1 GHz, Lo w -P o w e r, S OT2 3 ,
Cu rre n t -Fe e d b a c k Am p lifie rs w it h S h u t d o w n
_______________De t a ile d De s c rip t io n
The MAX4223–MAX4228 are ultra-high-speed, low-
p owe r, c urre nt-fe e d b a c k a mp lifie rs fe a turing -3d B
R
G
R
F
ba nd widths up to 1GHz, 0.1dB g a in fla tne ss up to
300MHz, a nd ve ry low d iffe re ntia l g a in a nd p ha s e
errors of 0.01% and 0.02°, respectively. These devices
op e ra te on d ua l ± 5V or ± 3V p owe r s up p lie s a nd
require only 6mA of supply current per amplifier. The
MAX4223/MAX4225/MAX4226 a re op timize d for
closed-loop gains of +1 (0dB) or more and have -3dB
b a nd wid ths 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).
IN-
R
IN-
T
Z
OUT
+1
+1
IN+
MAX4223
MAX4224
MAX4225
MAX4226
MAX4227
MAX4228
The current-mode feedback topology of these ampli-
fie rs a llows the m to a c hie ve s le w ra te s 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.
V
IN
3–MAX428
Th e o ry o f Op e ra t io n
Since the MAX4223–MAX4228 are current-feedback
a mp lifie rs , the ir op e n-loop tra ns fe r func tion is
expressed as a transimpedance:
Figure 1. Current-Feedback Amplifier
Lo w -P o w e r S h u t d o w n Mo d e
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 (100kΩ 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.
∆V
OUT
or T
Z
∆I
IN−
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:
T
S
( )
V
Z
OUT
= G x
V
T
S + G x R
( )
+ R
IN− F
IN
Z
R
F
where G = A = 1 +
.
V
R
G
At low gains, (G x R ) << R . Therefore, unlike tradi-
IN-
F
__________Ap p lic a t io n s In fo rm a t io n
tiona l volta g e -fe e db a c k a mp lifie rs, the c lose d -loop
bandwidth is essentially independent of the closed-
La yo u t a n d P o w e r-S u p p ly Byp a s s in g
The MAX4223–MAX4228 have an extremely high band-
width, and consequently require careful board layout,
inc lud ing the p os s ib le us e of c ons ta nt-imp e d a nc e
microstrip or stripline techniques.
loop gain. Note also that at low frequencies, T >> [(G
Z
x R ) + R ], so that:
IN-
F
V
R
OUT
F
= G = 1 +
V
R
IN
G
10 ______________________________________________________________________________________
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