Symmetrical differential vs symmetrical non-differential VAS

[suzyj]

I've been exploring (via simulation only, so far) the effect on THD that I get by employing a differential vs a non-differential symmetrical voltage amp. I'm using the 50W MOSFET amplifier that I've been playing with of late. Basically, I've simply removed one side of the VAS, then tweaked the bias to ensure the output MOSFETs are running the same quiescent current (70mA).

Performance at 1KHz was much as I expected. The non-differential circuit is slightly worse. When putting a 1KHz 50W sinewave into an 8 Ohm load, I measure 0.00014% THD for the differential VAS, and 0.00028% for the non-differential one. This is, BTW, a design optimised at 1KHz)

However at 10KHz it's a very different story. I'm seeing a dramatic improvement in THD performance with the non-differential VAS.

In a nutshell, putting a 10KHz, 50W sinewave into an 8 Ohm load, the THD for the differential VAS measures at 0.046%, whereas the non-differential VAS measures 0.0050%. At 25W (in case it's a headroom issue) it's 0.034% and 0.0044% respectively.

I guess the reduced capacitive loading being placed on the single-ended-to-differential stage that precedes it, along with running the remaining VAS transistors at close to twice the Ic, are responsible for the improvement.

Of course in real life the performance may be very different.

Anyway, I've attached a screen grab of the differential circuit. I'll put the non-differential one in the next post.

I'd appreciate people's thoughts on the mechanisms at work here, as I don't think I have a full understanding of what's happening.

[suzyj]

And the non-differential circuit:

[GK]

Originally posted by suzyj
And the non-differential circuit:

Hi Suzy.

Your single ended VAS has a quiescent current of approximately 9.3mA. Your differential VAS has a tail current of approximately 10.8mA, giving 5.4mA (contrasted with 9.3mA) per leg.
You need a high quiescent VAS current to drive the MOSFET input capacitance at high frequencies with low distortion.
Try increasing the differential VAS tail current to at least double the single ended VAS quiescent current (~20mA), but don't go too far with lowering the emitter degeneration.

Cheers,
Glen

[PB2]

Originally posted by suzyj
I've been exploring (via simulation only, so far) the effect on THD that I get by employing a differential vs a non-differential symmetrical voltage amp. I'm using the 50W MOSFET amplifier that I've been playing with of late. Basically, I've simply removed one side of the VAS, then tweaked the bias to ensure the output MOSFETs are running the same quiescent current (70mA).

Performance at 1KHz was much as I expected. The non-differential circuit is slightly worse. When putting a 1KHz 50W sinewave into an 8 Ohm load, I measure 0.00014% THD for the differential VAS, and 0.00028% for the non-differential one. This is, BTW, a design optimised at 1KHz)

However at 10KHz it's a very different story. I'm seeing a dramatic improvement in THD performance with the non-differential VAS.

In a nutshell, putting a 10KHz, 50W sinewave into an 8 Ohm load, the THD for the differential VAS measures at 0.046%, whereas the non-differential VAS measures 0.0050%. At 25W (in case it's a headroom issue) it's 0.034% and 0.0044% respectively.

I guess the reduced capacitive loading being placed on the single-ended-to-differential stage that precedes it, along with running the remaining VAS transistors at close to twice the Ic, are responsible for the improvement.

Of course in real life the performance may be very different.

Anyway, I've attached a screen grab of the differential circuit. I'll put the non-differential one in the next post.

I'd appreciate people's thoughts on the mechanisms at work here, as I don't think I have a full understanding of what's happening.

Hi Suzy,

It seems to me that in order to maintain balance in the diff pair VAS, you should float the collectors that are currently grounded, or actually load them with a resistor to ground so that they produce about the same signal level as the side driving the output stage. This should help to maintain balance at HF with regard to Miller capacitance in the devices. Seems it would make sense to use the same Cdom caps on both sides of the pairs, again to maintain HF symmetry.

I would then try looking at it open loop to see what's happening to the gain and bandwidth between the two cases, and of course the open loop distortion if the above suggestion doesn't work.

I prefer to run the VAS closer to 5 or 10 mA when possible since most of the popular devices have their best performance in this range. I also want there to be enough available current to swamp the non-linear parasitic capacitances. I always found that high bias current worked best in low complexity video amps that I've designed.

The diff pair VAS was starting to grow on me since it provides a source for a balanced output which is something I've been thinking about for some time now.

Once again, nice work.

Pete B.

[suzyj]

Originally posted by G.Kleinschmidt
Your single ended VAS has a quiescent current of approximately 9.3mA. Your differential VAS has a tail current of approximately 10.8mA, giving 5.4mA (contrasted with 9.3mA) per leg.

This is the core of what's confusing me. When I double the current in the differential VAS, such that the current in each side is equal to the current in the non-differential case, I'd expect the distortion figures to be better than the single ended case. That doesn't happen though. Instead I only get marginal improvement in THD (to 0.032%).

It's a little frustrating, as it's just not behaving as I think it ought to do.

