5.3 PRACTICAL PROPERTIES OF OPERATIONAL AMPLIFIERS 241zero with time also decays to zero. Instability can come about in various ways, and it is a
very common difficulty, which must be prevented. In almost all cases, op-amp circuits can be
classified as feedback circuits. Feedback of an improper kind can lead to instability or oscillation
of an op-amp circuit. In general, instability occurs when there is excessive phase shift in the
op amp and feedback loop, so that negative feedback is changed to positive feedback. The
frequency response of an op amp is usually designed to roll off smoothly at 20 dB decade,
as mentioned earlier. This type of frequency response ensures stability for the more common
op-amp circuits.
Frequency Response
As with all electronic circuits, op amps have limited frequency response. Because of the negative
feedback of the circuit, the passband of an op-amp circuit is usually much larger than that of the
op amp by itself. Typically with a given op amp, increasing the bandwidth of an op-amp circuit
will decrease the voltage gain in the same proportion. Thus for a given op amp, the product of gain
and bandwidth (i.e., the gain–bandwidth product) is a constant, as discussed earlier and shown in
Figure 5.2.4.
Table 5.3.1 lists some representative op-amp parameters for different amplifier types, only
to illustrate their characteristics. One can see from the table that a high-power op amp can deliver
a large output current, but has relatively large offset parameters. On the other hand, a precision
input amplifier has low offset and high gain, but at the price of bandwidth, slew rate, and output
current. The general-purpose amplifier, as the name suggests, strikes a balance between these
extremes.
TABLE 5.3.1Typical Op-Amp Characteristics
Input Input Gain– Maximum
Offset Offset Voltage Bandwidth Slew Output
Voltage Current Gain Product Rate Current
Type (mV) (pA) (V/mV) (MHz) (V/μs) (mA)High Power 5 200 75 1 3 500
Wide band 3 200 15 30 30 50
High slew rate 5 100 4 50 400 50
Precision input 0.05 2 500 0.4 0.06 1
General purpose 2 50 100 1 3 10
EXAMPLE 5.3.1
In order to illustrate the insensitivity of feedback circuits to variations of the op-amp parameters,
let us consider a simple feedback circuit using an op amp, as shown in Figure E5.3.1(a). Using
the op-amp model of Figure 5.1.1, find the output voltage in terms of the input voltage under two
sets of conditions:
(a)A= 105 ,Ri=10 k, RL=1k, RS=1k
(b)A= 2 × 105 ,Ri=30 k, RL=5k, RS=2kIn order to simplify the calculations, let us assume that the op amp’s output resistanceRois zero.