Re: LF: RE: [rsgb_lf_group] Single Stage Class-B

[Andy Talbot <andy.g4jnt@gmail.com>]

That is the task of the resonant tank circuit, to maintain the RF current flow throughout the device during the off period. 

During the off period, the drain volts have to fly up to 2.Vdd to keep it operating linearly, and the resonant network has to ensure that happens.

The concept is quite standard, used for most bipolar and FET output stages at low power (watts, tens of watts)  at VHF and UHF.   Although I've noticed that modern FET amplifier modules do run at a higher standing current and lower efficiency than bipolar ones.   All the old valved SSB transmitters of a bygone era use that topology

I may need another long walk to ponder in more depth ...

Andy  G4JNt

On 22 February 2016 at 18:53, Rik Strobbe <rik.strobbe@fys.kuleuven.be> wrote:

A single stage class B amplifies the signal over 180 degrees (ie. half the sine).

As long as the amplifier is not saturated the amplitude at the output is almost linear proportional to the input amplitude, but the signal is distorted due to the fact that half of the sine is missing.

In a class B push-pull each FET (or bipolar transistor, tube) will amplify the "opposite" 180 degrees of the input signal and these will combine at the output , so apart from the cross-over distortion the output signal in pretty undistorted.

 

73, Rik  ON7YD - OR7T

 

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Van: rsgb_lf_group@yahoogroups.co.uk [rsgb_lf_group@yahoogroups.co.uk]
Verzonden: maandag 22 februari 2016 19:39
Aan: rsgb_lf_group@yahoogroups.co.uk
CC: rsgb_lf_group@blacksheep.org
Onderwerp: Re: [rsgb_lf_group] Single Stage Class-B

 

There's nothing inherently "more linear" about push pull over a single stage, given you run the same device at the same power per stage (ie half the PP level)    

'jnt

On 22 February 2016 at 18:34, Rik Strobbe rik.strobbe@fys.kuleuven.be [rsgb_lf_group] <rsgb_lf_group@yahoogroups.co.uk> wrote:

 

Hello Andy,

 

the advantage of a push-pull class B amplifier is that it it pretty linear at higher power levels.

 

73, Rik  ON7YD - OR7T

 

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Van: rsgb_lf_group@yahoogroups.co.uk [rsgb_lf_group@yahoogroups.co.uk]
Verzonden: maandag 22 februari 2016 18:31
Aan: rsgb_lf_group@blacksheep.org; rsgb_lf_group@yahoogroups.co.uk
Onderwerp: [rsgb_lf_group] Single Stage Class-B

 

Whilst out walking and pondering, as one does....

There are hundreds of designs out there for  linear power amps using FETS.  They all use a similar design, push-pull with centre tapped ferrite cored transformers at input and output, often with resistive feedback - and all broadband.   The design is so standard it's become tedious ... and as I don't do tedious, it made me think:

Back in days of yore when those big bottles ruled the roost ,  apart from a few designs for UHF, they were all single ended.

So, why not a single ended MOSFET amp (for single band operation)?   We're used to seeing single device  for class-E, where the tank circuit does much of the waveform cleaning, and reduces the low pass filter requirements.    So what's wrong with a tank-based  design for class-B?

A single ended amp can even be made ferrite free (although a final stage DC feed choke would no doubt benefit from it).   Apart from a few very low power designs, I don't recall seeing any designs for single band ferrite free linear amps using power MOSFETS. 

Only major downsides I visualise is that load resistance at the device is 4 times lower for the same Vdd and Pout , and device dissipation is doubled

Has no one ever bothered, because broadband transformer push-pull is so straightforward?

Has a trick been missed?  

Andy  G4JNT

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Posted by: Andy Talbot <andy.g4jnt@gmail.com>

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