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Radio Frequency Integrated Circuit Design - Webs

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LNA <strong>Design</strong><br />

where VA is the Early voltage of the transistors. An ac current into the collector<br />

can be written as a function of ac current ic .<br />

ro_ac(i c ) = V A<br />

IC + ic<br />

181<br />

(6.88)<br />

Assuming for this analysis that there is no significant impedance in the<br />

circuit other than the transistor output resistance, the ac collector-emitter voltage<br />

can be written as<br />

Now from the relationship<br />

vce = i c r o_ac = i cV A<br />

IC + ic<br />

= V A<br />

ic<br />

IC<br />

1 + i c<br />

IC<br />

(6.89)<br />

x<br />

1 + x = x − x 2 + x 3 − x 4 + x 5 ... (6.90)<br />

(6.89) can be written out as a power series:<br />

vce = VA� ic<br />

IC� − VA� ic<br />

IC� 2<br />

+ VA� ic<br />

IC� 3<br />

= ro_dc ic − ro_dc The intermodulation current can now be easily determined.<br />

k 1<br />

i IP3 = 2 = 2<br />

1<br />

√3k 3 √<br />

3 r o_dc<br />

I 2<br />

C<br />

ro_dc Thus, the output intermodulation voltage is just<br />

v OP3 = 2IC<br />

ro<br />

(6.93)<br />

√3 = 2I C<br />

√ 3<br />

IC i 2 c + ro_dc I 2 i<br />

C<br />

3 c<br />

(6.91)<br />

(6.92)<br />

This is a fairly intuitive result. As the dc current is increased, the ac current<br />

is a smaller percentage of the total, and therefore the circuit behaves more<br />

linearly. Thus, designers have two choices if the current source is not linear<br />

enough. They can either increase the current or increase the output impedance.

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