question on RF amplifiers

Sep 08, 2006 5 Replies

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Strelnikov 19 years ago

Hi everybody,

I am a beginner in RF. Looking at several linear amplifier diagrams for the UHF range, I almost always see two active stages coupled like this: the collector or drain of a previous stage drives a coil connected to the supply rail, then via a capacitor, from the collector the signal goes to a matching element, like a tank or a transformer, and then to the base or gate of the following active element. My question is: why the primary of an eventual impedance adapting trafo couldn't be connected between the first stage collector and the positive rail, resuming in one component the coil and the matching element? Thanks in advance for the answers, and/ or for suggestions.

Mirko

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Joerg 19 years ago

Hello Mirko,

We all were at some point :-)

I have seen them both ways. However, two things can lead a designer to select the ground-based approach:

a. Reducing noise on the power rail to modulate into the signal path.

b. Unwanted feedback and instability resulting from that. If a board has to be done at rock-bottom cost you may only have two layers available. On RF boards there will (usually) still be a nearly complete ground plane but the supply rail is a wide trace and some bypass capacitors. This may not be enough to prevent RF from the last stage to make it into the supply rail and from there into the tank or matching circuit of a younger stage if these circuits hang on the rail. This can cause instability or oscillation.

Regards, Joerg http://www.analogconsultants.com

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PeteS 19 years ago

Indeed so.

This is one of the reasons that much communications equipment, even today, uses a negative power rail: i.e. the transistor collectors ultimately go to ground (which is in fact, the positive side of the power supply) and the emitters ultimately go to the negative rail.

This can simplify the design (depends on many things of course).

[Slightly O.T. for the OP] Although it's possible to use a positive supply with P-channel or PNP devices (so the collectors could return to ground with a positive supply although the only difference between a positive supply and a negative one is which way round the output is connected), the physics of the various transistors makes the N-channel / NPN devices preferable in RF amps [in general], but that's a separate subject :)

Cheers

PeteS

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Joerg 19 years ago

Hello Pete,

Yes, and then also you have to go with what's on the menu. Pretty much all the hot RF transistors are npn if you don't want to end up with hard to procure boutique parts.

Regards, Joerg http://www.analogconsultants.com

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PeteS 19 years ago

Hi Joerg - a fine weekend to you.

As you note, it's virtually all N-channel / NPN, but then that's probably because they are preferable because of their internal characteristics (chicken and egg problem :)

Cheers

PeteS

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Strelnikov 19 years ago

Thanks everybody for the answers. So, my pcb being a two layer, I'll have more than a reason to use an extra inductor and to stick the primary to the ground layer.

Bye

Mirko

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