AD8012
THEORY OF OPERATION
traditional single stage complementary devices. In addition,
because the AD8012 is 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 partially because of its wide common-mode
input and output voltage range capability. For 5 V supply
operation, the device consumes half the quiescent power (vs.
10 V supply) with little degradation in its ac and dc perfor-
mance characteristics. See data sheet comparisons.
The AD8012 is a dual, high speed CF amplifier that attains new
levels of bandwidth (BW), power, distortion, and signal swing
capability. Its wide dynamic performance (including noise) is
the result of both a new complementary high speed bipolar
process and a new and unique architectural design. The AD8012
uses 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, similar to that
of the AD9617. This design allows for the standing or quiescent
current to add to the high signal or slew current-induced stages.
In the time domain, the large signal output rise/fall time and
slew rate is typically controlled by the small signal BW of the
amplifier and the input signal step amplitude, respectively, and
not the dc quiescent current of the gain stages (with the excep-
tion 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, resulting in 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.
DC GAIN CHARACTERISTICS
Gain stages A1/A1B and A2/A2B combined provide negative
feedforward transresistance gain as shown in Figure 4. Stage A3
is a unity-gain buffer that provides external load isolation to A2.
Each stage uses a symmetrical complementary design (A3 is also
complementary though not explicitly shown). This is done to
reduce both second-order signal distortion and overall quiescent
power as previously described. In the quasi dc to low frequency
region, the closed-loop gain relationship can be approximated as:
G = 1+ RF /RN noninverting operation
G = –RF /RN
inverting operation
These basic relationships are common to all traditional opera-
tional amplifiers.
Overall, when high external load drive and low ac distortion is a
requirement, a twin-gain stage integrating amplifier like the
AD8012 will provide excellent results for lower power over the
C
A1
D
Z1 = R1 || C1
Z1
IPN
IPP
–V
I
A2
2
IQ1
Q3
C
1
P
C
P
ICQ + IO
IR + IFC
Q1
Q2
V
Ð
V
I
V
N
P
O
+
Z
I
A3
V
O
R
L
C
L
Z2
R
F
IE
R
N
IR – IFC
Q4
ICQ – IO
Z1
IQ1
A2
–V
I
C
1
P
AD8012
INP
IPN
A1
C
D
Figure 4. Simplified Block Diagram
–12–
REV. B
ADI [ ADI ]
