AD8024
–9–
REV. C
General
The AD8024 is a wide bandwidth, quad video amplifier. It offers a
high level of performance on 16 mA total quiescent supply cur-
rent for closed-loop gains of ±1 or greater.
Bandwidth up to 380 MHz, low differential gain and phase errors,
and high output current make the AD8024 an efficient video
amplifier.
The AD8024’s wide phase margin and high output current make it
an excellent choice when driving any capacitive load.
Choice of Feedback Resistor
Because it is a current feedback amplifier, the closed-loop
bandwidth of the AD8024 may be customized with the feed-
back resistor.
A larger feedback resistor reduces peaking and increases the
phase margin at the expense of reduced bandwidth. A smaller
feedback resistor increases bandwidth at the expense of increased
peaking and reduced phase margin.
The closed-loop bandwidth is affected by attenuation due to the
finite output resistance. The open-loop output resistance of ≈6 Ω
reduces the bandwidth somewhat when driving load resistors less
than ≈150 Ω. The bandwidth will be ≈10% greater for load resis-
tance above a few hundred ohms.
The value of the feedback resistor is not critical unless maintaining
the widest or flattest frequency response is desired. Table I shows
the bandwidth at different supply voltages for some useful closed-
loop gains when driving a 150 Ω load. The recommended resistors
are for the widest bandwidth with less than 2 dB peaking.
Table I. –3 dB Bandwidth vs. Closed-Loop Gain Resistor,
R
L
= 150
V
S
– Volts Gain R
F
– BW – MHz
±7.5 +1 5000 350
+2 750 275
+10 400 105
–1 750 165
±12 +1 8000 380
+10 215 150
–1 750 95
±2.5 +2 1125 125
Driving Capacitive Loads
When used in combination with the appropriate feedback resistor,
the AD8024 will drive any load capacitance without oscillation.
In accordance with the general rule for current feedback ampli-
fiers, increased load capacitance requires the use of a higher
feedback resistor for stable operation.
Due to the high open-loop transresistance and low inverting
input current of the AD8024, large feedback resistors do not
create large closed-loop gain errors. In addition, the high output
current allows rapid voltage slewing on large load capacitors.
For wide bandwidth and clean pulse response, an additional
small series output resistor of about 10 Ω is recommended.
–
+
AD8024
R
F
R
G
R
T
R
S
+V
S
–V
S
1.0F
0.1F
1.0F
0.1F
C
L
V
O
V
IN
Figure 3. Circuit for Driving a Capacitive Load
V
OUT
V
IN
1V 20nS
2V
Figure 4. Pulse Response Driving a Large Load
Capacitance, C
L
= 300 pF, G = +3, R
FB
= 2.32 k
Ω
,
R
S
= 10.5
Ω
, R
L
= 1 k
Ω
, V
S
=
±
7.5 V
