[Amps] designing and testing RF Bipolar Power bias circuit

[Dan Sawyer]

Manfred,

Thank you again for your input. The insight into the bias circuit was 
very helpful. I replaced the curve tracer with a variable power supply; 
that improved the response considerably. The plan is to proceed with a 
circuit based on individual diodes epoxied to the transistor caps.

Thanks again - Dan

Manfred Mornhinweg wrote:
> 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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