OPA682 datasheet(11/22 Pages) TI1

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OPA2681

SBOS091A

APPLICATIONS INFORMATION

WIDEBAND CURRENT FEEDBACK OPERATION

The OPA2681 gives the exceptional AC performance of a

wideband current feedback op amp with a highly linear, high

power output stage. Requiring only 6mA/ch. quiescent cur-

rent, the OPA2681 will swing to within 1V of either supply

rail and deliver in excess of 135mA guaranteed at room

temperature. This low output headroom requirement, along

with supply voltage independent biasing, gives remarkable

single (+5V) supply operation. The OPA2681 will deliver

greater than 200MHz bandwidth driving a 2Vp-p output into

100

Ω on a single +5V supply. Previous boosted output stage

amplifiers have typically suffered from very poor crossover

distortion as the output current goes through zero. The

OPA2681 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 perfor-

mance (bandwidth and distortion) is relatively independent

of signal gain. For similar AC performance with improved

DC accuracy, consider the high slew rate, unity gain stable,

voltage feedback OPA2680.

Figure 1 shows the DC coupled, gain of +2, dual power

supply circuit configuration used as the basis of the

±5V

Specifications and Typical Performance Curves. For test

purposes, the input impedance is set to 50

Ω with a resistor

to ground and the output impedance is set to 50

Ω with a

series output resistor. Voltage swings reported in the speci-

fications are taken directly at the input and output pins while

load powers (dBm) are defined at a matched 50

Ω load. For

the circuit of Figure 1, the total effective load will be 100

Ω

|| 804

Ω = 89Ω. The disable control line (DIS) is typically left

open (SO-14 only) to guarantee normal amplifier operation.

One optional component is included in Figure 1. In addition

to the usual power supply de-coupling capacitors to ground,

a 0.1

µF capacitor is included between the two power supply

pins. In practical PC board layouts, this optional added

capacitor will typically improve the 2nd harmonic distortion

performance by 3dB to 6dB.

Figure 2 shows the AC coupled, gain of +2, single supply

circuit configuration used as the basis of the +5V Specifica-

tions and Typical Performance Curves. Though not a “rail-

to-rail” design, the OPA2681 requires minimal input and

output voltage headroom compared to other very wideband

current feedback op amps. It will deliver a 3Vp-p output

swing on a single +5V supply with greater than 150MHz

bandwidth. The key requirement of broadband single supply

operation is to maintain input and output signal swings

within the usable voltage ranges at both the input and the

output. The circuit of Figure 2 establishes an input midpoint

bias using a simple resistive divider from the +5V supply

(two 806

Ω resistors). The input signal is then AC coupled

into this midpoint voltage bias. The input voltage can swing

to within 1.5V of either supply pin, giving a 2Vp-p input

signal range centered between the supply pins. The input

impedance matching resistor (57.6

Ω) used for testing is

adjusted to give a 50

Ω input match when the parallel

combination of the biasing divider network is included. The

gain resistor (RG) 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 has been

adjusted from the bipolar supply condition to re-optimize for

a flat frequency response in +5V, gain of +2, operation (see

Setting Resistor Values to Optimize Bandwidth). Again, on

a single +5V supply, the output voltage can swing to within

1V of either supply pin while delivering more than 75mA

output current. A demanding 100

Ω load to a midpoint bias

is used in this characterization circuit. The new output stage

used in the OPA2681 can deliver large bipolar output cur-

rents into this midpoint load with minimal crossover distor-

tion, as shown by the +5V supply, 3rd harmonic distortion

plots.

FIGURE 1. DC-Coupled, G = +2, Bipolar Supply, Specifi-

cation and Test Circuit.

FIGURE 2. AC-Coupled, G = +2, Single Supply Specifica-

tion and Test Circuit.

1/2

OPA2681

+5V

DIS

–5V

Ω Load

Ω

Ω Source

402

Ω

402

Ω

6.8µF

0.1µF

6.8µF

0.1µF

–V

1/2

OPA2681

+5V

DIS

S/2

806

Ω

100

Ω

57.6

Ω

806

Ω

499

Ω

499

Ω

0.1µF

6.8µF

0.1µF

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