1GHz, Low-Power, SOT23,

Current-Feedback Amplifiers with Shutdown

10

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_______________Detailed Description

The MAX4223–MAX4228 are ultra-high-speed, low-

power, current-feedback amplifiers featuring -3dB

bandwidths up to 1GHz, 0.1dB gain flatness up to

300MHz, and very low differential gain and phase

errors of 0.01% and 0.02°, respectively. These devices

operate on dual ±5V or ±3V power supplies and

require only 6mA of supply current per amplifier. The

MAX4223/MAX4225/MAX4226 are optimized for

closed-loop gains of +1 (0dB) or more and have -3dB

bandwidths of 1GHz. The MAX4224/MAX4227/

MAX4228 are optimized for closed-loop gains of +2

(6dB) or more, and have -3dB bandwidths of 600MHz

(1.2GHz gain-bandwidth product).

The current-mode feedback topology of these ampli-

fiers allows them to achieve slew rates of up to

1700V/µs with corresponding large signal bandwidths

up to 330MHz. Each device in this family has an output

that is capable of driving a minimum of 60mA of output

current to ±2.5V.

Theory of Operation

Since the MAX4223–MAX4228 are current-feedback

amplifiers, their open-loop transfer function is

expressed as a transimpedance:

The frequency behavior of this open-loop transimped-

ance is similar to the open-loop gain of a voltage-feed-

back amplifier. That is, it has a large DC value and

decreases at approximately 6dB per octave.

Analyzing the current-feedback amplifier in a gain con-

figuration (Figure 1) yields the following transfer func-

tion:

tional voltage-feedback amplifiers, the closed-loop

bandwidth is essentially independent of the closed-

Low-Power Shutdown Mode

The MAX4223/MAX4224/MAX4226/MAX4228 have a

shutdown mode that is activated by driving the SHDN

input low. When powered from ±5V supplies, the SHDN

input is compatible with TTL logic. Placing the amplifier

in shutdown mode reduces quiescent supply current to

350µA typical, and puts the amplifier output into a high-

impedance state (100k

Ω typical). This feature allows

these devices to be used as multiplexers in wideband

systems. To implement the mux function, the outputs of

multiple amplifiers can be tied together, and only the

amplifier with the selected input will be enabled. All of

the other amplifiers will be placed in the low-power

shutdown mode, with their high output impedance pre-

senting very little load to the active amplifier output. For

gains of +2 or greater, the feedback network imped-

ance of all the amplifiers used in a mux application

must be considered when calculating the total load on

the active amplifier output.

__________Applications Information

Layout and Power-Supply Bypassing

The MAX4223–MAX4228 have an extremely high band-

width, and consequently require careful board layout,

including the possible use of constant-impedance

microstrip or stripline techniques.

V

V

G

R

R

OUT

IN

F

G

=

=

+

1

.

V

V

G x

T

S

T

S

G x R

R

where G

A

R

R

OUT

IN

Z

Z

IN

F

V

F

G

=

( )

+

=

=

+

−

1

∆

∆

V

I

or T

OUT

IN

Z

−

MAX4223

MAX4224

MAX4225

MAX4226

MAX4227

MAX4228

IN-

OUT

+1

IN+

+1

Figure 1. Current-Feedback Amplifier