AD8051/AD8052/AD8054
THEORY OF OPERATION
The inputs of the device can handle voltages from −0.2 V below
the negative rail to within 1 V of the positive rail. Exceeding
these values do not cause phase reversal; however, the input
ESD devices begin to conduct if the input voltages exceed the
rails by greater than 0.5 V. During this overdrive condition, the
output stays at the rail.
CIRCUIT DESCRIPTION
The AD8051/AD8052/AD8054 are fabricated on the Analog
Devices, Inc. proprietary eXtra-Fast Complementary Bipolar
(XFCB) process, which enables the construction of PNP and
NPN transistors with similar fTs in the 2 GHz to 4 GHz region.
The process is dielectrically isolated to eliminate the parasitic
and latch-up problems caused by junction isolation. These
features allow the construction of high frequency, low distortion
amplifiers with low supply currents. This design uses a differential
output input stage to maximize bandwidth and headroom (see
Figure 40). The smaller signal swings required on the first stage
outputs (nodes SIP, SIN) reduce the effect of nonlinear currents
due to junction capacitances and improve the distortion per-
formance. This design achieves harmonic distortion of −80 dBc
@ 1 MHz into 100 Ω with VOUT = 2 V p-p (gain = +1) on a
single 5 V supply.
The rail-to-rail output range of the AD8051/AD8052/AD8054
is provided by a complementary common emitter output stage.
High output drive capability is provided by injecting all output
stage predriver currents directly into the bases of the output
devices Q8 and Q36. Biasing of Q8 and Q36 is accomplished by
I8 and I5, along with a common-mode feedback loop (not
shown). This circuit topology allows the AD8051/AD8052 to
drive 45 mA of output current and allows the AD8054 to drive
30 mA of output current with the outputs within 0.5 V of the
supply rails.
V
CC
I9
Q25
Q50
Q39
R26
Q4
R39
Q5
I10
I2
I3
Q36
I5
Q51
Q23
V
Q40
EE
R2
R15
Q13
Q22
R27
R23
Q21
V
EE
C3
C9
Q31
Q7
V
P
Q1
IN
V
OUT
Q27
V
N
IN
SIN
SIP
Q8
Q2
Q11
R3
Q3
Q24
I7
Q47
I8
CC
I11
V
C7
R21
R5
V
EE
Figure 40. AD8051/AD8052 Simplified Schematic
Rev. H | Page 16 of 24
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