AD8011ARZ-REEL Datasheet, PDF (9/16 Pages) Analog Devices – 300 MHz Current Feedback Amplifier

AD8011
THEORY OF OPERATION
The AD8011 is a revolutionary generic high speed CF amplifier
that attains new levels of BW, power, distortion, and signal swing
capability. If these key parameters were combined as a figure of
ac merit performance or [(frequency Ï« V_SIG)/(distortion Ï« power)],
no IC amplifier today would come close to the merit value of the
AD8011 for frequencies above a few MHz. Its wide dynamic
performance (including noise) is the result of both a new com-
plementary high speed bipolar process and a new and unique
architectural design. The AD8011 uses basically a two gain stage
complementary design approach versus the traditional “single
stage” complementary mirror structure sometimes referred to as
the Nelson amplifier. Though twin stages have been tried before,
they typically consumed high power since they were of a folded
cascade design much like the AD9617. This design allows for
the standing or quiescent current to add to the high signal or slew
current induced stages much like the Nelson or single-stage design.
Thus, in the time domain, the large signal output rise/fall time
and slew rate is controlled typically by the small signal BW of the
amplifier and the input signal step amplitude respectively, not the
dc quiescent current of the gain stages (with the exception of
input level shift diodes Q1/Q2). Using two stages versus one also
allows for a higher overall gain bandwidth product (GBWP) for
the same power, thus lower signal distortion and the ability to
drive heavier external loads. In addition, the second gain stage
also isolates (divides down) A3’s input reflected load drive and
the nonlinearities created resulting in relatively lower distortion
and higher open-loop gain.
Overall, when high external load drive and low ac distortion is a
requirement, a twin gain stage integrating amplifier like the AD8011
will provide superior results for lower power over the traditional
single-stage complementary devices. In addition, being a CF
amplifier, closed-loop BW variations versus external gain variations
(varying RN) will be much lower compared to a VF op amp, where
the BW varies inversely with gain. Another key attribute of this
amplifier is its ability to run on a single 5 V supply due in part to
its wide common-mode input and output voltage range capability.
For 5 V supply operation, the device obviously consumes half
the quiescent power (versus 10 V supply) with little degradation
in its ac and dc performance characteristics. See Specifications.
DC GAIN CHARACTERISTICS
Gain stages A1/A1B and A2/A2B combined provide negative
feedforward transresistance gain (see Figure 6). Stage A3 is a unity
gain buffer that provides external load isolation to A2. Each stage
uses a symmetrical complementary design. (A3 is also complemen-
tary though not explicitly shown.) This is done to reduce second
order signal distortion and overall quiescent power as discussed
previously. In the quasi dc to low frequency region, the closed-
loop gain relationship can be approximated as
G = 1 + R_F/R_N
G = –R_F/R_N
noninverting operation
inverting operation
These basic relationships are common to all traditional opera-
tional amplifiers. Due to the inverting input error current (I_E)
required to servo the output and the inverting I_E Ï« R_I drop
A1
Z1 = R1 || C1
Z1
IPP
IPN
–V_I
IQ1
C_P1
Q3
IR + IFC
Q1
V_P
V_N
Z_I
Q2
IE
Q4
IQ1
IR – IFC
Z1
INP
–V_I
IPN
C_P1
A1
C_D
A2
C_P2
ICQ + IO
V_O
Z2
A3
R_L
R_F
R_L
ICQ – IO
A2
AD8011
C_D
V_O
C_L
Figure 6. Simplified Block Diagram
REV. C
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