AD8012ARM-REEL7(1999) 데이터 시트보기 (PDF) - Analog Devices

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AD8012ARM-REEL7
(Rev.:1999)

Dual 350 MHz Low Power Amplifier

Analog Devices 

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.

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

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.

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.

IPP

VP

+

INP

A1

IPN

IQ1

Q3

Q1

VN

–

ZI

Z1 = R1 || C1

Z1

–VI

C P1

IR + IFC

Q2

IE

Q4

IR – IFC

Z1

IQ1

–VI

IPN

C P1

A1

CD

A2

CP2

ICQ + IO

VO ؅

A3

Z2

RL

RF

RN

ICQ – IO

A2

AD8012

CD

Figure 40. Simplified Block Diagram

VO

CL

REV. A

–11–

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