TPA152

75-mW STEREO AUDIO POWER AMPLIFIER

SLOS210A – JUNE 1998 – REVISED MARCH 2000

12

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

APPLICATION INFORMATION

gain setting resistors, RF and RI

The gain for the TPA152 is set by resistors RF and RI according to equation 1.

(1)

Gain

+*

R

F

R

I

Given that the TPA152 is a MOS amplifier, the input impedance is very high, consequently input leakage

currents are not generally a concern although noise in the circuit increases as the value of RF increases. In

addition, a certain range of RF values are required for proper start-up operation of the amplifier. Taken together

it is recommended that the effective impedance seen by the inverting node of the amplifier be set between 5

k

Ω and 20 kΩ. The effective impedance is calculated in equation 2.

(2)

Effective Impedance

+

R

FRI

R

F ) RI

As an example, consider an input resistance of 20 k

Ω and a feedback resistor of 20 kΩ. The gain of the amplifier

would be – 1 and the effective impedance at the inverting terminal would be 10 k

Ω, which is within the

recommended range.

For high performance applications, metal film resistors are recommended because they tend to have lower

noise levels than carbon resistors. For values of RF above 50 kΩ, the amplifier tends to become unstable due

to a pole formed from RF and the inherent input capacitance of the MOS input structure. For this reason, a small

compensation capacitor of approximately 5 pF should be placed in parallel with RF. This, in effect, creates a

low-pass filter network with the cutoff frequency defined in equation 3.

(3)

f

c(lowpass) +

1

2

pR

F CF

For example if RF is 100 kΩ and CF is 5 pF then fco(lowpass) is 318 kHz, which is well outside the audio range.

input capacitor, CI

In the typical application, an input capacitor, CI, is required to allow the amplifier to bias the input signal to the

proper dc level for optimum operation. In this case, CI and RI form a high-pass filter with the corner frequency

determined in equation 4.

(4)

f

c(highpass) +

1

2

pR

I

C

I

The value of CI is important to consider as it directly affects the bass (low frequency) performance of the circuit.

Consider the example where RI is 20 kΩ and the specification calls for a flat bass response down to 20 Hz.

Equation 4 is reconfigured as equation 5.

(5)

C

I +

1

2

pR

I

f

c(highpass)

In this example, CI is 0.40 µF, so one would likely choose a value in the range of 0.47 µF to 1 µF. A further

consideration for this capacitor is the leakage path from the input source through the input network (RI, CI) and

the feedback resistor (RF) to the load. This leakage current creates a dc offset voltage at the input to the amplifier

that reduces useful headroom, especially in high-gain applications (> 10). For this reason a low-leakage

tantalum or ceramic capacitor is the best choice. When polarized capacitors are used, the positive side of the

capacitor should face the amplifier input in most applications, as the dc level there is held at VDD/2, which is

likely higher that the source dc level. Please note that it is important to confirm the capacitor polarity in the

application.