MC33178_06 Datasheet, PDF (11/19 Pages) ON Semiconductor – Low Power, Low Noise Operational Amplifiers

To
Receiver
From
Microphone
MC33178, MC33179
10 k
A1
−
+
120 k
2.0 k
A2
−
+
200 k
10 k
10 k
1.0 mF
300
0.05 mF
820
V_R
1N4678
10 k
10 k
A3
−
+
V_R
Figure 34. Telephone Line Interface Circuit
Tip
Phone Line
Ring
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 24). The ability to drive a minimum
600 W load makes it particularly suitable for telecom
applications. Note that in the sample circuit in Figure 34
both A2 and A3 are driving equivalent loads of
approximately 600 W .
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.
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