I'm back! Tutorial on High-Power Balanced & Doherty Microwave Amplifie

[Hugoneus]

Sorry for the delay in my activities lately. I have moved and been very busy.

Don't forget to submit your questions for the Q/A I'll be doing soon.

Watch the video here: [30 Minutes]
youtu.be/beYDYuSMnoQ

More videos at The Signal Path:
http://www.TheSignalPath.com

[edpalmer42]

 Welcome back Shahriar!

You had us worried when you just vanished. 

[Hugoneus]

Thank you.

No I didn't move very far. The house just needed a lot of work and I was also pretty busy at work!

I hope to get the basement ready and the lab up and running soon.

[4CX35000]

Sorry for the delay in my activities lately. I have moved and been very busy.

Don't forget to submit your questions for the Q/A I'll be doing soon.

Watch the video here: [30 Minutes]
youtu.be/beYDYuSMnoQ

More videos at The Signal Path:
http://www.TheSignalPath.com

Doherty was originally applied to high powered medium wave AM transmitters as a method to try and achieve several tens of kilowatts at high efficiency. The last high powered Doherty amplifier I worked on was a Marconi transmitter with 2 tetrodes being used for the Doherty amplifier with RF applied to the control grid and AF being applied to the screen grids. The phasing between the anodes on the Doherty Amplifier was achieved using Pi network.

On the Marconi AM Doherty transmitter I recall the class AB stage (Carrier Valve) with a unmodulated carrier was driven into near saturation while the class C stage (Peaking Valve) was turned off.  When the AM carrier is 100% modulated the carrier valve and peaking valve are working together for the peaks of the modulation and only the carrier valve for the troughs of the modulation, the peaking being in class C is not doing anything below carrier level.

Marconi used to call the Doherty method - impedance modulation as the impedance as seen by the carrier valve is dynamic depending on what peaking valve was doing. As the peaking valve conducts the impedance as seen by carrier valve starts to decrease, therefore the power from that carrier stage increases.

Requirement for AM is at peak envelope power the transmitter is giving x4 the carrier power, so a 50 KW medium wave transmitter will output 200 KW at peak envelope power, that is x2 voltage and x2 current.

AM transmitters tend to be fixed frequency so at the RF was applied to the Doherty Circuit via a Pi impedance matching and phasing network, but for the AF being applied to the screen grid the impedance as seen by previous stages is dynamic. Therefore two extra small valves are used in cathode follower mode for both the peaking and carrier valves and the AF is applied to the screen grid via the cathode followers.

The Doherty as in your video can be used as a linear amplifier by applying the modulation directly to the control grid, but as a loss to the efficiency of the high level modulated Doherty amplifier.

Anyway I see you decided to avoid calling the Balanced Amplifier the Engelbrecht - Kurosawa amplifier, it can be a bit of a mouth full.

Regards

[Hugoneus]

Excellent additional information.

Thanks!