THS6204 Datasheet, PDF (24/43 Pages) Texas Instruments – Dual-Port, Differential VDSL2 Line Driver Amplifiers

THS6204
SBOS416C – OCTOBER 2007 – REVISED APRIL 2009 ................................................................................................................................................... www.ti.com
APPLICATION INFORMATION
WIDEBAND CURRENT-FEEDBACK
OPERATION
The THS6204 gives the exceptional ac performance
of a wideband current-feedback op amp with a highly
linear, high-power output stage. Requiring only
9mA/ch quiescent current, the THS6204 swings to
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
145MHz 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 THS6204 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
THS6204
R_F
1.24kW
V_I
R_G
R_F
274W 1.24kW
R_L V_O
This approach provides for 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 without interacting with the gain
setting. The differential signal gain for the circuit of
Figure 81 is:
A_D = 1 + 2 ´
R_F
R_G
(1)
Because the THS6204 is a current feedback (CFB)
amplifier, its bandwidth is primarily controlled with the
feedback resistor value; Figure 81 shows a value of
274Ω for the A_D = +10V/V design. The differential
gain, however, may be adjusted with considerable
freedom using just the R_G resistor. In fact, R_G may be
a reactive network providing 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 since 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 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.
1/2
THS6204
-12V
G_DIFF = 1 +
2 ´ R_F
R_G
= ^VO
V_I
Figure 81. Noninverting Differential I/O Amplifier
24
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