AD8022
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.
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.
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.
+V
S
15Ω
15Ω
+IN
–IN
OUTPUT
7.5pF
600μA
–V
S
Figure 38. Simplified Schematic
Rev. B | Page 12 of 16
ADI [ ADI ]
