Solid State PA Bias pot

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KA3EKH:
It’s on the schematic, Duh! Imagine that they bias the amplifiers just a bit on into a more linear state. Spent most of my time working with class C solid state and just starting to get further into solid state amplifiers for AM and SSB and now think the big thing may be to get a two tone test generator and start looking at SSB and AM response that way. Use to just use a distortion analyzer and a good demod back in the old days when you had to do “proofs” on AM broadcast stations, still a requirement but no longer required to do it so no one dose.
Thanks for the help; maybe someday I will grow up to be a knowledgeable Ham!

DMOD:
Quote from: KA3EKH on September 10, 2018, 09:53:13 PM

It’s on the schematic, Duh! Imagine that they bias the amplifiers just a bit on into a more linear state. Spent most of my time working with class C solid state and just starting to get further into solid state amplifiers for AM and SSB and now think the big thing may be to get a two tone test generator and start looking at SSB and AM response that way. Use to just use a distortion analyzer and a good demod back in the old days when you had to do “proofs” on AM broadcast stations, still a requirement but no longer required to do it so no one dose.
Thanks for the help; maybe someday I will grow up to be a knowledgeable Ham!

No problem, we learn by studying, experience, and a, "keep-at-it" attitude.


Phil - AC0OB

KD6VXI:
If you can independently set bias per active device then even a single tone generator can help.  At least set it for lowest crossover distortion.

Most bipolar devices 'act right' between 65 hundredths and 75 hundredths of a volt bias.  FETs can be in the range of volts of bias. 

The generally better method of setting it, without a tone generator and a scope is for quescient current.  Turn both pots to 0 mils, turn one up to the set point where the amp is  drawing half of specified current, increase the second pot to full quiescent.

The dataset will probably tell you a good ballpark value.  However, the actual circuit may cause these values to need to be adjusted.

Also, almost all solid state bias designs suck.  It's pretty hard to build a good regulator for both static and dynamic regulation for operation between a half and three quarters of a volt at multiple amps of current draw varying at a syllabic rate.

--Shane
KD6VXI

DMOD:
Quote from: KD6VXI on September 11, 2018, 06:04:27 AM


...Also, almost all solid state bias designs suck.  It's pretty hard to build a good regulator for both static and dynamic regulation for operation between a half and three quarters of a volt at multiple amps of current draw varying at a syllabic rate.

--Shane
KD6VXI   

It is actually quite easy and relatively simple for Bipolar circuitry:


Phil - AC0OB

KD6VXI:
Phil,

That circuit is one of the worst their is.  Unless the diode is bonded to the active device or the heat sink, it will run into thermal instability and can cause the amp to self destruct.

Unless you drop AMPS across the resistor, you won't be able to have good dynamic regulation.  The bias on a bipolar isn't like a fet, just apply some voltage and the device works.  The bipolar devices actually take current to stay on, especially when the device is being driven hard.  THIS circuit is what makes most biased CB amps IMD generators.

To quote Ian White:  "There are many problems with this simple circuit. The main one is that in order to maintain a constant voltage across D1, the permanent standing current through D1 must be several times higher than the maximum base current drawn by TR1 at the peak of RF drive. This requires an enormous standing current through RV1 and D1 - one ampere or even more. Most designers fail to provide this. Some commercial circuits even use a low-current signal diode such as a lN914 for D1! Bias regulation totally fails when peaks of RF drive create a heavy demand for base current, so this circuit is a real splatter-generator."

Rather than copy / paste a bunch of pages, Ian makes a pretty good presentation on his website, and particularly calls the circuit presented into question as a bad bias design.

http://www.ifwtech.co.uk/g3sek/tr-bias/tr-bias1.htm


To each his own.  Manufacturers have used that design for decades, as has many other "designers".  Doesn't mean it's a good design today, though.  Have I used that design.  YUP!!!!!!  In both single ended as well as push pull designs.   There are better.



For actual designs, spec an plots, comparisons between active and basic bias designs, this guy has a decent write up towards the bottom of his page.

The bias circuit contributed 12 dB to IMD.  That's very significant.

http://www.ok2kkw.com/00003016/bias/bias_new_en.htm


--Shane
KD6VXI

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