TLE2301

EXCALIBUR 3-STATE-OUTPUT WIDE-BANDWIDTH

POWER OPERATIONAL AMPLIFIER

SLOS131 – DECEMBER 1993

16

POST OFFICE BOX 655303

APPLICATION INFORMATION

component calculations

The following sections contain the calculations for input capacitors, gain resistors, coupling network, coupling

capacitors, transformer-leakage inductance, series resistors, decoupling, and frequency compensation.

input capacitor

The incoming signal is ac coupled to remove any incoming dc offset and to provide only unity gain for the

amplifier’s input offset voltage. The value of 100 nF is chosen for this input capacitor as it has very little influence

on the amplifier’s signal gain over the frequency band.

gain resistors

The gain-setting resistors are chosen for a gain of 1.96; i.e., choosing:

Gain

+

R

F

R

I

+ 4.7 kΩ

2.4 k

Ω

+ 1.96

R

F +

4.7 k

Ω and R

I +

2.4 k

Ω

The resistor values are low enough to ensure that the circuit does not suffer from stray capacitance and signal

pick-up problems but not too low as to significantly load the mains impedance when the amplifier is in its

high-impedance state.

coupling network

The function of the line interface is to provide isolation from the mains supply while coupling the communication

signals onto the mains network. As the mains voltage is large in comparison with the communication signals,

the mains voltage needs to be isolated from the electronic circuitry. The simple coupling network limits the

current flowing from the mains supply as well as providing a convenient point at which to implement the safety

isolation barrier between the mains supply and the communications circuitry. The transformer can easily

achieve an isolation of 4 kV between primary and secondary windings, and the capacitor provides the low

frequency roll-off to impede the mains voltage.

The transformer has two other useful properties. First, the turns ratio can be selected to provide efficient power

transfer between the TLE2301 amplifier and the mains network. Second, the transformer possesses leakage

inductance that can be tuned with the coupling capacitor to form a band-pass filter.

By altering the turns ratio, the power dissipated in the TLE2301 can be reduced while maintaining the required

voltage levels on the mains line. A turns ratio of 1.67:1 was selected in this design to apply a 120-

µdBV signal

onto the mains line. The calculation for the turns ratio is not straightforward due to the presence of numerous

complex impedances. The simplest method for deriving the turns ratio is to model the circuit with an analog

simulation program such as PSpice

™. It is from these simulations that the 1.67:1 turns ratio has been selected.

PSpice is a registered trademark of MicroSim Corporation.