[Amps] Designing the Cleanest Linear with RF Negative Feedback

[Steve Thompson]

Tom Cathey wrote:
>>They also say that IM product do not necessarily drop as drive is
>>reduced. It can actually be greater at lower power levels than at higher
>>power levels. IM products drop way down at one point midway in the power
>>range and then go back up again. They show a graph that is derived
>>mathematically and say that most tubes follow the graph pretty closely
>>to the predicted IM performance ratios of drive to bias amount.
>>
> 
> 
> Gary,
> 
> Yes, I've read that too, but then I've read the opposite.. That statement 
> above would make my question about using a chain of GG tubes running at 
> 1/10th reduced ratings a poor idea. I'm a little confused cuz I see it 
> written sometimes that for very linear operation, sometimes stages run at 
> 1/10th  their power ratings in class A.  Maybe that's for special purpose 
> receiver high performance preamps or something like that.
I think it's important to consider the difference between the class AB 
biassing that we are all used to in our amplifiers, and true class A 
which is usually reserved for specialist ultra-linear requirements. As a 
simple rule of thumb, ultra linear class A operation will have the 
maximum peak output power not much over 10% of the dc power dissipated 
in the device, and the current in the device will not alter with signal 
level.

Steve