AD8011AN(2000) Просмотр технического описания (PDF) - Analog Devices

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AD8011AN
(Rev.:2000)

300 MHz, 1 mA Current Feedback Amplifier

Analog Devices 

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 × VSIG)/(distortion

× 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 complementary 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 re-

ferred 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 ampli-

tude 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 band-

width 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 result-

ing 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 (vs. 10 V supply)

with little degradation in its ac and dc performance characteris-

tics. See data sheet comparisons.

DC GAIN CHARACTERISTICS

Gain stages A1/A1B and A2/A2B combined provide negative

feedforward transresistance gain. See Figure 28. 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 both reduce 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. Due to the inverting input error current

(IE) required to servo the output and the inverting I E × R I drop

A1

Z1 = R1 || C1

Z1

IPP

IPN

–VI

IQ1

CP1

Q3

IR + IFC

Q1

VP

VN

ZI

Q2

IE

Q4

IQ1

IR – IFC

Z1

INP

–VI

IPN

CP1

A1

CD

A2

CP2

ICQ + IO

VO

Z2

A3

RL

RF

RL

ICQ – IO

A2

AD8011

CD

VO

CL

Figure 28. Simplified Block Diagram

REV. B

–9–

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