MC33178 MC33179

9

MOTOROLA ANALOG IC DEVICE DATA

10 k

A1

To

Receiver

+

–

1.0

µF

300

200 k

120 k

2.0 k

A2

820

1N4678

Tip

Phone Line

Ring

A3

VR

From

Microphone

–

–

+

+

10 k

10 k

10 k

VR

10 k

0.05

µF

Figure 33. Telephone Line Interface Circuit

APPLICATION INFORMATION

This unique device uses a boosted output stage to

combine a high output current with a drain current lower than

similar bipolar input op amps. Its 60

° phase margin and 15 dB

gain margin ensure stability with up to 1000 pF of load

capacitance (see Figure 23). The ability to drive a minimum

600

Ω load makes it particularly suitable for telecom

applications. Note that in the sample circuit in Figure 33 both

A2 and A3 are driving equivalent loads of approximately

600

Ω.

The low input offset voltage and moderately high slew rate

and gain bandwidth product make it attractive for a variety of

other applications. For example, although it is not single

supply (the common mode input range does not include

ground), it is specified at +5.0 V with a typical common mode

rejection of 110 dB. This makes it an excellent choice for use

with digital circuits. The high common mode rejection, which

is stable over temperature, coupled with a low noise figure

and low distortion, is an ideal op amp for audio circuits.

The output stage of the op amp is current limited and

therefore has a certain amount of protection in the event of a

short circuit. However, because of its high current output, it is

especially important not to allow the device to exceed the

maximum junction temperature, particularly with the

MC33179 (quad op amp). Shorting more than one amplifier

could easily exceed the junction temperature to the extent of

causing permanent damage.

Stability

As usual with most high frequency amplifiers, proper lead

dress, component placement, and PC board layout should be

exercised for optimum frequency performance. For example,

long unshielded input or output leads may result in unwanted

input/output coupling. In order to preserve the relatively

low input capacitance associated with these amplifiers,

resistors connected to the inputs should be immediately

adjacent to the input pin to minimize additional stray input

capacitance. This not only minimizes the input pole

frequency for optimum frequency response, but also

minimizes extraneous “pick up” at this node. Supplying

decoupling with adequate capacitance immediately adjacent

to the supply pin is also important, particularly over

temperature, since many types of decoupling capacitors

exhibit great impedance changes over temperature.

Additional stability problems can be caused by high load

capacitances and/or a high source resistance. Simple

compensation schemes can be used to alleviate these

effects.