AD8023
+1
G
GAIN
ACL ꢁ
0
1+ SCT (RF + Gn rin)
–1
where:
CT = transcapacitance ꢂ 1 pF
V
= ꢂ7.5V
RF = feedback resistor
S
–2
–3
–4
–5
G = ideal closed loop gain
V
= ꢂ2.5V
S
R
R
G
PHASE
1+
0
Gn =
F = noise gain
rin = inverting input resistance ꢂ 150 Ω
ACL = closed loop gain
–6
–7
–90
–180
G = –10
R
= 150ꢀ
V
= ꢂ2.5V
The –3 dB bandwidth is determined from this model as:
L
S
–8
–9
1
f3 ꢁ
2 π CT (RF + Gn rin)
1
10
FREQUENCY – MHz
100
500
This model will predict –3 dB bandwidth to within about
10% to 15% of the correct value when the load is 150 Ω and
VS = 7.5 V. For lower supply voltages there will be a slight
decrease in bandwidth. The model is not accurate enough to
predict either the phase behavior or the frequency response
peaking of the AD8023.
Figure 30. Closed-Loop Gain and Phase vs. Frequency,
G = –10, RL = 150 Ω
General
The AD8023 is a wide bandwidth, triple video amplifier that
offers a high level of performance on less than 9.0 mA per
amplifier of quiescent supply current. The AD8023 achieves
bandwidth in excess of 200 MHz, with low differential gain and
phase errors and high output current making it an efficient video
amplifier.
It should be noted that the bandwidth is affected by attenuation
due to the finite input resistance. Also, the open-loop output
resistance of about 6 Ω reduces the bandwidth somewhat when
driving load resistors less than about 150 Ω. (Bandwidths will
be about 10% greater for load resistances above a couple
hundred ohms.)
The AD8023’s wide phase margin coupled with a high output
short circuit current make it an excellent choice when driving
any capacitive load up to 300 pF.
Table I. –3 dB Bandwidth vs. Closed-Loop Gain and Feedback
Resistor, RL = 150 ꢀ (SOIC)
It is designed to offer outstanding functionality and performance
at closed-loop inverting or noninverting gains of one or greater.
VS – Volts
Gain
RF – Ohms
BW – MHz
7.5
+1
+2
+10
–1
–10
+1
+2
+10
–1
–10
2000
750
300
750
250
2000
1000
300
460
240
50
150
60
250
90
30
Choice of Feedback and Gain Resistors
Because it is a current feedback amplifier, the closed-loop band-
width of the AD8023 may be customized using different values
of the feedback resistor. Table I shows typical bandwidths at
different supply voltages for some useful closed-loop gains when
driving a load of 150 Ω.
2.5
The choice of feedback resistor is not critical unless it is desired
to maintain the widest, flattest frequency response. The resistors
recommended in the table (chip resistors) are those that will
result in the widest 0.1 dB bandwidth without peaking. In
applications requiring the best control of bandwidth, 1%
resistors are adequate. Resistor values and widest bandwidth
figures are shown. Wider bandwidths than those in the table can
be attained by reducing the magnitude of the feedback resistor
(at the expense of increased peaking), while peaking can be
reduced by increasing the magnitude of the feedback resistor.
750
250
95
50
Driving Capacitive Loads
When used in combination with the appropriate feedback
resistor, the AD8023 will drive any load capacitance without
oscillation. The general rule for current feedback amplifiers is
that the higher the load capacitance, the higher the feedback
resistor required for stable operation. Due to the high open-loop
transresistance and low inverting input current of the AD8023,
the use of a large feedback resistor does not result in large closed-
loop gain errors. Additionally, its high output short circuit current
makes possible rapid voltage slewing on large load capacitors.
Increasing the feedback resistor is especially useful when driving
large capacitive loads as it will increase the phase margin of the
closed-loop circuit. (Refer to the Driving Capacitive Loads
section for more information.)
For the best combination of wide bandwidth and clean pulse
response, a small output series resistor is also recommended.
Table II contains values of feedback and series resistors which
result in the best pulse responses. Figure 28 shows the AD8023
driving a 300 pF capacitor through a large voltage step with
virtually no overshoot. (In this case, the large and small signal
pulse responses are quite similar in appearance.)
To estimate the –3 dB bandwidth for closed-loop gains of 2 or
greater, for feedback resistors not listed in the following table,
the following single pole model for the AD8023 may be used:
REV. A
–9–
ADI [ ADI ]
