Issue link: https://resources.system-analysis.cadence.com/i/1326562
CHAPTER 2 - Conveying Power at Radio Frequency 96 The difference between the case of AC/DC circuits and RF circuits is of course that, in the latter, a transmission line is used to connect R S and R L and its characteristic impedance must also be equal to R S and R L in order for maximum power transfer to be achieved. Instead of looking at three circuit elements however, we could just say that must be equal to R S (Figure 2.9-1) in order for maximum power to be transferred to the actual load R L and, to this end, equation (2.9-1) helps greatly! Let us get back to it and explore other interesting values of . For , the modulus of becomes ( ) ( ) ( ) ( ) Now that is a very interesting result, one that you will come across in one form or another, very often! ( ⁄ ) Equation (2.9-2) shows that, if our quarter-wave line is short circuited at the load end, i.e. , the impedance will be equal to infinity. This means that, at the generator end, we will see an open circuit! Equation (2.9-2) also shows that if , i.e. the line is open-circuited, becomes equal to zero. This means that at the generator end we will see a short circuit! This impedance inversion is an extremely useful property of quarter-wave lines! Of course these statements are only true at the frequency corresponding to the wavelength . Let us now take a look at one important application of this and its practical realisation. When we design an amplifier at Radio Frequency, much in the same way as we would at audio frequency, we need to bias our active device i.e. supply the correct DC voltages for it to operate in the desired class of amplification. At Radio Frequency it is extremely important to prevent the amplifier's input and output signals from flowing back into the bias network since this may cause the amplifier to become unstable, start oscillating and, often enough, burn out! Figure 2.9-5 A MOSFET Amplifier. The red arrows show RF signal power flowing back into the bias network. DC DC V IN V OUT V GG V DD (2.9-2) G S D Conquer Radio Frequency 96 www.cadence.com/go/awr