Different Differential Pairs

[dsavitsk]

Does anyone have a reason to use one of these over the other? (A) has the advantage of not needing a negative rail, of running on a lower PS, and of not needing the PS to be of any particular voltage, and (B) has the advantage of needing a few less parts. Beyond that, does anyone have experience with performance differences between the two?

[jcx]

do you want input signal common mode rejection

the "long tailed" part of "long tailed pair" refers to the common mode independance of the bias - how good an aproxiamation to a ccs it is - a large value R to a negative supply can be an adequate "ccs"

if the R is too small you get larger common mode signal on the plates

bjt ccs can run down to 1 V or less - so depending on signal, tube bias you can get the "long tail" bias independance even without a negative supply

[SpreadSpectrum]

A and B operate on completely different loadlines.

Putting CCS in tail forces balance but you are still operating across resistive load so the slope of the load line is determined by the load resistor (obviously).

Putting CCS in plate circuit gives you horizontal load lines. However, since you have CCS plate loads there is hardly any signal across the tail resistor to enforce balance so balance will be degraded. Distortion per phase will be excellent, though.

[DF96]

Yes, put CCS in tail if you want low common-mode gain (e.g. as phase splitter). Put CCS in anodes if you already have differential drive and want low distortion. Don't put CCS in both!

[SY]

There is a way to get away with using both- if the tail current source is a bit sloppy (say, a single transistor CCS) and the plate load CCS are very tight (say, a cascode CCS), surprisingly, one can get excellent balance and CMR while realizing maximum gain and linearity. I didn't believe it myself until I was urged to breadboard an example and it worked just fine. There will be more about this in the soon-to-appear 4th edition of Valve Amplifiers.

[dsavitsk]

Alternatively, could you just parallel the tail CCS with a large value resistor that is just there to pick up the slop?

[DF96]

Yes, but then you start adding common-mode gain. A similar issue came up in another thread. Diff-mode gain is roughly mu. Common-mode gain is roughly Ranode/Rtail (factor of 2 somewhere too?). For a phase splitter you need low common-mode gain, so if CCS are used in the anodes then you need sloppy ones here and a good one in the tail. SY says the opposite, so one of us is wrong!

[bigwill]

At risk of hijacking the thread SLIGHTLY (sorry), but still on the topic of differential pairs, could anyone explain this phenomenon:

In a simulation I built a bog-standard long tailed pair with 6SN7s, sane anode resistors and a CCS tail. Given a single ended signal on one input and earthing the other produces exactly what you'd expect - a fairly low distortion differential output on the anodes. This being a Class-A circuit the distortion will fall with the level of the input/output to nearly immeasurable in the 10s of mV output range.

Why is it when you feed both grids the EXACT same signal (ie connected together) you get a very low residual (fair enough, imperfect CCS, capacitance, etc) BUT this residual has a surprisingly high percentage of distortion despite the signal being so low in amplitude? Even in simulation with perfectly matched tubes.

[SpreadSpectrum]

Originally Posted by SY

There is a way to get away with using both- if the tail current source is a bit sloppy (say, a single transistor CCS) and the plate load CCS are very tight (say, a cascode CCS), surprisingly, one can get excellent balance and CMR while realizing maximum gain and linearity. I didn't believe it myself until I was urged to breadboard an example and it worked just fine. There will be more about this in the soon-to-appear 4th edition of Valve Amplifiers.

SY, you wouldn't happen to know how soon?

[tubelab.com]

Don't put CCS in both!

Hey, the big dumb blonde one spent a whole day finding out that this is a most excellent way to make a vacuum tube flip flop! Reducing the quality of each CCS can reduce the instability but I had the best luck with a pure CCS in the tail. Adding CCS's with parallel resistors in the plate circuit improves the gain a bit but too much resistance makes the circuit unstable. The tubes were 6SN7's and the CCS's were 10M45's.