THS6214 Datasheet, PDF (24/41 Pages) Texas Instruments – Dual-Port, Differential, VDSL2 Line Driver Amplifiers

THS6214
SBOS431 – MAY 2009 ....................................................................................................................................................................................................... www.ti.com
APPLICATION INFORMATION
WIDEBAND CURRENT-FEEDBACK
OPERATION
The THS6214 provides the exceptional ac
performance of a wideband current-feedback op amp
with a highly linear, high-power output stage.
Requiring only 21mA/port quiescent current, the
THS6214 swings to within 1.9V of either supply rail
on a 100Ω load and delivers in excess of 416mA at
room temperature. This low-output headroom
requirement, along with supply voltage independent
biasing, provides remarkable ±6V supply operation.
The THS6214 delivers greater than 140MHz
bandwidth driving a 2V_PP output into 100Ω on a ±6V
supply. Previous boosted output stage amplifiers
typically suffer from very poor crossover distortion as
the output current goes through zero. The THS6214
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 81 shows the dc-coupled, gain of +10V/V, dual
power-supply circuit configuration used as the basis
of the ±12V 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 81, the total effective load is
100Ω || 1.24kΩ || 1.24kΩ = 86.1Ω.
+12V
1/2
THS6214
R_F
1.24kW
V_IN
R_G
R_F
274W 1.24kW
R_L V_OUT
This approach allows the user to set a source
termination impedance at the input that is
independent of the signal gain. For instance, simple
differential filters may be included in the signal path
right up to the noninverting inputs with no interaction
with the gain setting. The differential signal gain for
the circuit of Figure 81 is:
A_D = 1 + 2 ´
R_F
R_G
(1)
Where A_D = differential gain.
Figure 81 shows a value of 274Ω for the A_D = +10V/V
design. Because the THS6214 is a current feedback
(CFB) amplifier, its bandwidth is primarily controlled
with the feedback resistor value; the differential gain,
however, may be adjusted with considerable freedom
using just the R_G resistor. In fact, R_G may be reduced
by a reactive network that provides a very isolated
shaping to the differential frequency response.
Various combinations of single-supply or ac-coupled
gain can also be delivered using the basic circuit of
Figure 81. Common-mode bias voltages on the two
noninverting inputs pass on to the output with a gain
of +1V/V because an equal dc voltage at each
inverting node creates no current through R_G. This
circuit does show a common-mode gain of +1V/V
from input to output. The source connection should
either remove this common-mode signal if undesired
(using an input transformer can provide this function),
or the common-mode voltage at the inputs can be
used to set the output common-mode bias. If the low
common-mode rejection of this circuit is a problem,
the output interface can also be used to reject that
common-mode. For instance, most modern
differential input analog-to-digital converters (ADCs)
reject common-mode signals very well, while a line
driver application through a transformer also
attenuates the common-mode signal through to the
line.
1/2
THS6214
-12V
G_DIFF = 1 +
2 ´ R_F
R_G
= ^VOUT
V_IN
Figure 81. Noninverting Differential I/O Amplifier
24
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