AD8012
traditional single stage complementary devices. In addition,
being a CF amplifier, closed-loop BW variations versus exter-
nal gain variations (varying RN) will be much lower compared
to a VF op amp, where the BW varies inversely with gain. An-
other 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 (vs.
10 V supply) with little degradation in its ac and dc perfor-
mance characteristics. See data sheet comparisons.
THEORY OF OPERATION
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
basically 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 much like
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, not
the dc quiescent current of the gain stages (with the exception
of input level shift diodes Q1/Q2). Using two stages vs. 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 dis-
tortion and higher open-loop gain.
DC GAIN CHARACTERISTICS
Gain stages A1/A1B and A2/A2B combined provide negative
feedforward transresistance gain. See Figure 40. Stage A3 is a
unity gain buffer which 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 quies-
cent power as discussed above. In the quasi dc to low frequency
region, the closed loop gain relationship can be approximated
as:
G = 1+RF /RN
G = –RF /RN
noninverting operation
inverting operation
These basic relationships above are common to all traditional
operational 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
V
N
P
V
O
+
–
Z
I
A3
V
O
R
L
C
L
Z2
R
F
IE
R
N
IR – IFC
Q4
ICQ – IO
Z1
IQ1
A2
C
–V
I
C
1
P
AD8012
INP
IPN
A1
D
Figure 40. Simplified Block Diagram
REV. A
–11–
ADI [ ADI ]
