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AD8011 THEORY OF OPERATION

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 characteristics. See data sheet comparisons.

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 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 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 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 distortion and higher open-loop gain.

A1

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 quiescent 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

CD

–V I IQ1

A2 C P1 CP2

Q3

ICQ + IO

IR + I FC

Q1 VN

VP

V O´

ZI Z2

Q2

VO

A3 RF

IE

RL IR – IFC

Q4

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

Z1 = R1 || C1 Z1

IPN

IPP

DC GAIN CHARACTERISTICS

ICQ – IO

Z1 IQ1 –V I

INP

IPN

A2 C P1

AD8011

A1

CD

Figure 28. Simplified Block Diagram

REV. 0

–9–

RL

CL


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