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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> http://ludens.cl
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