Choose gain-setting resistors RIN and RF according to
the amount of desired gain, keeping in mind the maximum
output amplitude. The output coupling capacitor, COUT,
blocks the DC component of the amplifier output, prevent-
ing DC current flowing to the load. The output capacitor
and the load impedance form a highpass filer with the
-3dB point determined by:
3dB IN OUT
f2R C
−=
π
For a 32Ω load, a 100μF aluminum electrolytic capacitor
gives a low-frequency pole at 50Hz.
Bridge Amplier
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 eliminates
the need for the large coupling capacitor required by the
single op-amp speaker driver when single-supply opera-
tion is necessary. Voltage gain is set to 10V/V; however,
it can be changed by adjusting the 82kΩ resistor 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 extending
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
single-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 maxi-
mum load (32Ω to ground).
Observe the Absolute Maximum Ratings for power dis-
sipation and output short-circuit duration (10s, max)
because the output current can exceed 200mA (see the
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 com-
bination 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′ << 2kΩ is recommended.
Figure 3. Dual MAX4230/MAX4231 Bridge Amplifier for 200mW
at 3V
Figure 4. Rail-to-Rail Input/Output Range
1
6
7
5
28
3V
3V
34
1/2
MAX4232
1/2
MAX4232
0.5VP-P
R5
51kΩ
R1
16kΩ
R2
82kΩ
R4
10kΩ
32W
fS = 100Hz
R3
10kΩ
R6
51kΩ
C2
0.1µF
C1
0.1mF
IN
1V/div
OUT
1V/div
5µs/div
VCC = 3.0V
RL = 100kΩ
MAX4230–MAX4234 High-Output-Drive, 10MHz, 10V/μs,
Rail-to-Rail I/O Op Amps with Shutdown in SC70
www.maximintegrated.com Maxim Integrated
│
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