On 8/13/2011 8:53 AM, Ron Youvan wrote:
> Dr. David Kirkby wrote:
>
<snip>
>> I don't know how close we are to the theoretical limits of what is possible,
>> but
>> perhaps not too far.
> I've never heard of this, but I see no theoretical limit problem, we can
> tolerate any output
> impedance if we are allowed to use a transformer/s.
There are physical limitations, but I'm embarrassed to say that although
I used to teach semiconductor circuit design It's been so long that I do
not remember any formulas. The limitation being talked about sounds like
it applies to bipolar transistors rather than today's and would apply to
a "given" transistor where the gain is inversely proportional to
frequency and dissipation goes up dramatically near the upper end of the
operating range.
Like tubes frequency limitations are due to transit times, however the
transistor has junction temperature, gate temp, gate to source and drain
capacity in addition to temp limitations.
Like David I see no reason for a theoretical limit. The main limitation
in today's RF Power transistors is getting rid of the heat fast enough.
They've made great strides, but the thermal mass of the internal
components is so small that even with the best of heat sinking
techniques the internal delta T can easily become the power limiting
factor well below the device's actual power rating.
Those same small pieces that limit heat dissipation are what give the
transistor the ability to go to such high frequencies. The relatively
large size of tubes limits their upper frequency due to transient times,
although in some systems the tube itself becomes part of the resonant
circuit or even the actual circuit. (I phrased that poorly but I think
the microwave guys will understand what I was trying to say). For many
tubes the internal lead length limits their usefulness to HF or very low
VHF.
73
Roger (K8RI)
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