AD8022

Rev. B | Page 12 of 16

THEORY OF OPERATION

The AD8022 is a voltage-feedback op amp designed especially

for ADSL or other applications requiring very low voltage and

current noise along with low supply current, low distortion, and

ease of use.

The AD8022 is fabricated on Analog Devices’ proprietary

eXtra-Fast Complementary Bipolar (XFCB) process, which

enables the construction of PNP and NPN transistors with

similar fTs in the 4 GHz region. The process is dielectrically

isolated to eliminate the parasitic and latch-up problems caused

by junction isolation. These features enable the construction of

high frequency, low distortion amplifiers with low supply

currents.

15

Ω

15

Ω

OUTPUT

+IN

–IN

7.5pF

600

μA

Figure 38. Simplified Schematic

As shown in Figure 38, the AD8022 input stage consists of an

NPN differential pair in which each transistor operates a

300 μA collector current. This gives the input devices a high

transconductance and therefore gives the AD8022 a low input

noise of 2.5 nV/√Hz @ 100 kHz. The input stage drives a folded

cascode that consists of a pair of PNP transistors. These PNPs

then drive a current mirror that provides a differential input to

single-ended output conversion. The output stage provides a

high current gain of 10,000 so that the AD8022 can maintain a

high dc open-loop gain, even into low load impedances.