REV. A–12–
AD8047/AD8048
THEORY OF OPERATION
General
The AD8047 and AD8048 are wide bandwidth, voltage feed-
back amplifiers. Since their open-loop frequency response follows
the conventional 6 dB/octave roll-off, their gain bandwidth
product is basically constant. Increasing their closed-loop gain
results in a corresponding decrease in small signal bandwidth.
This can be observed by noting the bandwidth specification
between the AD8047 (gain of 1) and AD8048 (gain of 2).
Feedback Resistor Choice
The value of the feedback resistor is critical for optimum perfor-
mance on the AD8047 and AD8048. For maximum flatness at a
gain of 2, R
F
and R
G
should be set to 200 Ω for the AD8048.
When the AD8047 is configured as a unity gain follower, R
F
should be set to 0 Ω (no feedback resistor should be used) for
the plastic DIP and 66.5 Ω for the SOIC.
V
IN
V
OUT
+V
S
–V
S
6
7
2
4
3
R
F
R
TERM
R
G
G = 1
+
R
F
R
G
AD8047/
AD8048
10F
10F
0.1F
0.1F
Figure 3. Noninverting Operation
V
IN
V
OUT
+V
S
–V
S
6
7
2
4
3
R
F
R
TERM
R
G
10F
10F
0.1F
0.1F
G =
R
F
R
G
–
AD8047/
AD8048
Figure 4. Inverting Operation
When the AD8047 is used in the transimpedance (I to V) mode,
such as in photodiode detection, the values of R
F
and diode
capacitance (C
I
) are usually known. Generally, the value of R
F
selected will be in the kΩ range, and a shunt capacitor (C
F
)
across R
F
will be required to maintain good amplifier stability.
The value of C
F
required to maintain optimal flatness (<1 dB
peaking) and settling time can be estimated as
C
F
≅ (2 ω
O
C
I
R
F
–1)/ω
O
2
R
F
2
[]
1/2
where
O
is equal to the unity gain bandwidth product of
the amplifier in rad/sec, and C
I
is the equivalent total input
capacitance at the inverting input. Typically,
O
= 800 × 10
6
rad/sec
(see Open-Loop Frequency Response curve, TPC 15).
As an example, choosing R
F
= 10 kΩ and C
I
= 5 pF requires
C
F
to be 1.1 pF (Note: C
I
includes both source and parasitic
circuit capacitance). The bandwidth of the amplifier can be
estimated using the C
F
calculated as
f
3 dB
≅
1. 6
2πR
F
C
F
For general voltage gain applications, the amplifier bandwidth
can be closely estimated as
f
3 dB
≅
ω
O
2π 1 +
R
F
R
G
This estimation loses accuracy for gains of +2/–1 or lower due
to the amplifier’s damping factor. For these low gain cases, the
bandwidth will actually extend beyond the calculated value (see
Closed-Loop BW plots, TPCs 13 and 25).
As a general rule, capacitor C
F
will not be required if
(R
F
R
G
) × C
I
≤
NG
4 ω
O
where NG is the Noise Gain (1 + R
F
/R
G
) of the circuit. For
most voltage gain applications, this should be the case.
V
OUT
R
F
I
I
C
I
C
F
AD8047
Figure 5. Transimpedance Configuration
Pulse Response
Unlike a traditional voltage feedback amplifier, where the slew
speed is dictated by its front end dc quiescent current and gain
bandwidth product, the AD8047 and AD8048 provide on
demand current that increases proportionally to the input step
signal amplitude. This results in slew rates (1000 V/µs) compa-
rable to wideband current feedback designs. This, combined
with relatively low input noise current (1.0 pA/√Hz), gives the
AD8047 and AD8048 the best attributes of both voltage and
current feedback amplifiers.
Large Signal Performance
The outstanding large signal operation of the AD8047 and
AD8048 is due to a unique, proprietary design architecture.
In order to maintain this level of performance, the maximum
180 V-MHz product must be observed (e.g., @ 100 MHz,
V
O
≤ 1.8 V p-p) on the AD8047 and the 250 V-MHz product
must be observed on the AD8048.
Power Supply Bypassing
Adequate power supply bypassing can be critical when optimiz-
ing the performance of a high frequency circuit. Inductance in
the power supply leads can form resonant circuits that produce
peaking in the amplifier’s response. In addition, if large current
transients must be delivered to the load, then bypass capacitors
(typically greater than 1 µF) will be required to provide the best
settling time and lowest distortion. A parallel combination of at
least 4.7 µF, and between 0.1 µF and 0.01 µF, is recommended.
Some brands of electrolytic capacitors will require a small series
damping resistor ≈4.7 Ω for optimum results.
Driving Capacitive Loads
The AD8047/AD8048 have excellent cap load drive capability
for high speed op amps, as shown in Figures 7 and 9. However,
when driving cap loads greater than 25 pF, the best frequency
response is obtained by the addition of a small series resistance.
