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Class15.pdf - EE163A Introductory Microwave Circuits Fall...

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Unformatted text preview: EE163A Introductory Microwave Circuits Fall, 2015 Prof. Y. Ethan Wang Electrical Engineering Dept. UCLA Lesson 15 •  ReducCon of ConducCon Angle •  Class A, AB and B Reducing ConducCon Angle • The complete sinusoid like drain current in Class-­‐A contains too much DC component • Reducing the conducCon angle or using non-­‐sinusoid current may help to reduce the DC component, at the price of reduced gain • The fundamental component of the current can be selected with a bandpass filter and only this component will be passed to the load • The amplitude of each harmonic component is determined by Fourier expansion coefficients of the drain current and a proper terminaCon is required for each harmonic component ConducCon Angle Analysis Conduc<on Angle Waveform quiescent voltage Gate Voltage time quiescent current Drain Current 0 Classical Modes of Opera<on Mode Bias Quiescent current ConducCon angle 2q point A 0.5 AB 0-0.5 0.5 0-0.5 2p p-2p B 0 0 p C <0 0 0-p time Quiescent current as funcCon of the half-­‐conducCon angle q From Class A to Class AB, B, C, the amplifier goes into deeper cut-­‐off Harmonic TerminaCon High Q tank at the fundamental time D G S 0 • Harmonic components of the output current is bypassed by the LC tank • Only the fundamental component current is delivered to the load • Drain voltage is close to a sinusoid as the harmonics are short circuited time time Other Harmonic TerminaCon Approaches RF ground D G S TerminaCon condiCon: D G S The slightly shorter than quarter wave transmission line should transform the RF ground to an inductance OpCmum Load Fundamental frequency component of the current The maximum RF voltage swing is from 0 to 2Vdc The maximum RF current swing is given by 2I1, where The opCmum load impedance is thus, 1, when θ = π Previously for class A opCmum load, The opCmum load impedance for any Conduc<on Ropt For class B , when A ngle , θ conducCon angle is thus Class A RA Class AB Class B θ =π θ = 3π 4 θ =π 2 0.92RA RA Maximum Efficiency and Power The valid RF output power is given by, The DC supply is given by Therefore, the drain efficiency is Note that The PAE is: It yields η D( A)=50%, η D(=B)78.5%, Power efficiency increases when the conduc?on angle reduces, but the maximum output power reduces when the bias goes into Class C. Example of Class-­B Design A FET device has a maximum raCng of drain-­‐to-­‐source voltage of 10V and a maximum raCng of drain current of 200mA. The output capacitance Cds is 2pF. Find the opCmum load impedance at 2GHz and the maximum output power of a Class B PA. What is the bandwidth of the opCmum matching? ...
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