Hi,
Quote:With step-up and step-down networks, do you mean that this:
step-up network: voltage gain > 1
step-down network: voltage gain < 1
LC networks consist of only reactive devices. Hence they do not dissipate energy (ideally).
P
in = V
in2/R
in = P
out = V
out2/R
loadSo from the above equation you can see that any increase in the impedance is accompanied by an increase in the voltage and vice versa.
Quote:Intuitively, I can see why the use of step-down network decreases gain and increases linearity but I can't get about efficiency.
Can you explain it or tell me where can I read this?
I think most texts on PAs will cover this. Basically power is dissipated in a device when there is simultaneously a voltage across the device and a current flowing through the device. For a MOSFET, the current and drain voltage waveforms are out-of-phase (for a real load), so if the signal is very large, the voltage waveform will reach a minimum when the current waveform reaches a maximum. If the voltage minimum can be made to be zero, then no energy is dissipated in the device at that point of time. Ideally in a switching PA, the MOSFET is completely ON for half of the time and completely OFF for half of the time. It acts like a switch. When completely ON, the current is maximum, but the drain voltage should be zero to ensure no power dissipation in the device. When completely OFF, the voltage can swing high, but the current flowing through the device should be zero.
Note that because in a switching PA the device is switched completely ON or OFF, the amplitude information in the signal is lost. So such amplifiers can't normally be used to amplify signals with amplitude modulation. There are of course ways around this, but this also kind of exposes the basic tradeoff between efficiency and linearity.
Quote:Yes, I thought that but I also read that inductors come with parasitics and cause self resonances at a variety of frequencies
It really depends on how you implement your inductor. If you are implementing it on-chip, you probably have a good case. From my experience (which may or may not really apply in your case), if you are implementing it on chip, transformers are a good choice. But then if your requirements are very relaxed, it might not matter.
One of the most important points which I don't think you revealed in your previous message is the required output power. This will be a big factor in your design. For high power designs, you will quickly find that every little trace of metal counts and has to be modeled using an EM simulator. It will also be very important to carefully design and optimize your matching networks. You may even need to chose an architecture whose main benefit is to relax the requirements on the device modeling.
regards,
Aaron