0195136047.pdf

240 ANALOG BUILDING BLOCKS AND OPERATIONAL AMPLIFIERS

−

+

2

3

1

8

μA 709 5

6

C 1

C 2

R 2 (use 50 Ω when amplifier is

operated with capacitive loading)

R 1

Range of C 1 10–5000 pF

Range of R 1 0–1.5 kΩ

Range of C 2 3–200 pF

Figure 5.3.8Typical frequency-compensation

circuit for FairchildμA 709.

and is therefore usually associated with low closed-loop voltage gain. For a 741 the slew rate is

0.5 V/μsatA=1. For more recently developed op amps, the slew rate ranges from 5 to 100

V/μs. The effect of the slew rate in response to an input step voltage is shown in Figure 5.3.9.

If one attempts to make the output voltage change faster than the slew rate, nonlinearity will be

introduced. When specifying such output voltage requirements as rise time, output voltage, and

frequency, it is necessary to choose an op amp with a slew rate that meets the specifications. With

a sine-wave input, the slew rate limits a combination of maximum operating frequency and output

voltage magnitude.

The slew rate occurs because at some stage in the amplifier a frequency-compensating

capacitor will have to be charged, and the available limited charging current restricts the maximum

rate of change of the capacitor voltage. With externally compensated op amps, such as the 709,

slew rates will depend on the value of the compensating capacitors, which are in turn chosen

on the basis of the closed-loop gain needed. The lower the gain, the higher the compensating

capacitors, and hence the lower the slew rate. For aμA 709, the slew rate is 0.3 V/μsatA= 1

and 1.5 V/μsatA=10.

Noise

This refers to the small, rapidly varying, random spurious signals generated by all electronic

circuits. Noise places a limit on the smallness of signals that can be used. The subject of random

signals and noise belongs to a branch of electrical engineering known as communication theory.

Stability

The amplifier is said to be stable when it performs its function reliably under all normal operating

conditions. By definition, a system is stable if its response to an excitation that decays to

0.5 μs

0

0

vd

1 V

v 0

1 V

Figure 5.3.9Effect of op-amp slew rate with step voltage input.