MAX4180–MAX4187
Single/Dual/Quad, 270MHz, 1mA, SOT23,
Current-Feedback Amplifiers with Shutdown
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Detailed Description
The MAX4180–MAX4187 are ultra-low-power current-
feedback amplifiers featuring bandwidths up to
270MHz, 0.1dB gain flatness to 90MHz, and low differ-
ential gain (0.08%) and phase (0.03°) errors. These
amplifiers achieve ultra-high bandwidth-to-power ratios
with low distortion, wide signal swing, and excellent
load-driving capabilities. They are optimized for ±5V
supplies but also operate from a single +5V supply
while consuming only 1mA per amplifier. With ±60mA
output current drive capability, the devices achieve low
distortion even while driving 150Ω loads.
Wide bandwidth, low power, low differential phase and
gain error, and excellent gain flatness make the
MAX4180–MAX4187 ideal for use in portable video
equipment such as cameras, video switchers, and
other battery-powered applications. Their two-stage
design provides higher gain and lower distortion than
conventional single-stage, current-feedback topolo-
gies. This feature, combined with fast settling time,
makes these devices suitable for buffering high-speed
analog-to-digital converters (ADCs).
The MAX4180/MAX4181/MAX4183/MAX4185 have a
low-power shutdown mode that is activated by driving
the amplifiers’ SHDN input low. Placing them in shut-
down reduces quiescent supply current to 135µA (typ)
and places amplifier outputs in a high-impedance
state. These amplifiers can be used to implement a
high-speed multiplexer by connecting together the out-
puts of multiple amplifiers and controlling the SHDN
inputs to enable one amplifier and disable all the oth-
ers. The disabled amplifiers present very little load
(0.1µA leakage current and 4pF capacitance) to the
active amplifiers’ output. Note that the feedback net-
work impedance of all the disabled amplifiers must be
considered when calculating the total load on the
active amplifier output.
Application Information
Theory of Operation
The MAX4180–MAX4187 are current-feedback ampli-
fiers, and their open-loop transfer function is expressed
as a transimpedance, ∆V
OUT
/∆I
IN
, or T
Z
. The frequency
behavior of the open-loop transimpedance is similar to
the open-loop gain of a voltage-mode feedback
amplifier. That is, it has a large DC value and decreas-
es at approximately 6dB per octave.
Analyzing the follower with gain, as shown in Figure 1,
yields the following transfer function:
V
OUT
/ V
IN
= G x [(T
Z
(S) / T
Z
(s) + G x (R
IN
+ R
F
)]
where G = A
VCL
= 1 + (R
F
/ R
G)
, and R
IN
= 1 /g
M
≅ 160Ω.
At low gains, G x R
IN
< R
F
. Therefore, the closed-loop
bandwidth is essentially independent of closed-loop
gain. Similarly, T
Z
> R
F
at low frequencies, so that:
Layout and Power-Supply Bypassing
The MAX4180–MAX4187 have an RF bandwidth and,
consequently, require careful board layout, including
the possible use of constant-impedance microstrip or
stripline techniques.
To realize the full AC performance of these high-speed
amplifiers, pay careful attention to power-supply bypass-
ing and board layout. The PC board should have at least
two layers: a signal and power layer on one side, and a
large, low-impedance ground plane on the other side.
The ground plane should be as free of voids as possible.
With multilayer boards, locate the ground plane on a
layer that incorporates no signal or power traces.
Regardless of whether a constant-impedance board is
used, observe the following guidelines when designing
the board:
• Do not use wire-wrap boards. They are too inductive.
• Do not use breadboards. They are too capacitive.
• Do not use IC sockets. They increase parasitic ca-
pacitance and inductance.
• Use surface-mount components rather than through-
hole components. They give better high-frequency
performance, have shorter leads, and have lower
parasitic reactances.
