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

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_ _

50Ω

50Ω COAX

SOURCE

MAX4200–MAX4205

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

SOT23 Open-Loop Buffers

www.maximintegrated.com

Maxim Integrated │ 9