SBOU006 datasheet(12/33 Pages) TI2

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OPA3691

SBOS227E

www.ti.com

APPLICATIONS INFORMATION

WIDEBAND CURRENT-FEEDBACK OPERATION

The OPA3691 gives the exceptional AC performance of a

wideband current-feedback op amp with a highly linear, high-

power output stage. Requiring only 5.1mA/ch quiescent

current, the OPA3691 will swing to within 1V of either supply

rail and deliver in excess of 160mA at room temperature.

This low output headroom requirement, along with supply

voltage independent biasing, gives remarkable single (+5V)

supply operation. The OPA3691 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 OPA3691 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 +2, dual power-

supply circuit configuration used as the basis of the

±5V

Electrical Characteristics and Typical Characteristics. 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

electrical characteristics are taken directly at the input and

output pins while load powers (dBm) are defined at a matched

Ω load. For the circuit of Figure 1, the total effective load

will be 100

Ω || 998Ω. The disable control line (DIS) is

typically left open to ensure normal amplifier operation. One

optional component is included in Figure 1. In addition to the

usual power-supply decoupling capacitors to ground, a 0.01

µF

capacitor is included between the two power-supply pins. In

practical PC board layouts, this optionally added capacitor

will typically improve the 2nd-harmonic distortion perfor-

mance 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 Characteristics. Though not a

rail-to-rail

design, the OPA3691 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 be-

tween the supply pins. The input impedance matching resis-

tor (57.6

Ω) 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 (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. Again, on a single +5V

supply, the output voltage can swing to within 1V of either

supply pin while delivering more than 120mA output current.

A demanding 100

Ω load to a midpoint bias is used in this

characterization circuit. The new output stage used in the

OPA3691 can deliver large bipolar output currents into this

midpoint load with minimal crossover distortion, as shown by

the +5V supply, 3rd-harmonic distortion plots.

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

tion and Test Circuit.

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

tion and Test Circuit.

1/3

OPA3691

+5V

DIS

–5V

Ω Load

Ω

Ω Source

499

Ω

499

Ω

6.8

µF

0.1

µF

6.8

µF

0.1

µF

0.01

µF

–V

1/3

OPA3691

+5V

DIS

S/2

806

Ω

100

Ω

57.6

Ω

806

Ω

499

Ω

499

Ω

0.1

µF

0.1

µF

6.8

µF

0.1

µF

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