REV. B

–12–

AD8012

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

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.

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

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.

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:

These basic relationships are common to all traditional opera-

tional amplifiers.

Q1

Q2

IPP

IPN

INP

IPN

A1

A1

IQ1

Q3

Q4

IE

Z2

A2

ICQ – IO

ICQ + IO

IQ1

AD8012

A2

Z1 = R1 || C1

Z1

A3

Z1

IR – IFC

IR + IFC

+

Ð

I

Figure 4. Simplified Block Diagram

G= + R R

G= –R R

FN

FN

1/

/

noninverting operation

inverting operation