AD8017

–11–

REV. C

THEORY OF OPERATION

The AD8017 is a dual high speed CF amplifier that attains new

levels of bandwidth (BW), power, distortion and signal swing,

under heavy current loads. 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 AD8017 basically uses a two gain stage comple-

mentary 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

cascode 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 quies-

cent current of the gain stages (with the exception of input level

shift diodes Q1/Q2). Using two stages as opposed to one, also

allows for a higher overall gain bandwidth product (GBWP) for

the same power, thus providing 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. See Figure 6.

Overall, when “high” external load drive and low ac distortion is

a requirement, a twin gain stage integrating amplifier like the

AD8017 will provide excellent results for low power over the

traditional single stage complementary devices. In addition,

being a CF amplifier, closed-loop BW variations versus external

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 less than half the quiescent power (versus

12 V supply) with little degradation in its ac and dc performance

characteristics. See specification pages for comparisons.

DC GAIN CHARACTER

Gain stages A1/

A1 and A2/A2 combined provide negative feed-

forward transresistance gain. See Figure 6. 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 quiescent

power as discussed above. In the quasi dc-to-low frequency

region, the closed loop gain relationship can be approximated as:

These basic relationships above are common to all traditional

operational amplifiers.

A1

IPN

IQ1

Q3

IPP

+

Q1

Q2

Z1

Q4

–

INP

IPN

A1

IR + IFC

IR – IFC

IQ1

IE

AD8017

–A2

ICQ + IO

–A3

–

A2

Z1

ICQ – IO

Z2

Z1 = R1 || C1

Z1

Figure 6. Simplified Block Diagram