Improving the linearity of an N-ch MOSFET output stage

[x-pro]

I am happy to invite forum members to have a look at my short article:

http://www.ant-audio.co.uk/Theory/N-...0linearity.pdf

In this thread I will try to answer questions about that circuit configuration and also about the last diagram from the article - a practical example of an output stage that can be used with or without an overall NFB with either tube or solid-state VAS. I attach this circuit diagram here as well.

Alex Nikitin

[x-pro]

I did a simulation in LT spice of two versions of my circuit configuration, using andy_c N-MOSFET model for 2SK1530 from this post:

http://www.diyaudio.com/forums/showt...83#post1316183

Here is the result. The idle current of the output transistors is set for 70mA, DC conditions on the output are set at less than 1mV offset. The rest is clear from the SPICE parameters. From the FFT graph the distortion reduction due to the additional FET on the left is up to 40 dB for odd order components.

Alex

[Francois G]

Thanks for sharing your circuit. It looks very interesting, especially due to my interest in tube voltage amplification. Do you have any test or simulation results for the final circuit?

I'm not an electrical engineer and I'm looking forward to learn more in this discussion group about you circuit and similar ones. That said, your design looks similar to the amplifier Wim De Haan published in audioXpress about 18 months ago and a higher power version recently in Elektor and I thought you might enjoy comparing them.

Both the NISHIKI and MUGEN designs are hybrid amplifiers without global feedback using quasi-complementary output stages. The website does not show the schematic because it was published in the Elektor October 2007 issue. The topology is shown on his website at http://www.wimdehaan.nl/ under the NISHIKI heading.

The website has extensive test results for these amplifiers using different tubes and other options.

[Francois G]

You posted the sim results as I was writing my post (#3)! Thanks.

[Francois G]

I realized that the circuits are not so similar after reading your article. It seems that the means of achieving phase splitting is quite different, but I admit that I don't fully understand either circuit. So, I still looking forward to learning.

[x-pro]

Here is the result of another simulation run for the same two circuits - now for 20kHz.

Cheers

Alex

[sajti]

Hi,

this circuit looks nice! I will try to build it, with IRL540N, and IRF9520N.
My only question is: How can I use more output devices parallel? I need source resistors for that. But the voltage drop on this resistors can saturate the drivers, at the peak currents. Do You have any idea about it?

Sajti

[x-pro]

Originally posted by sajti
Hi,

this circuit looks nice! I will try to build it, with IRL540N, and IRF9520N.
My only question is: How can I use more output devices parallel? I need source resistors for that. But the voltage drop on this resistors can saturate the drivers, at the peak currents. Do You have any idea about it?

Sajti

Hi Sajti,

yes, you can build the circuit from the first post with IRL540N on the output, providing you would not increase P/S voltage, however IRF9520 could be a bit too big - better to use 9510 or 9610. You are quite right that it is difficult to parallel devices in this configuration. Every circuit has it's strong and weak points. To increase the power of this circuit I had to use a series connection of two devices (what is usually called "totem pole"). Modern MOSFETs can pass a lot of current (HUF76639 is 50A device) so the current is not much of a problem - as shown the circuit can deliver about 20A peak current into 1 Ohm load. And the amplifer sounds better when devices are not connected in parallel.

The maximum output current in this circuit is quite accurately and symmetrically limited by 2Vth*S - that limit has negative temperature coefficient (as both Vth and S drop with temperature) which helps the amp to survive overload conditions. The circuit as shown would handle a short on the output for a second or two - enough for AC fuses on the output of the transformer to blow.

Few more important notes:

1) output devices should be mounted on a heatsink with less than 1C/W using mika or AlO insulation with thermal grease and not far one from another (about 25-30 mm between mounting screws) . No silicon impegnated washers, please - the amp will blow.

2) thermal sensing transistor Q1 should be attached to the heatsink between output devices and thermally connected to it using thermal grease.

3) VR1 bias adjustement pot should be a multiturn cermet type and before switching the amp on first time it needs to be in the "top" (i.e. max resistance) position. Adjust bias slowly - it is a very sharp adjustement. It is useful to look at the bias behavior during warm-up. It may be necessary to change the value of R4 a bit (i.e. 200-220-240-270 Ohm) to correct the current source temperature coefficient for a particular type of the output devices. If the bias increases with temperature you'll need to decrease the value of R4 and vise versa. Every time you change R4 you'll need to reset the pot and re-adjust the bias. It is useful to have a slightly negative TC.

Cheers

Alex

[lumanauw]

Hi, Xpro,

Is it possible to use bipolar in position of M3-M4 and omit D5-D6? Bipolar has bigger Gm than mosfet, will this affect something?
Oops, sorry. After reading your article, M3 and M2's Vgs must be matched. Bipolar only have 0V6, while mosfets need about 3-4V.

[x-pro]

Originally posted by lumanauw
Is it possible to use bipolar in position of M3-M4 and omit D5-D6? Bipolar has bigger Gm than mosfet, will this affect something?

There are several problems using bipolars for the differential stage. First, you'll need additional biasing (at least) for M4 position otherwise there would be not enough votage swing to operate M2 FET. More important that the base currents of bipolars would introduce an additional temperature dependant component into the bias of the output devices and will make it less stable. Without source resistors the performance and sound much better but the biasing is critical. There are some other issues as well. And you can not omit D5-D6 either with FETs or bipolars - otherwise o/p stage would destroy itself easily in a fault condition.

Originally posted by lumanauw
Oops, sorry. After reading your article, M3 and M2's Vgs must be matched. Bipolar only have 0V6, while mosfets need about 3-4V.

Vgs for M4 should be at least twice of Vgs for M1, M2 in idle condition. For devices as shown HUF76639 has Vgs about 1.7V and ZVP3310 - about 3.5V. M3 Vgs is not that important however it is useful to get it the same as for M4 - providing for near 0 DC offset from the input to the output and good circuit symmetry.

Cheers

Alex