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Radio Frequency Integrated Circuit Design - Webs

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Voltage-Controlled Oscillators<br />

This can be solved for v�′ noting that g m ≈ 1/re .<br />

v�′ =<br />

i i<br />

j�C 2<br />

Another equation can be written for vce.<br />

vce = i i + g mv�′<br />

j�C 1<br />

Substituting (8.41) into (8.42) gives<br />

vce =� 1<br />

j�C1��ii + g m ii<br />

j�C2�<br />

263<br />

(8.41)<br />

(8.42)<br />

(8.43)<br />

Now using (8.41) and (8.43) and solving for Z i = vi /ii with some<br />

manipulation,<br />

Z i = vi ii = v�′ +vce =<br />

ii<br />

1<br />

j�C1 +<br />

1<br />

j�C2 −<br />

g m<br />

� 2 C 1C2<br />

(8.44)<br />

this is just a negative resistor in series with the two capacitors. Thus, a necessary<br />

condition for oscillation in this oscillator is<br />

rs <<br />

g m<br />

� 2 C 1C2<br />

(8.45)<br />

where rs is the equivalent series resistance on the resonator. It will be shown<br />

in Example 8.4 that the series negative resistance is maximized for a given fixed<br />

total series capacitance when C 1 = C 2. An identical expression to (8.45) can<br />

be derived for the Colpitts common-collector circuit.<br />

8.8.2 Negative Resistance for Series and Parallel <strong>Circuit</strong>s<br />

Equation (8.44) shows the analysis results for the oscillator circuit shown in<br />

Figure 8.18 when analyzed as an equivalent series circuit of C 1, C 2, and R neg.<br />

Since the resonance is actually a parallel one, the series components need to be<br />

converted back to parallel ones. However, if the equivalent Q of the RC circuit<br />

is high, the parallel capacitor C p will be approximately equal to the series<br />

capacitor C s , and the above analysis is valid. Even for low Q, these simple<br />

equations are useful for quick calculations.

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