistors). The input
signal is then AC-coupled into this midpoint voltage bias. The
input voltage can swing to within 1.3V of either supply pin,
giving a 2.4V
PP
input signal range centered between the
supply pins. The input impedance matching resistor (57.6Ω)
used for testing is adjusted to give a 50Ω input match when
the parallel combination of the biasing divider network is
included. The gain resistor (R
G
) is AC-coupled, giving the
circuit a DC gain of +1—which puts the input DC bias voltage
(2.5V) on the output as well. The feedback resistor value is
adjusted from the bipolar supply condition to re-optimize for
a flat frequency response in +5V, gain of +4, operation.
Again, on a single +5V supply, the output voltage can swing
to within 1V of either supply pin while delivering more than
200mA output current. A demanding 100Ω load to a midpoint
bias is used in this characterization circuit. The new output
stage used in the OPA2677 can deliver large bipolar output
currents into this midpoint load with minimal crossover distor-
tion, as shown by the +5V supply, harmonic distortion plots.
1/2
OPA2677
+6V
+
–6V
50Ω Load
50Ω
50ΩV
O
V
I
50Ω Source
R
G
133Ω
R
F
402Ω
+
6.8µF
0.1µF 6.8µF
0.1µF
+V
S
–V
S
1/2
OPA2677
+5V
–5V
50Ω Load
50ΩV
O
V
I
50Ω
Source
R
M
100Ω
R
F
402Ω
R
F
402Ω
Power-supply
decoupling
not shown.
FIGURE 1. DC-Coupled, G = +4, Bipolar Supply, Specifica-
tion and Test Circuit.
FIGURE 2. DC-Coupled, G = –4, Bipolar Supply, Specifica-
tion and Test Circuit.