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SLOS417B – OCTOBER 2003 – REVISED JULY 2006
APPLICATION INFORMATION
FULLY DIFFERENTIAL AMPLIFIER
The TPA2010D1 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 ensures 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 TPA2010D1 can still be used with a
single-ended input; however, the TPA2010D1 should be used with differential inputs when in a noisy
environment, like a wireless handset, to ensure maximum noise rejection.
Advantages of Fully DIfferential Amplifiers
•
Input-coupling capacitors not required:
– The fully differential amplifier allows the inputs to be biased at voltage other than mid-supply. For example,
if a codec has a midsupply lower than the midsupply of the TPA2010D1, the common-mode feedback
circuit will adjust, and the TPA2010D1 outputs will still be biased at midsupply of the TPA2010D1. The
inputs of the TPA2010D1 can be biased from 0.5 V to V
DD
– 0.8 V. 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. This is because 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
much better than the typical audio amplifier.
COMPONENT SELECTION
shows the TPA2010D1 typical schematic with differential inputs and
shows the TPA2010D1
with differential inputs and input capacitors, and
shows the TPA2010D1 with single-ended inputs.
Differential inputs should be used whenever possible because the single-ended inputs are much more
susceptible to noise.
Table 1. Typical Component Values
REF DES
R
I
C
S
C
I (1)
(1)
VALUE
150 kΩ (±0.5%)
1 µF (+22%, -80%)
3.3 nF (±10%)
EIA SIZE
0402
0402
0201
MANUFACTURER
Panasonic
Murata
Murata
PART NUMBER
ERJ2RHD154V
GRP155F50J105Z
GRP033B10J332K
C
I
is only needed for single-ended input or if V
ICM
is not between 0.5 V and V
DD
– 0.8 V. C
I
= 3.3 nF
(with R
I
= 150 kΩ) gives a high-pass corner frequency of 321 Hz.
Input Resistors (R
I
)
The input resistors (R
I
) set the gain of the amplifier according to
V
Gain
+
2 x 150 kW
R
V
I
(1)
Resistor matching is very important in fully differential amplifiers. The balance of the output on the reference
voltage depends on matched ratios of the resistors. CMRR, PSRR, and cancellation of the second harmonic
distortion diminish if resistor mismatch occurs. Therefore, it is recommended to use 1% tolerance resistors or
better to keep the performance optimized. Matching is more important than overall tolerance. Resistor arrays
with 1% matching can be used with a tolerance greater than 1%.
Place the input resistors very close to the TPA2010D1 to limit noise injection on the high-impedance nodes.
For optimal performance the gain should be set to 2 V/V or lower. Lower gain allows the TPA2010D1 to operate
at its best, and keeps a high voltage at the input making the inputs less susceptible to noise.
9
TI [ TEXAS INSTRUMENTS ]
