Page 41 - Op Amps Design, Applications, and Troubleshooting

24 BASIC CONCEPTS OF THE INTEGRATED OPERATIONAL AMPLIFIER

1.4.9 Power Supply Requirements
When ideal op amps were discussed, we briefly indicated that all op amps require
an external DC power supply. Many op amps are designed for dual supply opera-
tion with ±15 volts being the most common. Other op amps are specifically
designed for single-supply operation. However, in all cases, we must provide a
DC supply for the op amp in order for it to operate.
The power supply connections on the op amp are generally labeled +V CC or
V+ for the positive connection and -V cc or V~ for the negative connection. You will
recall from Figure 1.2 that the DC power source provides the bias and operating
voltages for the op amp's internal transistors. The magnitude of the power supply
voltage is determined by the application and limited by the specific op amp being
considered. A typical op amp will operate with supply voltages as low as 6 volts
and as high as 18 volts, although neither of these values should be viewed as
extremes. Certain devices in specific applications can operate on less than 5 volts.
Other high-voltage op amps are designed to operate normally with voltages sub-
stantially higher than 18 volts.
The current capability of the power supply is another consideration. The
actual op amp draws fairly low currents with 1-3 milliamperes being typical.
Some low-power op amps require only a few microamperes of supply current to
function properly. In most applications, the external circuitry plays a greater role
in power supply current requirements than the op amp itself.
Yet another power supply consideration involves the amount of noise con-
tributed to the circuit by the power supply. There are several forms of power-
supply noise including

1. Power line ripple caused by incomplete filtering
2. High-frequency noise generated within the supply circuitry
3. Switching transients produced by switching regulators
4. Noise coupled to the DC supply line from other circuits in the system
5. Externally generated noise that is coupled onto the DC supply line
6. Noise caused by poor voltage regulation

Noise that appears on the DC power supply lines can be passed through the
internal circuitry of the op amp and appear at the output. Depending on the type
and in particular the frequency of the noise voltages, they will undergo varying
amounts of attenuation as they pass through the op amp's components. Frequen-
cies below 100 hertz are severely attenuated with losses as great as 10,000 being
typical. As the noise frequencies increase, however, the attenuation in the op amp
is less. Frequencies greater than 1.0 megahertz may be coupled from the DC sup-
ply line to the output of the op amp with no significant reduction in amplitude.
The degree to which the output is affected by noise on the DC supply lines is
called the power supply rejection ratio (PSRR).
Power distribution is a very important consideration in circuit design, yet
frequently receives only minimal attention. This issue will be addressed in the fol-
lowing section with regard to circuit construction.