What makes a good top tube in a mu follower? Transconductance? Any rules of thumb for choosing the top tube?
Definitely Gm. Ideally, the top tube would have a cathode follower Av= 1. That way, the bottom would see a pure CCS.
"Any rules of thumb for choosing the top tube?"
No, not really, though it would seem that a MOSFET works best up top.
There's more to making a good mu follower than picking the right tube for the top. A mu follower effectively provides a plate load to the bottom tube that is equal to the middle resistor times the mu of the top tube so you might argue a hi mu tube is what you need, but then again you probably want a nice low output impedance so you want hi gm for that.
One of the reasons for using a mu follower is that the high effective plate load means the bottom tube produces lower distortion. However, once the effective plate load of the bottom tube gets past about 10*ra, the improvement in distortion from the lower tube is tiny so you could say you should aim for a top tube mu such that mu*middle resistor > 10*ra. Alternatively you can change the value of the middle resistor but don't forget this changes the operating point of both tubes because their plate voltages will have changed and of course the maximum output swing will have changed too.
And we haven't even touched on what idle current to choose. This depends partly on the 'best' operating point for the bottom tube and the expected load. Don't forget that just because the mu follower has a low output impedance it does not mean it can drive low impedance loads.
So, the short answer is it depends on a whole lot of things, not the least of which is what is your mu follower intended to do?
Cheers
Ian
One of the reasons for using a mu follower is that the high effective plate load means the bottom tube produces lower distortion. However, once the effective plate load of the bottom tube gets past about 10*ra, the improvement in distortion from the lower tube is tiny so you could say you should aim for a top tube mu such that mu*middle resistor > 10*ra. Alternatively you can change the value of the middle resistor but don't forget this changes the operating point of both tubes because their plate voltages will have changed and of course the maximum output swing will have changed too.
And we haven't even touched on what idle current to choose. This depends partly on the 'best' operating point for the bottom tube and the expected load. Don't forget that just because the mu follower has a low output impedance it does not mean it can drive low impedance loads.
So, the short answer is it depends on a whole lot of things, not the least of which is what is your mu follower intended to do?
Cheers
Ian
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