READING AMPLIFIER FREQUENCY-RESPONSE CURVES

Figure 2-2 shows the frequency-response curves for four different amplifiers. View (A) is the same frequency-response curve as shown in figure 2-1. View (B) is the frequency-response curve of an amplifier that would also be classified as an audio amplifier, even though the curve is not "flat" from 15 hertz to 20 kilohertz and does not drop off sharply at the frequency limits. From the curve, you can see that the lower frequency limit of this amplifier (f₁) is 100 hertz. The upper frequency limit (f₂) is 10 kilohertz. Therefore, the bandwidth of this amplifier must be 10 kilohertz minus 100 hertz or 9900 hertz. Most amplifiers will have a frequency-response curve shaped like view (B) if nothing is done to modify the frequency-response characteristics of the circuit. (The factors that affect Frequency response and the methods to modify the Frequency response of an amplifier are covered a little later in this chapter.)

Figure 2-2A. - Frequency response curves.

Figure 2-2B. - Frequency response curves.

Figure 2-2C. - Frequency response curves.

Figure 2-2D. - Frequency response curves.

Now look at view (C). This frequency-response curve is for an rf amplifier. The frequency limits of this amplifier are 100 kilohertz (f₁) and 1 megahertz (f₂); therefore, the bandwidth of this amplifier is 900 kilohertz.

View (D) shows another audio amplifier. This time the frequency limits are 30 hertz (f₁) and 200 hertz (f₂). The bandwidth of this amplifier is only 170 hertz. The important thing to notice in view (D) is that the frequency scale is different from those used in other views. Any frequency scale can be used for a frequency-response curve. The scale used would be determined by what frequencies are most useful in presenting the frequency-response curve for a particular amplifier.

Q.1 What is the bandwidth of an amplifier?
Q.2 What are the upper and lower frequency limits of an amplifier?
Q.3 What are the upper and lower frequency limits and the bandwidth for the amplifiers that have frequency-response curves as shown in figure 2-3?