MAX4230–MAX4234
Choose gain-setting resistors RIN and RFaccording to
the amount of desired gain, keeping in mind the maxi-
mum output amplitude. The output coupling capacitor,
COUT, blocks the DC component of the amplifier out-
put, preventing DC current flowing to the load. The out-
put capacitor and the load impedance form a highpass
filer with the -3dB point determined by:
For a 32Ωload, a 100µF aluminum electrolytic capaci-
tor gives a low-frequency pole at 50Hz.
Bridge Amplifier
The circuit shown in Figure 3 uses a dual MAX4230 to
implement a 3V, 200mW amplifier suitable for use in
size-constrained applications. This configuration elimi-
nates the need for the large coupling capacitor
required by the single op-amp speaker driver when sin-
gle-supply operation is necessary. Voltage gain is set
to 10V/V; however, it can be changed by adjusting the
82kΩresistor value.
Rail-to-Rail Input Stage
The MAX4230–MAX4234 CMOS op amps have parallel-
connected N- and P-channel differential input stages
that combine to accept a common-mode range extend-
ing to both supply rails. The N-channel stage is active
for common-mode input voltages typically greater than
(VSS + 1.2V), and the P-channel stage is active for
common-mode input voltages typically less than (VDD -
1.2V).
Rail-to-Rail Output Stage
The minimum output is within millivolts of ground for sin-
gle-supply operation, where the load is referenced to
ground (VSS). Figure 4 shows the input voltage range
and the output voltage swing of a MAX4230 connected
as a voltage follower. The maximum output voltage
swing is load dependent; however, it is guaranteed to
be within 500mV of the positive rail (VDD = 2.7V) even
with maximum load (32Ωto ground).
The MAX4230–MAX4234 incorporate a smart short-cir-
cuit protection feature. When VOUT is shorted to VDD or
VSS, the device detects a fault condition and limits the
output current, therefore protecting the device and the
application circuit. If VOUT is shorted to any voltage
other than VDD or VSS, the smart short-circuit protection
is not activated. When the smart short circuit is not
active, the output currents can exceed 200mA (see
Typical Operating Characteristics.)
Input Capacitance
One consequence of the parallel-connected differential
input stages for rail-to-rail operation is a relatively large
input capacitance CIN (5pF typ). This introduces a pole
at frequency (2πR′CIN)-1, where R′is the parallel combi-
nation of the gain-setting resistors for the inverting or
noninverting amplifier configuration (Figure 5). If the pole
frequency is less than or comparable to the unity-gain
bandwidth (10MHz), the phase margin is reduced, and
the amplifier exhibits degraded AC performance through
either ringing in the step response or sustained oscilla-
tions. The pole frequency is 10MHz when R′= 2kΩ. To
maximize stability, R′<< 2kΩis recommended.