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R Vcc using barcode integrated for visual studio .net control to generate, create qr image in visual studio .net applications. GS1 General Specifications Figure 11.12. A negativeresistance oscillator. Vee (a) (b). Radio-frequency electronics: Circuits and applications Poulsen arc transmitters, circa World War I, provided low-frequency RF power exceeding 100 kW. 11.3 Oscillator dynamics These reso nant oscillators are basically linear amplifiers with positive feedback. At turn-on they can get started by virtue of their own noise if they run class A. The tiny amount of noise power at the oscillation frequency will grow exponentially into the full-power sine wave.

Once running, the signal level is ultimately limited by some nonlinearity. This could be a small-signal nonlinearity in the transistor characteristics. Otherwise, the finite voltage of the dc power provides a severe large-signal nonlinearity, and the operation will shift toward class-C conditions.

The fact that amplitude cannot increase indefinitely shows that some nonlinearity is operative in every real oscillator. Any nonlinearity causes the transistor s low- frequency 1/ noise to mix with the RF signal, producing more noise close to the carrier than would exist for linear operation. An obvious way to mitigate large-signal nonlinearity is to detect the oscillator s output power and use the detector voltage in a negative feedback arrangement to control the gain.

This can maintain an amplitude considerably lower than the power supply voltage. Alternatively, if the oscillator uses a device (transistor or op-amp circuit) with a soft saturation characteristic, the amplitude will reach a limit while the operation is still nearly linear. For example, the amplifier in the oscillator of Figure 11.

10 might have a small cubic term, i.e., VOUT = AVIN BVIN3, where B/A is very small (see Problem 11.

5).. 11.4 Frequency stability Long-term (seconds to years) frequency fluctuations are due to component aging and changes in ambient temperature and are called drift. Short-term fluctuations, known as oscillator noise, are caused by the noise produced in the active device, the finite loaded Q of the resonant circuit, and nonlinearity in the operating cycle. The higher the Q, the faster the loop phase-shift changes with frequency.

Any disturbances (transistor fluctuations, power supply variations changing the transistor s parasitic capacitances, etc.) that tend to change the phase shift will cause the frequency to move slightly to reestablish the overall 360 shift. The higher the resonator Q, the smaller the frequency shift.

Note that this is the loaded Q, so the most stable oscillators, besides having the highest Q resonators, are loaded as lightly as possible. In LC oscillators, losses in the inductor almost always determine the resonator Q. A shorted piece of transmission line is sometimes used as a high-Q inductor.

24 treats oscillator noise in detail.. Oscillators 11.5 Colpitts oscillator theory Let us loo visual .net qr bidimensional barcode k in some detail at the operation of the Colpitts oscillator. Figure 11.

13 shows the Colpitts oscillator of Figure 11.6(c) redrawn as a small-signal equivalent circuit (compare the figures). The still-to-be-biased transistor is represented as a voltage-controlled current source.

The resistor rbe represents the small-signal base-to-emitter resistance of the transistor. The parallel combination of L and the load resistor, R, is denoted as Z, i.e.

, Z = j LR/(j L+R) = j LS +RS, where LS and RS are the component values for the equivalent series network. Likewise, it is convenient to denote rbe 1 as g. The voltage Vbe, a phasor, is produced by the current I (a phasor) from the current source.

This is a linear circuit, so Vbe can be written as Vbe = I ZT, where ZT is a function of . We will calculate this transfer impedance using standard circuit analysis. Since the current I is proportional to Vbe, we can write an equation expressing that, in going around the loop, the voltage Vbe exactly reproduces itself : gm Vbe ZT Vbe or 1 gm : ZT (11:1).

This equat ion will let us find the component values needed for the circuit to oscillate at the desired frequency, i.e., the values that will make the loop gain equal to unity and the phase shift equal 360 .

We can arbitrarily select L, choosing an inductor whose Q is high at the desired frequency. Equation (11.1), really two equations (real and imaginary parts), will then provide values for C1 and C2.

To derive an expression for ZT, we will assume that Vbe = 1 and work backward to find the corresponding value of I. With this assumption, inspection of Figure 11.13 shows that the current I1 is given by I1 j C2 g: (11:2).

Now the vo .net vs 2010 Quick Response Code ltage Vc is just the 1 volt assumed for Vbe plus I1Z, the voltage developed across Z: Vc 1 j C2 g Z: (11:3).
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