Oscillator circuits – INCOMPLETE

Chapter 9.4 Oscillator circuits – INCOMPLETE

Lessons In Electric Circuits Volume III – Semiconductors Book
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Lessons In Electric Circuits Volume III – Semiconductors Book

  • 424CHAPTER 9. PRACTICAL ANALOG SEMICONDUCTOR CIRCUITSefficiency. High efficiency, light weight, and small size are the reasons switching power suppliesare almost universally used for powering digital computer circuitry.9.2.4Ripple regulatedA ripple-regulated power supply is an alternative to the linear regulated design scheme: a”brute force” power supply (transformer, rectifier, filter) constitutes the ”front end” of the cir-cuit, but a transistor operated strictly in its on/off (saturation/cutoff) modes transfers DC powerto a large capacitor as needed to maintain the output voltage between a high and a low set-point. As in switchers, the transistor in a ripple regulator never passes current while in its”active,” or ”linear,” mode for any substantial length of time, meaning that very little energywill be wasted in the form of heat. However, the biggest drawback to this regulation scheme isthe necessary presence of some ripple voltage on the output, as the DC voltage varies betweenthe two voltage control setpoints. Also, this ripple voltage varies in frequency depending onload current, which makes final filtering of the DC power more difficult.Ripple regulator circuits tend to be quite a bit simpler than switcher circuitry, and theyneed not handle the high power line voltages that switcher transistors must handle, makingthem safer to work on.9.3Amplifier circuits – PENDINGNote, Q3 and Q4 in Figure 434,9.4 are complementary, NPN and PNP respectively. This circuitworks well for moderate power audio amplifiers. For an explanation of this circuit see “Directcoupled complementary-pair,”(page 266,257).9.4Oscillator circuits – INCOMPLETEThe phase shift oscillator of Figure 434,9.5 produces a sinewave output in the audio frequencyrange. Resistive feedback from the collector would be negative feedback due to 180o phasing(base to collector phase inversion). However, the three 60o RC phase shifters ( R1C1, R2C2,and R3C3) provide an additional 180o for a total of 360o. This in-phase feedback constitutespositive feedback. Oscillations result if transistor gain exceeds feedback network losses.9.4.1Varactor multiplierA Varactor or variable capacitance diode with a nonlinear capacitance vs frequency character-istic distorts the applied sinewave f1 in Figure 435,9.6, generating harmonics, f3.The fundamental filter passes f1, blocking the harmonics from returning to the generator.The choke passes DC, and blocks radio frequencies (RF) from entering the Vbias supply. Theharmonic filter passes the desired harmonic, say the 3rd, to the output, f3. The capacitor atthe bottom of the inductor is a large value, low reactance, to block DC but ground the inductor