APPLICATION INFORMATION
FULLY DIFFERENTIAL AMPLIFIER
Advantages of Fully Differential Amplifiers
SHORT CIRCUIT AUTO-RECOVERY
COMPONENT SELECTION
Decoupling Capacitor (C
S
)
Input Capacitors (C
I
)
TPA2035D1
SLOS562 – AUGUST 2008
................................................................................................................................................................................................
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The device is a fully differential amplifier with differential inputs and outputs. The fully differential amplifier
consists of a differential amplifier and a common-mode amplifier. The differential amplifier ensures that the
amplifier outputs a differential voltage on the output that is equal to the differential input times the gain. The
common-mode feedback specifies that the common-mode voltage at the output is biased around V
DD
/2
regardless of the common-mode voltage at the input. The fully differential TPA2035D1 can still be used with a
single-ended input; however, the TPA2035D1 should be used with differential inputs when in a noisy
environment, like a wireless handset, to ensure maximum noise rejection.
•
Input-coupling capacitors not required:
–
The fully differential amplifier allows the inputs to be biased at voltage other than mid-supply. The inputs of
the TPA2035D1 can be biased anywhere within the common mode input voltage range listed in the
Recommended Operating Conditions table. If the inputs are biased outside of that range, input-coupling
capacitors are required.
•
Midsupply bypass capacitor, C
(BYPASS)
, not required:
–
The fully differential amplifier does not require a bypass capacitor. Any shift in the midsupply affects both
positive and negative channels equally and cancels at the differential output.
•
Better RF-immunity:
–
GSM handsets save power by turning on and shutting off the RF transmitter at a rate of 217 Hz. The
transmitted signal is picked-up on input and output traces. The fully differential amplifier cancels the signal
better than the typical audio amplifier.
When a short circuit event happens, the TPA2035D1 goes to shutdown mode and tries to reactivate itself after
20
µ
s. This auto-recovery will continue until the short circuit event stops. This feature can protect the device
without affecting the device's long term reliability.
Figure 29
shows the TPA2035D1 typical schematic with differential inputs, while
Figure 30
shows the
TPA2035D1 with differential inputs and input capacitors.
Figure 31
shows the TPA2035D1 with a single-ended
input.
The TPA2035D1 is a high-performance class-D audio amplifier that requires adequate power supply decoupling
to ensure the efficiency is high and total harmonic distortion (THD) is low. For higher frequency transients,
spikes, or digital hash on the line, a good low equivalent-series-resistance (ESR) ceramic capacitor, typically
1
µ
F, placed as close as possible to the device V
DD
lead works best. Placing this decoupling capacitor close to
the TPA2035D1 is important for the efficiency of the class-D amplifier, because any resistance or inductance in
the trace between the device and the capacitor can cause a loss in efficiency. For filtering lower-frequency noise
signals, a 10
µ
F or greater capacitor placed near the audio power amplifier would also help, but it is not required
in most applications because of the high PSRR of this device. Typically, the smaller the capacitor's case size, the
lower the inductance and the closer it can be placed to the TPA2035D1.
The TPA2035D1 does not require input coupling capacitors if the design uses a differential source that is biased
within the common-mode input voltage range. That voltage range is listed in the Recommended Operating
Conditions table. If the input signal is not biased within the recommended common-mode input range, such as in
needing to use the input as a high pass filter, shown in
Figure 30
, or if using a single-ended source, shown in
Figure 31
, input coupling capacitors are required. The same value capacitors should be used on both IN+ and
IN– for best pop performance.
10
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