REV. A
AD8018
–8–
THEORY OF OPERATION
The AD8018 is composed of two current feedback amplifiers
capable of delivering 400 mA of output current while swinging
to within 0.5 V of either power supply, and maintaining low
distortion. A differential line driver using the AD8018 can provide
CPE performance on a single 5 V supply. This performance is
enabled by Analog Device’s XFCB process and a novel, two-
stage current feedback architecture featuring a patent-pending
rail-to-rail output stage.
A simplified schematic is shown in Figure 4. Emitter followers
buffer the positive input, V
P
, to provide low input current and
current noise. The low impedance current feedback summing
junction is at the negative input, V
N
. The output stage is another
high-gain amplifier used as an integrator to provide frequency
compensation. The complementary common-emitter output
provides the extended output swing.
A current feedback amplifier’s dynamic and distortion performance
is relatively insensitive to its closed-loop signal gain, which is
a distinct advantage over a voltage-feedback architecture. Figure
5 shows a simplified model of a current feedback amplifier. The
feedback signal is a current into the inverting node. R
IN
is inversely
proportional to the transconductance of the amplifier’s input stage,
g
mi
. Circuit analysis of the pictured follower with gain yields:
VV G
T
TRGR
OUT IN
ZS
ZS F IN
/
()
()
=×
++×
where:
GRR
T
R
CR
Rg
FG
ZS
T
S
TT
IN mi
=+
=
+
=≅
1
1
1 125
/
()
/
()
Ω
Recognizing that G ⫻ R
IN
< R
F
, and that the –3 dB point is set
when T
Z(S)
= R
F
, one can see that the amplifier’s bandwidth
depends primarily on the feedback resistor. There is a value of
R
F
below which the amplifier will be unstable, as an actual ampli-
fier will have additional poles that will contribute excess phase
shift. The optimum value for R
F
depends on the gain and the
amount of peaking tolerable in the application.
V
O
BIAS
V
N
V
P
Figure 4. Simplified Schematic
G = 1
I
T
= I
IN
C
T
R
T
I
IN
V
OUT
R
G
R
F
R
IN
+
V
IN
–
V
O
+
–
Figure 5. Model of Current Feedback Amplifier
FEEDBACK RESISTOR SELECTION
In current feedback amplifiers, selection of the feedback and gain
resistors will impact the MTPR performance, bandwidth, noise,
and gain flatness. Care should be exercised in the selection of these
resistors so that the optimum performance is achieved. Table I
shows the recommended resistor values for use in a variety of gain
settings for the test circuit in TPC 1. These values are intended
to be a starting point when designing for any application.
FREQUENCY – Hz
CROSSTALK – dB
–90
1M 10M
100M
1G
–80
–70
–60
–50
–40
–30
–20
–10
R
L
= 5⍀
SIDE A DRIVEN
R
L
= 5⍀
SIDE B DRIVEN
V
IN
= 2V p-p
G = 2
V
S
= ⴞ2.5
100k
R
L
= 100⍀
SIDE A DRIVEN
R
L
= 100⍀
SIDE B DRIVEN
–100
–110
TPC 25. Crosstalk vs. Frequency
