Re: About stability and feedback

Eeyore wrote:
> In another discussion with Iain Churches, the subject of stability with
> feedback came into the conversation.
> 
> The basic rules for stability with feedback applied are quite simple.
> Namely that the feedback should not become 'in phase' with the input
> (due to phase lag issues typically in the circuit in question) AND have
> a value of greater than one.
> 
> Now, how does the LOAD affect this phase lag (or lead) ? In a typical
> semiconductor based amplifier which I am of course rather more used to
> working with, using an emitter follower output, quite simply it dosen't
> very much since the ouput Z is inherently low compared to the load. So
> this topology suffers relatively little from load related instability
> problems.
> 

Output Z does not affect the phase lead or lag at all. All it does is 
change the frequency at which it occurs. A lower output Z simply means 
the turnover frequency is higher, so if a  particular load cause 45 
degrees phase ****ft at 100KHz when connected to a 1 ohm source it will 
just have 45 degrees phase ****ft at 1MHz when connected to a 0.1 ohm 
source. Whether or not the topology is likely to suffer or not because 
of this depends principally on the gain bandwidth product. Since SS amps 
can easily achieve much greater GB products than tube amps it is 
probably a good job SS amps can achieve a very low output Z otherwise 
they might be even more difficult to keep stable


Cheers

Ian