TLE2301

EXCALIBUR 3-STATE-OUTPUT WIDE-BANDWIDTH

POWER OPERATIONAL AMPLIFIER

SLOS131 – DECEMBER 1993

18

POST OFFICE BOX 655303

APPLICATION INFORMATION

series resistor

current through the filter. With dual supply rails, there is always a potential problem of large turn-on currents as

the amplifier powers up. If one supply rail turns on before the other, the output of the TLE2301 amplifier could

saturate near to the applied supply rail, causing a large current to flow through the transformer winding

its rails could rise to the minimum operating voltage of the amplifier, at which point the amplifier is ensured to

have returned to stable operation.

With a series resistor of 3.3

Ω and assuming the output saturates at the maximum peak-to-peak voltage

= 0.91 A). Further reduction of this turn-on current by raising the value of the series resistor deteriorates the

filter’s performance into low signalling impedances on the mains network.

Alternatively, this turn-on current could be blocked by means of a series capacitor, but for this frequency band

the capacitor has to be large in value (

≥3.3 µF) so as not to adversely affect the filter. A nonpolarized capacitor

of this value is relatively expensive, and the resistor is still required to fulfill other functions.

Another way of preventing overcurrent at power up is to use the TLE2301 3-state mode. As the TRS2 control

line is intended to be tied to the microprocessor’s 0-V rail, the TRS1 control line must be taken high to activate

the 3-state mode, which implies that the positive rail is required to turn on first. Other schemes could be devised

to take TRS2 below the 0-V rail until the power supply has stabilized if the negative rail turns on first. Instead

of relying on a definite power-supply sequence or elaborate control circuitry, it is simpler to limit the current either

with a series resistor or capacitor.

The second function of the series resistor is to limit the dc current flow through the transformer winding due to

the dc offset at the amplifier’s output, which is caused by its input offset voltage. For a worst case input offset

of 20 mV, the output offset is also 20 mV as the dc gain of the circuit is unity. Offsets due to input bias currents

are negligible since the values of the gain-setting resistors are low. The dc current through the transformer is

therefore less than 7 mA (20 mV/3.3

Ω). This low level of dc current does not appreciatively increase the power

dissipation of the amplifier or noticeably diminish the harmonic performance of the transformer.

The final function of the series resistor is to limit the signalling current in the event that the mains impedance

might appear as solely reactive; i.e., without a resistive component. As a rough estimate, the peak signal current

from the amplifier is:

I

OM +

V

O (PP)

R

S

+

5.5 V

2

3.3

Ω +

833 mA

V

O (PP) +

Peak-to-peak output voltage swing

where:

I

OM +

Peak-output-signalling current from amplifier