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2.9 Transmission Lines Applied to High Frequency Circuits 97 It would also take useful RF power away from the input and output of the amplifier thereby decreasing its gain and efficiency. The bias arrangement shown in Figure 2.9-5 is of course greatly simplified and it would normally be implemented by a resistive network, fed by a single power supply (section 5.3.1), but this will do for our example. What we would like to avoid is RF power flowing back into the bias network. This may be achieved if the RF signal sees a very high impedance at the point where the bias supply joins the signal path. The RF current would then only be able to flow along the signal path. The quarter wave line is ideally suited to this purpose. Let us see how. Figure 2.9-6 Short-circuited quarter wave line As we have seen, a quarter wave line terminated with a short circuit, looks like an open circuit. In the circuit shown in Figure 2.9-6, is therefore very high! So if we connected this transmission line to the gate terminal (G) of our device for instance, there would be no chance of any RF current flowing into it! Now if we connect the power supply to the other end of the line as shown in Figure 2.9-7, then no RF current will be able to flow back into the power supply itself. However the DC voltage will still be able to reach the gate of the transistor since, at DC, a transmission line is just like a wire (section 2.3) Figure 2.9-7 Quarter-wave line terminated with an RF short circuit, feeds bias voltage to gate Looking at Figure 2.9-7, you may be thinking, "Is he mad? He will short the power supply to ground!". Well, you would be right, were it not for the fact that the short circuit at the end of the line (circled in red), is not a DC short-circuit but an RF one. This is another very important concept which is encountered again and again in RF circuits. We cannot just use a wire connected to ground to create an RF short because, as we have seen, the length of wire alone would change the impedance seen by the line. Also, and more importantly, a wire would not be suited to contain the electromagnetic fields which are created by high frequency signals. Z 0 Transmission Line Z IN Z 0 Transmission Line Z IN DC V GG Gate Terminal, G Conquer Radio Frequency 97 www.cadence.com/go/awr