I might try replicating more of the load on the other side (as per Pete B's suggestion) and see what effect that has.

[GK]

Originally posted by suzyj


This is the core of what's confusing me. When I double the current in the differential VAS, such that the current in each side is equal to the current in the non-differential case, I'd expect the distortion figures to be better than the single ended case. That doesn't happen though. Instead I only get marginal improvement in THD (to 0.032%).

It's a little frustrating, as it's just not behaving as I think it ought to do.

I might try replicating more of the load on the other side (as per Pete B's suggestion) and see what effect that has.

I think your differential VAS problems would go away if you were to simply cascode all four VAS transistors.
Replicating the voltage swing on the currently grounded side would be quite difficult, as the MOSFET input capacitance and the miller compensation capacitors will demand higher drive current at HF.
This means that, for example, if you resistively loaded the idle side to match the voltage swing at 1kHz, at 20kHz the idle side will well and truly saturate.

Cheers,
Glen

[PB2]

Originally posted by G.Kleinschmidt



I think your differential VAS problems would go away if you were to simply cascode all four VAS transistors.
Replicating the voltage swing on the currently grounded side would be quite difficult, as the MOSFET input capacitance and the miller compensation capacitors will demand higher drive current at HF.
This means that, for example, if you resistively loaded the idle side to match the voltage swing at 1kHz, at 20kHz the idle side will well and truly saturate.

Cheers,
Glen

Glen, that would probably work, but it is complex and reduces the swing of the VAS.

I suggest a resistive load to match the diff VAS outputs at say 10 Hz, and a capacitor to match at 10 kHz, and then see if this is a good enough approximation. I'd say that this has to be the problem since it is obviously frequency dependent. Might be a model problem also with the VAS transistors.

You could also remove the output devices and just put a resistive load on the VAS to try the different test cases. This way you could have perfect symmetry on the two sides of the diff pair.

Pete B.

[Terry Demol]

Originally posted by suzyj


This is the core of what's confusing me. When I double the current in the differential VAS, such that the current in each side is equal to the current in the non-differential case, I'd expect the distortion figures to be better than the single ended case. That doesn't happen though. Instead I only get marginal improvement in THD (to 0.032%).

It's a little frustrating, as it's just not behaving as I think it ought to do.

I might try replicating more of the load on the other side (as per Pete B's suggestion) and see what effect that has.

Suzy,

It is possible the distortion is originating somewhere else and the
VAS component is only part of the picture.

The basic topology goes to heroic lengths to linearise the VAS
with 2 cascaded / degenerated approach and then attempts
to drive the highly non linear capacitance of the OP MOS devices
with a high Z current drive.

It could be well worth to simplify the VAS and insert a small sig
high, BW (ie; very low c) bjt follower in front of the OP MOS
devices.

This will effectively isolate them and provide a much more linear &
frequency independant load to VAS.

Greg Ball did this in his SKA to such good effect with the addition
of some clever bootstrapping that he did away with VAS all
together and still retained VG linearity.

cheers

Terry

[GK]

Originally posted by PB2


Glen, that would probably work, but it is complex and reduces the swing of the VAS.

I suggest a resistive load to match the diff VAS outputs at say 10 Hz, and a capacitor to match at 10 kHz, and then see if this is a good enough approximation. I'd say that this has to be the problem since it is obviously frequency dependent. Might be a model problem also with the VAS transistors.

I think that could be made to work to an extent in simulation, but in real life you also have the MOSFET input capacitance to compensate for as well, and that varies wildly with load current.
I agree that cascoding would limit the output swing though, which is already limited due to the high Vgs of the lateral MOSFETs. On the other hand though, a design of this complexity probably deserves seperate, higher voltage rails to the low-level circuitry to compensate.

You could also remove the output devices and just put a resistive load on the VAS to try the different test cases. This way you could have perfect symmetry on the two sides of the diff pair.

Pete B.

I agree. In fact, just simulating the different VAS stages by themselves would probably be better than simulating the entire amplifier.


Cheers,
Glen

[PB2]

Originally posted by G.Kleinschmidt



I think that could be made to work to an extent in simulation, but in real life you also have the MOSFET input capacitance to compensate for as well, and that varies wildly with load current.
I agree that cascoding would limit the output swing though, which is already limited due to the high Vgs of the lateral MOSFETs. On the other hand though, a design of this complexity probably deserves seperate, higher voltage rails to the low-level circuitry to compensate.





I agree. In fact, just simulating the different VAS stages by themselves would probably be better than simulating the entire amplifier.


Cheers,
Glen

Some voltage swing on the side that was grounded is much better than zero if you know what I mean, and often close enough works very well when you're trying to keep things simple. If this approximate load produces a diff VAS with half the distortion, or less, than the single ended version then we might say close enough and go with it. Design is a compromise as I'm sure you know, the distortion is already low just trying to make it a bit better. Obviously, design involves compromises and choosing among the many options is part of the art as I'm sure most here know.

Pete B.