MAX4200–MAX4205
Ultra-High-Speed, Low-Noise, Low-Power,
SOT23 Open-Loop Buffers
_______________________________________________________________________________________ 9
• 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 possi-
ble. 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 impedance 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 usu-
ally be minimized by using a properly terminated trans-
mission 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 usu-
ally 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 sensitiv-
ity to load variations. This feedback also produces
demand-driven current bias to the output transistors for
±90mA (MAX4200/MAX4203) drive capability that is rel-
atively independent of the output voltage (see Typical
Operating Characteristics).
Output Capacitive Loading and Stability
The MAX4200–MAX4205 provide maximum AC perfor-
mance with no load capacitance. This is the case when
the load is a properly terminated transmission line.
However, these devices are designed to drive any load
capacitance without oscillating, but with reduced AC per-
formance.
Since the MAX4200–MAX4205 operate in an open-loop
configuration, there is no negative feedback to be
transformed into positive feedback through phase shift
introduced by a capacitive load. Therefore, these
devices will not oscillate with capacitive loading, unlike
similar buffers operating in a closed-loop configuration.
However, a capacitive load reacting with the buffer’s
output impedance can still affect circuit performance. A
capacitive load will form a lowpass filter with the
buffer’s output resistance, thereby limiting system
bandwidth. With higher capacitive loads, bandwidth is
dominated by the RC network formed by R
T
and C
L
;
the bandwidth of the buffer itself is much higher. Also
note that the isolation resistor forms a divider that
decreases the voltage delivered to the load.
Another concern when driving capacitive loads results
from the amplifier’s output impedance, which looks
inductive at high frequency. This inductance forms an
L-C resonant circuit with the capacitive load and caus-
es peaking in the buffer’s frequency response.
Figure 2 shows the frequency response of the
MAX4200/MAX4203 under different capacitive loads. To
settle out some of the peaking, the output requires an iso-
lation resistor like the one shown in Figure 3. Figure 4 is a
plot of the MAX4200/MAX4203 frequency response with
capacitive loading and a 10Ω isolation resistor. In many
applications, the output termination resistors included in
the MAX4201/MAX4202/ MAX4204/MAX4205 will serve
this purpose, reducing component count and board
space. Figure 5 shows the MAX4201/MAX4202/
MAX4204/MAX4205 frequency response with capacitive
loads of 47pF, 68pF, and 120pF.
Coaxial Cable Drivers
Coaxial cable and other transmission lines are easily dri-
ven when properly terminated at both ends with their
characteristic impedance. Driving back-terminated
transmission lines essentially eliminates the line’s capaci-
tance. The MAX4201/MAX4204, with their integrated 50Ω
output termination resistors, are ideal for driving 50Ω
cables. The MAX4202/MAX4205 include integrated 75Ω
termination resistors for driving 75Ω cables. Note that the
output termination resistor forms a voltage divider with
the load resistance, thereby decreasing the amplitude of
the signal at the receiving end of the cable by one half
(see the Typical Application Circuit).
