[Amps] designing and testing RF Bipolar Power bias circuit

[Manfred Mornhinweg]

Hi Dan,

> Things are proceeding, however the bias seems 
> unduly noisy. I measure over two volts of peak to peak ripple at the 
> bias source.

Such extreme noise would be strongly driving the transistors, fully into 
saturation! Are you sure you really have that much noise on the bias?

> The bias is a current source,

That's a big mistake. To bias bipolar transistors you need a stable 
VOLTAGE source, of about 0.6V, that compensates for the thermal 
coefficient of the transistors, and it must be of very low impedance, 
meaning that it must be able to provide a widely varying current, with 
almost no voltage drop. The current you have to expect is the collector 
current divided by the RF current amplification ratio. In some cases, it 
could be as high as 1/3 the collector current, but typically is more 
like 1/10 the collector current.

 > When power is applied the bias sags.

Of course... and you end up still being in class C! In the worst case, 
you damage the transistors from reverse conduction of the base-emitter 
junctions. You need the stiff voltage source, not a current source!

 >  This may not be
> the 'best' circuit, however it is convenient because it is possible to 
> easily make measurable adjustments.

But it's useless for actual operation!

> 1. any thoughts on the source of the noise?

I would think it's measurement error. Probably the amplifier is 
oscillating, and you are taking that for bias noise.

> 2. would a standard voltage divider and diode circuit be better?

Yes, much better. But still not optimal. It's best to use a reference 
diode thermally coupled to the transistors, in a regulator circuit that 
compensates enough to make the idling current go slightly DOWN when the 
transistors heat up.

> 3. would it be less noisy.

I would think so. Be sure to bypass it for both RF and AF.

> 4. would a capacitor across bias input help?

Yes. That's what I mean by bypassing it.

> Sorry for the low level questions, but your analysis would be helpful. 

Even the biggest gurus started with low level questions!

Manfred.

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