Ultra-High-Speed, Low-Noise, Low-Power,

SOT23 Open-Loop Buffers

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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

500k

Ω 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. 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

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 isolation resistor like the one shown in Figure 3.

Figure 4 is a plot of the MAX4200/MAX4203 frequency

response with capacitive loading and a 10

Ω 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

capacitance. The MAX4201/MAX4204, with their inte-

grated 50

Ω output termination resistors, are ideal for dri-

ving 50

Ω cables. The MAX4202/MAX4205 include

integrated 75

Ω 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).

MAX42_ _

50

Ω

50

Ω COAX

SOURCE

Figure 1. Using a Properly Terminated Input Source