AD8042 View Datasheet(PDF) - Analog Devices
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AD8042 Datasheet PDF : 16 Pages
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AD8042
APPLICATIONS INFORMATION
CIRCUIT DESCRIPTION
The AD8042 is 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 35). 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 performance. With this design, harmonic distortion
of better than −77 dB @ 1 MHz into 100 Ω with VOUT = 2 V p-p
(gain = +2) on a single 5 V supply is achieved.
VCC
VINP
VINN
I1
R26
Q4
I10
R39
Q5
R15 R2
Q13 Q17
Q40
VEE
I2 I3 Q25
Q51
Q50
I9
Q36
Q39
I5
Q23 VEE
Q22
R23 R27
Q7
Q31
Q21 Q27
C3
VOUT
SIP
SIN
C9
Q2
Q11
Q3
Q24
C7
VEE
R5
R21 R3
I7
Q8
Q47
I8
VCC
Figure 35. Simplified Schematic
The rail-to-rail output range of the AD8042 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 AD8042 to drive 40 mA of output current
with the outputs within 0.5 V of the supply rails.
On the input side, the device can handle voltages from 0.2 V
below the negative rail to within 1.2 V of the positive rail.
Exceeding these values does not cause phase reversal; however,
the input ESD devices do begin to conduct if the input voltages
exceed the rails by greater than 0.5 V.
DRIVING CAPACITIVE LOADS
The capacitive load drive of the AD8042 can be increased by
adding a low valued resistor in series with the load. Figure 36
shows the effects of a series resistor on capacitive drive for
varying voltage gains. As the closed-loop gain is increased, the
larger phase margin allows for larger capacitive loads with less
overshoot. Adding a series resistor with lower closed-loop gains
accomplishes the same effect. For large capacitive loads, the
frequency response of the amplifier is dominated by the roll-off
of the series resistor and capacitive load.
1000
VS = 5V
200mV STEP WITH 90% OVERSHOOT
RS
CL
RS = 5Ω
RS = 0Ω
100
RS = 20Ω
10
1
2
3
4
5
CLOSED-LOOP GAIN (V/V)
Figure 36. Capacitive Load Drive vs. Closed-Loop Gain
OVERDRIVE RECOVERY
Overdrive of an amplifier occurs when the output and/or input
range are exceeded. The amplifier must recover from this overdrive
condition. As shown in Figure 37, the AD8042 recovers within
30 ns from negative overdrive and within 25 ns from positive
overdrive.
5.0V
2.5V
0V
1V
G = +2
VS = 5V
VIN = 5V p-p
RL = 1kΩ TO 2.5V
Figure 37. Overdrive Recovery
50ns
Rev. E | Page 12 of 16
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