Electronics > RF, Microwave, Ham Radio

RF output transformers variations.

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G0FTD:
Greetings all !

Attached to this  post is a drawing showing to methods of taking out the RF from an output stage.
The example I drew represents a FET in this case.

I have seen to styles of taking RF out of a device.

1 -  The combined RF choke and transformer output method. Where of course you can alter the output impedance with
turns ratio as normal.

2 - Where the device keeps the RF Choke on it's own, and uses a separate RF output transformer, with suitable turns ratio
to suit impedance.

So why the two methods ?

At first glance, you might just say that it means you save on another toroid by having the combined version on example
#1, but is that really it ?

Is there something that I am missing ?

73 de Andy G0FTD

uncle_bob:
Hi

The first version puts DC through the core and thus *may* saturate the core. You also see push pull / two device versions that try to reduce this effect.

The second version lets you have a high impedance RF choke to B+ that *might* help you keep RF off the supply. You have to get into the details of both the RFC and transformer in each case to really see what the cost impact and matching range really is.

Bob

G0FTD:
You're the second person to mention the saturation issue.

It's looking like the preference to using the combined RF Choke come impedance transformer is in low signal level
circuits, whereas large PA stages are best with having a separate RF Choke with a proper and separate rf transformer
on the output.

Regards,

Andy

uncle_bob:

--- Quote from: G0FTD on July 03, 2016, 07:29:38 pm ---You're the second person to mention the saturation issue.

It's looking like the preference to using the combined RF Choke come impedance transformer is in low signal level
circuits, whereas large PA stages are best with having a separate RF Choke with a proper and separate rf transformer
on the output.

Regards,

Andy

--- End quote ---


Hi

A very common approach in a high power amp is to use a pair of output devices. The RF choke is set up to both transform and have balanced (opposed) DC in the windings to reduce the saturation issue. You have a three winding device, but still only have one core. Wire is cheap ....

Bob

vk6zgo:

--- Quote from: G0FTD on July 03, 2016, 10:55:20 am ---Greetings all !

Attached to this  post is a drawing showing to methods of taking out the RF from an output stage.
The example I drew represents a FET in this case.

I have seen to styles of taking RF out of a device.

1 -  The combined RF choke and transformer output method. Where of course you can alter the output impedance with
turns ratio as normal.

2 - Where the device keeps the RF Choke on it's own, and uses a separate RF output transformer, with suitable turns ratio
to suit impedance.

So why the two methods ?

At first glance, you might just say that it means you save on another toroid by having the combined version on example
#1, but is that really it ?

Is there something that I am missing ?

73 de Andy G0FTD

--- End quote ---

In Number (1), the primary of the transformer is part of the tuned drain load------not sure what happened to the "C" part of the LC circuit in your drawing,but if it is VHF/UHF,it is possibly made up of stray circuit capacitance.
It is a "series fed" circuit,because current from the dc supply flows through the resonant drain load.

In Number (2),the RFC is not part of the drain load at all,but is simply a "decoupling" component which has a high impedance at the operating frequency.
The LC circuit making up the transformer primary is the drain load.
This is known as a "shunt fed" circuit.

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