Why transistor with higher impedance is easier for matching?

[kidbany]

I am reading a good RF book called "Practical RF Circuit Design For Modern Wireless System", there is a statement "The lower-power devices are easier to match because their impedance is higher" in section 2.6 "Available Gain Design Technique". I can't understand as in my mind, the lower impedance located at the left side of smitch chart, and the higher impedance located at the right side, they could be matched by 2 elements: L and C, so I think the difficulty is similar.

Anyone who can give some explanation on this topic, thank you very much.

 

[tyassin]

Re: Why transistor with higher impedance is easier for match

Hi

I think in "higher", they mean closer to 50 ohm.

High power devices have very small impedances at the output. Simply because high power devices are made from several devices put in parallel.

Regards

 

[kidbany]

Re: Why transistor with higher impedance is easier for match

Thanks. But even the "higher" means closer to 50Ohm compared with the low resistance, the matching is still needed and the difficulty should be similar also.

 

[WimRFP]

Hello,

Imagine a class C amplifier with 5V supply and 2W output.

Assuming 1V saturation at the drain or collector, you have a swing of 4V. To get 2W into a load, the drain or collector has to see R = 4^2/(2*2) = 4 Ohms. Of course you need a parallel inductive component also, but I regard this for now.

A 100 mW device with same swing needs 80 Ohms.

To have a broadband match from 80 ohms to 50 Ohms you need fewer components with respect to the matching from 4 ohms to 50 Ohms.

 

[kidbany]

Re: Why transistor with higher impedance is easier for match

Hello,

Imagine a class C amplifier with 5V supply and 2W output.

Assuming 1V saturation at the drain or collector, you have a swing of 4V. To get 2W into a load, the drain or collector has to see R = 4^2/(2*2) = 4 Ohms. Of course you need a parallel inductive component also, but I regard this for now.

A 100 mW device with same swing needs 80 Ohms.

To have a broadband match from 80 ohms to 50 Ohms you need fewer components with respect to the matching from 4 ohms to 50 Ohms.

I think application here is narrowband, not broadband. For a narrow band matching, as I mentioned before, the difficulty should be similar. Could you recommend some paper for the matching difference between low and high impedence if possible?

 

[WimRFP]

Hello,

I do not have a link to a paper at hand, but you may try the application notes section from manufacturers of UHF power transistors (NXP, ST, Freescale, NEC-Renesas, etc).

Regarding the original question, matching from very low impedance to 50 Ohms in a single step, requires higher loaded Q factor of the L and C used. In other words, the blind energy/s in the components (product of Irms*Urms) will be higher. To enable good overall efficiency, the Q factor of the components must be >> loaded Q factor of circuit.