Hello all.
I'm studying about impedance matching for RF amplifiers.
● I'd like to ask a
conceptual question. I understand that a transistor with an actual 0F capacitance doesn't exist, but let's make an exception for the sake of this discussion.
Let's assume there is an
RF front-end system shown in the attached slide, and it is under the following
four conditions:
1. The input impedance of the common source amplifier is infinite over all frequencies.
2. The signal source is a single-tone continuous wave with a single wavelength (λ).
3. Between the signal source and the input of the common source amplifier, there is a 50-ohm transmission line that is much longer than the signal wavelength (λ « l).
4. All components are noiseless. Noise is not an issue to deal with here.
● Under the circuit conditions described above, I have reached the following
five conclusions. Are these interpretations
theoretically correct?
1. All signal power is reflected at the transistor gate and then dissipated entirely in the source impedance (Rsrc). Therefore, no signal distortion occurs due to cumulative and successive bouncing.
2. Since the input impedance of the common source amplifier is infinite, the behavior can be analyzed using the voltage transfer principle, not the power transfer principle.
3. With the infinite input impedance of the common source amplifier, a voltage standing wave is formed due to perfect reflection, doubling the voltage. As a result, vgs equals vrf.
4. There is no need to add a noiseless 50-ohm shunt resistor to the gate for impedance matching. If impedance matching is done in this manner, vgs would be halved, reducing the final output voltage (vout).
5. If the load impedance is ideally infinite over all frequencies, the length of the transmission line does not matter at all. Whether it is long or short, it makes no difference.
Thanks.