Question about SSB Biasing.

[BC Coyote]

Hmmmmm... I thought Class C applied any time the base of a power transistor is grounded through a transformer in push/pull operation. Or any time the base is held at ground potential for that matter.....
Applying a low bias voltage around 0.5 to 1 VDC on the base puts it, I think in class A/B territory....... I'm no expert though !

 

[brandon7861]

Go to School on RF Power Amplifier Classes - Mini-Circuits Blog

Learn about the differences in how Class-A, B, AB, C and other classes of solid-state power amplifiers (SSPAs) are biased and how they perform with respect to linearity, efficiency and output power.

blog.minicircuits.com

This is a very good read about the different classes of rf amps.
I think you are getting confused by push/pull operation and actual classes of bias.

73
Jeff

You are right, the class of operation is about conduction angle and not necessarily the configuration. I did word that incorrectly,

But lets assume that the push pull pair are both in class C, each having typically less than 90° of conduction. That gives the push pull pair a total of less than 180° conduction for the whole cycle if they are in fact class C. I think they are on longer than that, but I admit, I never tested it. One thing I do know is that the 1200w xforce I used to have did not like my 100A supply. Not at all. I had to run two car batteries with really heavy cable.

The next time I get one of these big boxes in front of me, I will look at the conduction angle and find out.

 

[brandon7861]

You know what, why not have a little fun with this...............

Lets take the input impedance of a 2SC2879 right off the datasheet, match it to 50Ω, give it 4 watts, then see what the RMS voltage is at that impedance.

Looks like the voltage at the base with 4w of drive is 2.829v RMS, or 8v pk-pk. With a 0.7v Vbe drop on each transistor, each sees 4v, that leaves 3.3v after it turns on. Now to excel to see what that conduction angle would be. 0.7v to 0.7v, rough estimate, its a lot more than 90° just by looking at this simple sine wave graph. Thats why I said there would have to be some negative DC bias established for it to be class C

Again, I could be wrong, it just doesn't seem intuitive looking at any of those schematics that they would be class C.

edit: Maybe with a fraction of a watt drive it might enter class C

 

[brandon7861]

We could go further and consider how many pairs of transistors that amount of power is split between as it would depend on the number of pills and drive level..

My Xforce was an 8 pill and took 35w drive. 35w split between 4 push pull pairs is around 8w for each pair, so that gives an even higher conduction angle when you match that 8w into the transistors input impedance. Therefore, I don't think I need to do the math on every single box layout, I think my point is clear. I don't think any of them are class C.

A cobra 25 with a single final and self-bias, class C all day long, but not a box with push-pull transistors getting hit with 4-8w on each pair.

Edit: sneaking in one more comment... What is one thing every class C amp absolutely needs????? The one thing none of these amps even have. A filter.

 

[brandon7861]

Didn't mean to kill/jack the thread, mods can delete my posts if they are not appropriate. I guess all this class C talk bugged me enough to want to have that conversation, but I'm no amp builder, so maybe I am doing the math wrong or not taking something into account.

If someone knows how to calculate the duty cycle differently than I did, other than putting the input watts across the transistor's input impedance and looking at how much of the resulting waveform is high enough to turn on the transistor, please chime in and correct me.

SSB needs to be biased class AB, because with no bias, only the loud sounds that result in enough RF voltage to make the transistor conduct would get amplified, and if the RF going into the transistor from a whisper or quiet syllable was only reaching 1v peak at the transistor's base, that would result in a low duty cycle for those quiet sounds because only the very tip of the signal would be high enough to turn on the transistor. During those quiet sounds, the amp would surely be running in class C, but not during loud sounds. So, relay delay aside, a SSB radio on an AM only amp with no filtering would create lots of harmonics on weak sounds and little distortion on loud sounds.

In AM, there is a carrier, so the quiet sounds can be amplified normally because the transistor is already on due to the carrier.

In FM, there is full RF voltage present all the time, so just like AM with a dead key, the RF is enough to get the transistor close enough to class B to not worry about it.

I never put much thought into why people lower the deak key for use with amps, but I will try now. Again, correct me if I am wrong. Wouldn't doing so put the carrier closer to class C operation which allows the transistor to run cooler during quiet periods thus allowing it to be pushed harder during the loud periods providing more punch/swing (at the expense of distortion during the quiet sounds)? Thats what makes sense to me anyhow.

I just cannot imagine an an AM only push pull amp running in class C when it has a proper dead key. Maybe this class C talk comes from the fact everyone drops their dk and thats what ends up happening, idk....

 

[AudioShockwav]

Quote:
Transistor Biasing is the process of setting a transistors DC operating voltage or current conditions to the correct level so that any AC input signal can be amplified correctly by the transistor.

And end quote

From here:https://www.electronics-tutorials.ws/amplifier/transistor-biasing.html

It's AC that we are amplifying if that helps and when a transistor turns on and off there is noise on top of everything else.

As for the math, one of the more engaging engineering individuals can help you with that, never been my rabbit hole to go down.
If you have heard this on the air you don't forget it.
Years ago I had a single transistor amp with the base grounded through a choke.
You could hear the distortion in the audio, it was unusable on ssb.
Two in push pull helps but you still need some bias.

73
Jeff