Detailed Description
The MAX4200–MAX4205 wide-band, open-loop buffers
feature high slew rates, high output current, low 2.1nV√Hz
voltage-noise density, and excellent capacitive-load-driv-
ing capability. The MAX4200/MAX4203 are single/dual
buffers with up to 660MHz bandwidth, 230MHz 0.1dB
gain flatness, and a 4200V/μs slew rate. The MAX4201/
MAX4204 single/dual buffers with integrated 50Ω output
termination resistors, up to 780MHz bandwidth, 280MHz
gain flatness, and a 4200V/μs slew rate, are ideally suited
for driving high-speed signals over 50Ω cables. The
MAX4202/MAX4205 provide bandwidths up to 720MHz,
230MHz gain flatness, 4200V/μs slew rate, and integrated
75Ω output termination resistors for driving 75Ω cables.
With an open-loop gain that is slightly less than +1V/V,
these devices do not have to be compensated with the
internal dominant pole (and its associated phase shift)
that is present in voltage-feedback devices. This feature
allows the MAX4200–MAX4205 to achieve a nearly con-
stant group delay time of 405ps over their full frequency
range, making them well suited for a variety of RF and IF
signal-processing applications.
These buffers operate with ±5V supplies and consume
only 2.2mA of quiescent supply current per buffer while
providing up to ±90mA of output current drive capability.
Applications Information
Power Supplies
The MAX4200–MAX4205 operate with dual supplies from
±4V to ±5.5V. Both VCC and VEE should be bypassed to
the ground plane with a 0.1μF capacitor located as close
to the device pin as possible.
Layout Techniques
Maxim recommends using microstrip and stripline tech-
niques to obtain full bandwidth. To ensure that the PC
board does not degrade the amplifier’s performance,
design it for a frequency greater than 6GHz. Pay care-
ful attention to inputs and outputs to avoid large para-
sitic capacitance. Whether or not you use a constant-
impedance board, observe the following guidelines when
designing the board:
●Do not use wire-wrap boards, because they are too
inductive.
● Do not use IC sockets, because they increase para-
sitic capacitance and inductance.
● Use surface-mount instead of through-hole compo-
nents for better high-frequency performance.
● Use a PC board with at least two layers; it should be
as free from voids as possible.
●Keep signal lines as short and as straight as pos-
sible. Do not make 90° turns; round all corners.
Input Impedance
The MAX4200–MAX4205 input impedance looks like
a 500kΩ resistor in parallel with a 2pF capacitor. Since
these devices operate without negative feedback, there
is no loop gain to transform the input impedance upward,
as in closed-loop buffers. As a consequence, the input
impedance is directly related to the output impedance. If
the output load impedance decreases, the input imped-
ance also decreases. Inductive input sources (such as an
unterminated cable) may react with the input capacitance
and produce some peaking in the buffer’s frequency
response. This effect can usually be minimized by using
a properly terminated transmission line at the buffer input,
as shown in Figure 1.
Output Current and Gain Sensitivity
The absence of negative feedback means that open-loop
buffers have no loop gain to reduce their effective output
impedance. As a result, open-loop devices usually suffer
from decreasing gain as the output current is decreased.
The MAX4200–MAX4205 include local feedback around
the buffer’s class-AB output stage to ensure low output
impedance and reduce gain sensitivity to load variations.
This feedback also produces demand-driven current bias
to the output transistors for ±90mA (MAX4200/MAX4203)
drive capability that is relatively independent of the output
voltage (see Typical Operating Characteristics).
Figure 1. Using a Properly Terminated Input Source
MAX42_ _
RL
50Ω
*MAX4201/4202/4204/4205 ONLY
RT*
50Ω COAX
SOURCE
MAX4200–MAX4205 Ultra-High-Speed, Low-Noise, Low-Power,
SOT23 Open-Loop Buers
www.maximintegrated.com Maxim Integrated
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