Datasheet
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SBOS270C − AUGUST 2003 − REVISED AUGUST 2008
www.ti.com
16
APPLICATION INFORMATION
WIDEBAND CURRENT-FEEDBACK OPERATION
The OPA2674 gives the exceptional AC performance of a
wideband current-feedback op amp with a highly linear,
high-power output stage. Requiring only 9mA/ch quies-
cent current, the OPA2674 swings to within 1V of either
supply rail and delivers in excess of 380mA at room tem-
perature. This low output headroom requirement, along
with supply voltage independent biasing, gives remark-
able single (+5V) supply operation. The OPA2674 delivers
greater than 150MHz bandwidth 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
OPA2674 achieves a comparable power gain with much
better linearity. The primary advantage of a current-feed-
back op amp over a voltage-feedback op amp is that AC
performance (bandwidth and distortion) is relatively inde-
pendent 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 ef-
fective load is 100Ω || 535Ω = 84Ω.
1/2
OPA2674
+6V
+
−
6V
50
Ω
Load
50
Ω
50
Ω
V
O
V
I
50
Ω
Source
R
G
133
Ω
R
F
402
Ω
+
6.8
µ
F
0.1
µ
F6.8
µ
F
0.1
µ
F
+V
S
−
V
S
Figure 1. DC-Coupled, G = +4, Bipolar Supply,
Specification and Test Circuit
Figure 2 shows the DC-coupled, bipolar supply circuit in-
verting gain configuration used as the basis for the ±6V
Electrical and Typical Characteristics. Key design consid-
erations of the inverting configuration are developed in the
Inverting Amplifier Operation discussion.
1/2
OPA2674
+6V
−
6V
50
Ω
Load
50
Ω
V
O
V
I
50
Ω
Source
R
M
100
Ω
R
F
402
Ω
R
G
100
Ω
Power−supply
decoupling
not shown.
Figure 2. DC-Coupled, G = −4, Bipolar Supply,
Specification and Test Circuit
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 de-
sign, the OPA2674 requires minimal input and output volt-
age headroom compared to other 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 opera-
tion is to maintain 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 with-
in 1.3V of either supply pin, giving a 2.4V
PP
input signal
range centered between the supply pins. The input imped-
ance 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
+1which 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 mid-
point bias is used in this characterization circuit. The new
output stage used in the OPA2674 can deliver large bipolar
output currents into this midpoint load with minimal cross-
over distortion, as shown by the +5V supply, harmonic dis-
tortion plots in the Typical Characteristics charts.