Figure 20 Open Loop Gain and phase of Breadboarded Amplifier. This figure includes two graphs for the price of one. The horizontal scale is the same for both of them and extends from 1 Hz to 1 MHz. The top graph is the gain of the amplifier in dB minus 40 dB. It extends from minus 90 to plus 10 dB. The vertical scale on the bottom graph extends from minus 180 degrees to plus 180 degrees. On the circle of angles the values minus 180 degrees and plus 180 degrees are one in the same. All values are approximate having been visually picked off the graph. The gain graph starts out at 1 Hz and minus 60 dB. Remember to algebraically add 40 dB. It goes up on a gradual slope until it reaches minus 1 dB at 2 kHz. The peak is smoothly rounded. The graph starts down on a similar slope ending at minus 55 dB at 1 MHz. The phase graph starts out at 1 Hz and minus 5 degrees. It descends for a distance than almost levels off at minus 90 degrees then falls to minus 180 degrees at 2 kHz. At minus 180 degrees the line goes straight up to plus 180 degrees then continues to fall on approximately the same slope. It passes through zero degrees at about 110 kHz and continues on down to minus 180 degrees again at 810 kHz. Once again it jumps to plus 180 and finally ends at 165 degrees and 1 MHz. The zero phase angle is important because it is generally where oscillation takes place if it is going to. At 110 kHz the gain is minus 27 dB. Minus 27 dB plus 40 dB = plus 13 dB. That's enough to make an oscillator.
End of verbal description.
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