Datasheet

APPLICATION INFORMATION
WIDEBAND CURRENT-FEEDBACK
A =1+
D
2 ´
R
F
R
G
(1)
R
L
R
F
1.24kW
R
F
1.24kW
-12V
+12V
1/2
THS6214
1/2
THS6214
R
G
274W
V
OUT
V
IN
G =1+
DIFF
=
2 R´
F
R
G
V
OUT
V
IN
THS6214
SBOS431 – MAY 2009 ....................................................................................................................................................................................................... www.ti.com
This approach allows the user to set a source
termination impedance at the input that is
OPERATION
independent of the signal gain. For instance, simple
The THS6214 provides the exceptional ac
differential filters may be included in the signal path
performance of a wideband current-feedback op amp
right up to the noninverting inputs with no interaction
with a highly linear, high-power output stage.
with the gain setting. The differential signal gain for
Requiring only 21mA/port quiescent current, the
the circuit of Figure 81 is:
THS6214 swings to within 1.9V of either supply rail
on a 100 Ω load and delivers in excess of 416mA at
room temperature. This low-output headroom
requirement, along with supply voltage independent
Where A
D
= differential gain.
biasing, provides remarkable ± 6V supply operation.
The THS6214 delivers greater than 140MHz
Figure 81 shows a value of 274 Ω for the A
D
= +10V/V
bandwidth driving a 2V
PP
output into 100 Ω on a ± 6V
design. Because the THS6214 is a current feedback
supply. Previous boosted output stage amplifiers
(CFB) amplifier, its bandwidth is primarily controlled
typically suffer from very poor crossover distortion as
with the feedback resistor value; the differential gain,
the output current goes through zero. The THS6214
however, may be adjusted with considerable freedom
achieves a comparable power gain with much better
using just the R
G
resistor. In fact, R
G
may be reduced
linearity. The primary advantage of a
by a reactive network that provides a very isolated
current-feedback op amp over a voltage-feedback op
shaping to the differential frequency response.
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 because an equal dc voltage at each
test purposes, the input impedance is set to 50 Ω with
inverting node creates no current through R
G
. This
a resistor to ground and the output impedance is set
circuit does show a common-mode gain of +1V/V
to 50 Ω with a series output resistor. Voltage swings
from input to output. The source connection should
reported in the Electrical Characteristics are taken
either remove this common-mode signal if undesired
directly at the input and output pins, whereas load
(using an input transformer can provide this function),
powers (dBm) are defined at a matched 50 Ω load.
or the common-mode voltage at the inputs can be
For the circuit of Figure 81 , the total effective load is
used to set the output common-mode bias. If the low
100 Ω || 1.24k Ω || 1.24k Ω = 86.1 Ω .
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.
Figure 81. Noninverting Differential I/O Amplifier
24 Submit Documentation Feedback Copyright © 2009, Texas Instruments Incorporated
Product Folder Link(s): THS6214