Solid State PA Bias pot

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DMOD:
Quote from: KD6VXI on September 11, 2018, 11:06:46 PM

Phil,


...Unless you drop AMPS across the resistor, you won't be able to have good dynamic regulation...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 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.

--Shane
KD6VXI


Thanks and I am well aware of bipolar vs. FET biasing requirements, but I thought we were speaking specifically to bipolar biasing in linear amplifiers.

And that was the basis for the simple bias source, i.e., source about 10 times more current than the needed bias current. I never stated it was the best and there are much more complicated designs out there that do much the same thing except they regulate the currents and voltages much more precisely. The design does work as a simple biasing source.

E.g., look at the Ameritron AL-500M which uses bipolar RF transistors. The bias current is sourced by simply adjusting the base voltage of the 2N3055 pass transistors via a voltage divider, sampling temps, crow barring, etc. I.e., The circuit is just a regulated version of the dropping resistor/honkin' diode circuit.  :)

The source of IMD in most SS CB amps was the fact they were biasing stages in an approx. ClassBC (more C than B) operating area (in contrast to biasing in the AB1,2 area) in order to squeeze out as much "dirty"   :o  power as was possible.


Phil - AC0OB

KD6VXI:
I was speaking to bipolar devices.  The problem is, the diode clamp fed through a resistor and then using a pot to pick the sweet spot falls apart under load.

Unless you drop a few watts across the potentiometer. This is in addition to the amps of current dropped across R1.

A fet doesn't require this, hence my statement pointing the difference out.

The page I posted takes a cb amplifier using the exact circuit you posted, replaces it with an active circuit and the amplifier cleans up in all regards. IMD, saturated power output, etc.

I've used the diode clamp.  It will work.  Im just pointing out that it is by far not the best circuit to use. 



Try this:  simulate the diode clamp, and what does the regulation do when pulling an amp through the wiper of the pot?  Do you maintain voltage regulation?  What's the variance?  Don't have Sim software at work or I'd try.


--Shane
KD6VXI

DMOD:
Quote from: KD6VXI on September 14, 2018, 05:44:45 AM

...I was speaking to bipolar devices.  The problem is, the diode clamp fed through a resistor and then using a pot to pick the sweet spot falls apart under load...

Not if you use the formula I gave in the attachment.

Rdrop = (Vs - Vfdiode)/(Idiode + Isupply + Ipot)

You have to take into account all current sinks.   8)


Quote

I've used the diode clamp. It will work. Im just pointing out that it is by far not the best circuit to use.  

Which I what I stated as well and we agree so no need to repeat.   :)


Phil - AC0OB

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