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APPLICATION INFORMATION

FULLY DIFFERENTIAL AMPLIFIER

Advantages of Fully Differential Amplifiers

COMPONENT SELECTION

_

+

IN–

IN+

PWM

H–

Bridge

Internal

Oscillator

To Battery

GND

Bias

Circuitry

+

–

Differential

Input

TPA2005D1

Filter-Free Class D

SHUTDOWN

TPA2005D1-Q1

SLOS474 – AUGUST 2005

The TPA2005D1 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

regardless of the common-mode voltage at the input. The fully differential TPA2005D1 can still be used with a

single-ended input; however, the TPA2005D1 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 a voltage other than midsupply. For

example, if a codec has a midsupply lower than the midsupply of the TPA2005D1, the common-mode

feedback circuit adjusts, and the TPA2005D1 outputs still is biased at midsupply of the TPA2005D1. The

range, input-coupling capacitors are 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 26 shows the TPA2005D1 typical schematic with differential inputs, and Figure 27 shows the TPA2005D1

with differential inputs and input capacitors, and Figure 28 shows the TPA2005D1 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

150 k

Ω (±0.5%)

0402

Panasonic

ERJ2RHD154V

1

µF (+22%, -80%)

0402

Murata

GRP155F50J105Z

3.3 nF (

±10%)

0201

Murata

GRP033B10J332K

(1)

high-pass corner frequency of 321 Hz.

Figure 26. Typical TPA2005D1 Application Schematic With Differential Input for a Wireless Phone

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