which FET for these operating conditions ?

[Bigun]

Hi,

I want to build an amplifier with a FET source-follower output, operated single ended and so a familiar sight in this forum. I simply would like some good advice on choice of a suitable FET

I will operate it with low Vds (e.g. 5V) and idle current should be consistent with providing 10W into nominal 8 Ohm loads.

I don't want to parallel multiple devices, it should be a single device. MOSFET (vertical or lateral) and JEFTs, and SiC are all OK solutions.

The output impedance of the gate drive circuit will be relatively high, e.g. 1K so a FET with low gate capacitance is very desirable.

No nfb will be employed (other than it being a source follower).

Thoughts ???

[transistormarkj]

Hmm, 10W (RMS) into 8 ohms requires a waveform at the load whose RMS voltage is 8.94V and whose peak value is plus-or-minus 12.65 volts. I suggest you allow at least 4 volts for the source follower VGS drop, thus the power supplies are +16.65 and -12.65 if the source follower is N-channel. It is very difficult to see how you can meet your power-into-8-ohm-load spec with Vds <= 5 volts, when the source follower has +16.65 on its drain and -12.65 on its source.

[Bigun]

Originally Posted by transistormarkj

It is very difficult to see how you can meet your power-into-8-ohm-load spec with Vds <= 5 volts

I will restrict the voltage across drain-source not by having low power supply rails but through the use of active devices such as coscode. I mention the low Vds because I assume it's an important parameter to know in helping recommend a linear FET.

[transistormarkj]

The IRF-510 (DATASHEET AND PURCHASE INFO FROM DIGI KEY) appears to meet your needs

• VDS = 100V
• IDS = 5.6 A continuous
• Qgate = 8.3 nC
• CISS = 180 pF
• PD(max) = 43 watts

Better yet, Linear Technology ships a SPICE simulation model of it, in their "standard.mos" library with their LTSPICE simulator.

Although I confess I don't understand your Ninja Circuit Trickery

I will restrict the voltage across drain-source not by having low power supply rails but through the use of active devices such as coscode

since I don't see how a "coscode" can help a source follower. Thus I have an opportunity to learn!!

[h_a]

Hi,

I wouldn't want to use a mosfet at 5V VDS in linear applications; the parasitic capacitances become really large and non-linear at such low VDSs. Better try a BJT in such a case. If it really has to be a mosfet, better go for a lateral mosfet like the Renesas.

Hannes

[EUVL]

If you are prepared to use cascode, and only wants 10W into 8R, then there is no better than LU1014 for linearity (see Zen Version 9).
May not be low enough Zout, so you need to play some tricks, which adds some complications.
But not impossible.

If you want low Zout, then you need to use a Semisouth or UHC Mosfet with high Yfs.

Or use 500x 2SK170BL in parallel (with cascode).


Patrick

[buzzforb]

I am going to be attempting a Semisouth RO85 version of B1 with dual rails.

[VladimirK]

Have a look at these fancy russian parts, they are still available.
have quite unique parameters, not typical for modern parts.

[EUVL]

With Vds=20V, 5V vgs gives 1A current. That is a transconductance of 200mS.

To get to a power amp of Zout of say 0.1R, you would need 50 of them in parallel, wouldn't you ?
And that is only for the driver device.
4 of them for a pair of source followers => 200 devices ?


Patrick

[Bigun]

A low output impedance is not critical, the types of speakers I plan to use don't perform at their best with high damping factors. A single FET source follower will very likely meet my needs in this regard. The SEWA amplifier by MadsK is a good example but I think there are even more examples from Nelson...

Vladimirk, you've travelled this road a lot further than I, so I pay attention to your recommendation. But I don't see where these devices are available, unless you are offering to supply a few ???

Transistormarkj, The IRF-510 specs look nice, but the Vgs vs Id curves look, well, really curved ! - not at all linear ?
Cascoding the device reduces power dissipation allowing use of smaller device with lower capacitance.

Hannes - which lateral FET do you like the look of ?