THS6204IPWPR datasheet(24/43 Pages) TI

24 / 43 page

APPLICATION INFORMATION

WIDEBAND CURRENT-FEEDBACK

=1+

2 ´

(1)

1.24kW

-12V

+12V

1/2

THS6204

1/2

THS6204

274W

=1+

DIFF

´ F

THS6204

SBOS416C – OCTOBER 2007 – REVISED APRIL 2009 ................................................................................................................................................... www.ti.com

This approach provides for a source termination

OPERATION

impedance at the input that is independent of the

signal gain. For instance, simple differential filters

The THS6204 gives the exceptional ac performance

may be included in the signal path right up to the

of a wideband current-feedback op amp with a highly

noninverting inputs without interacting with the gain

linear,

high-power

output

stage.

Requiring

only

setting. The differential signal gain for the circuit of

9mA/ch quiescent current, the THS6204 swings to

Figure 81 is:

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 ±6V supply

operation.

The

THS6204

delivers

greater

than

Because the THS6204 is a current feedback (CFB)

145MHz bandwidth driving a 2VPP output into 100Ω

amplifier, its bandwidth is primarily controlled with the

on a ±6V supply. Previous boosted output stage

feedback resistor value; Figure 81 shows a value of

amplifiers typically suffer from very poor crossover

274

Ω for the AD = +10V/V design. The differential

distortion as the output current goes through zero.

gain, however, may be adjusted with considerable

The THS6204 achieves a comparable power gain

freedom using just the RG resistor. In fact, RG may be

with much better linearity. The primary advantage of

a reactive network providing a very isolated shaping

a current-feedback op amp over a voltage-feedback

to the differential frequency response.

op amp is that ac performance (bandwidth and

Various combinations of single-supply or ac-coupled

distortion) is relatively independent of signal gain.

gain can also be delivered using the basic circuit of

Figure 81 shows the dc-coupled, gain of +10V/V, dual

Figure 81. Common-mode bias voltages on the two

power-supply circuit configuration used as the basis

noninverting inputs pass on to the output with a gain

of the ±12V Electrical and Typical Characteristics. For

of +1V/V since an equal dc voltage at each inverting

test purposes, the input impedance is set to 50

Ω with

node creates no current through RG. This circuit does

a resistor to ground and the output impedance is set

show a common-mode gain of +1V/V from input to

to 50

Ω with a series output resistor. Voltage swings

output. The source connection should either remove

reported in the electrical characteristics are taken

this common-mode signal if undesired (using an input

directly at the input and output pins, whereas load

transformer

can

provide

this

function),

the

powers (dBm) are defined at a matched 50

Ω load.

common-mode voltage at the inputs can be used to

For the circuit of Figure 81, the total effective load is

set the output common-mode bias. If the low

100

Ω || 1.24kΩ || 1.24kΩ = 86.1Ω.

common-mode rejection of this circuit is a problem,

the output interface may also be used to reject that

common-mode.

For

instance,

most

modern

differential input ADCs reject common-mode signals

very well, while a line driver application through a

transformer will also attenuate the common-mode

signal through to the line.

Figure 81. Noninverting Differential I/O Amplifier

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