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APPLICATION INFORMATION
FULLY DIFFERENTIAL AMPLIFIER
Advantages of Fully DIfferential Amplifiers
COMPONENT SELECTION
Input Resistors (RI)
Gain +
2 x 150 kW
R
I
V
(1)
SLOS417C – OCTOBER 2003 – REVISED SEPTEMBER 2007
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 VDD/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.
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 VDD –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.
Figure 27 shows the TPA2010D1 typical schematic with differential inputs and
Figure 28 shows the TPA2010D1
with differential inputs and input capacitors, and
Figure 29 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
VALUE
EIA SIZE
MANUFACTURER
PART NUMBER
RI
150 k
(±0.5%)
0402
Panasonic
ERJ2RHD154V
CS
1
μF (+22%, –80%)
0402
Murata
GRP155F50J105Z
CI
(1)
3.3 nF (±10%)
0201
Murata
GRP033B10J332K
(1)
CI is only needed for single-ended input or if VICM is not between 0.5 V and VDD – 0.8 V. CI = 3.3 nF
(with RI = 150 k) gives a high-pass corner frequency of 321 Hz.
The input resistors (RI) set the gain of the amplifier according to Equation 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.
Copyright 2003–2007, Texas Instruments Incorporated
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