____________________________________________________ Video and Radio Frequency Amplifiers
At 70.7 percent of the output current (.14 A) use the following formula:
True Power = I2R
True Power = (.14 A)2 (50Ω)
True Power = (.02 x 50) watts
True Power = 1 watt
6-12. In Figure 6-1, the two points marked f1 and f2 will enable you to determine the
frequency-response limits of the amplifier. In this case, the limits are 15 Hz and 20 KHz.
You should now know how a frequency-response curve could enable you to determine the
frequency limits and the bandwidth of an amplifier.
READING AMPLIFIER FREQUENCY-RESPONSE CURVES
6-13. Figure 6-2 shows the frequency-response curves for four different amplifiers. View
(A) is the same frequency-response curve as shown in Figure 6-1.
6-14. Figure 6-2, 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 Hz to
20 KHz 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 Hz. The upper frequency limit
(f2) is 10 KHz. Therefore, the bandwidth of this amplifier must be 10 KHz minus 100 Hz
or 9,900 Hz. 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.
6-15. Figure 6-2, view (C) shows the frequency-response curve for an RF amplifier. The
frequency limits of this amplifier are 100 KHz (f1) and 1 MHz (f2). Therefore, the
bandwidth of this amplifier is 900 KHz.
6-16. Figure 6-2, view (D) shows another audio amplifier. This time the frequency limits
are 30 Hz (f1) and 200 Hz (f2). The bandwidth of this amplifier is only 170 Hz. The
important thing to notice in this view 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.
Figure 6-2. Frequency-response Curves
23 June 2005