Bob Cordell Interview: BJT vs. MOSFET

[PMA]

Exactly. And a lot of other newer transitors, quite often used in US amplifiers .

[Wavebourn]

Originally posted by PMA
Exactly. And a lot of other newer transitors, quite often used in US amplifiers .

Even mine?

[Wavebourn]

This beast sounds very delicate driven by a vacuum tube, but evaporizes screwdrivers and binding posts.

[Bob Cordell]

Originally posted by Ultima Thule
Bob,

I guess your thoughts aplies mostly to vertical FET's, whats your take on lateral ones, not just paramters where the main differences like even lower transconductance, lower Cre and negative temp-co are the most noticeable but as well in practice?

Cheers Michael

Michael,

Much of what I said applies as well to the lateral power MOSFETs. There are many designs out there that use the laterals, and some people assert that they sound better. They are easier to use and are a more forgiving device than the vertical MOSFETs, but they don't perform as well as the verticals in my opinion.

The vertical MOSFETs are much faster, and some people have had bad experiences with the verticals when they did not exercise enough care in high frequency design and layout. Indeed, one of the few things that can cause a vertical MOSFET to destruct is a high-frequency parasitic oscillation, often above 100 MHz. The cause of the failure is the buildup of a.c. oscillation voltages internally on the gate in excess of 20 volts peak, which will punch through the oxide and destroy the device. This can happen even if the external gate-source voltage is clamped to be less than 20 volts as a result of the bondwire inductance working in resonance against the gate capacitance.

Although the vertical MOSFETs have significantly better temperature stability than bipolars, they are not as inherently temperature stable (bias-wise) as the laterals. The current at which the Id for fixed Vgs characteristic goes from a positive temperature coefficient (bad) to a negative coefficient (good) is around 100 mA for the laterals, while it is up around 1 amp for the verticals.

Cheers,
Bob

[Bob Cordell]

Originally posted by Wavebourn


What???

Yes these transistors are quite good. Looks like you can get a worst-case beta of 50 at 7 amps per device (I use the typical low-temperature curve as worst-case room temperature). So if you parallel four devices, you can get 28 A into a pesky 2-ohm load, for a voltage of 56V peak. The total base current will then be about 560 mA, still not trivial, but certainly manageable. Still quite a bit more than would be required to drive a MOSFET at that level.

Under more forgiving conditions, you might put 60V peak into a 4-ohm resistive load, for 15A peak and an average power of 450 watts. Beta might be closer to 75. Peak base current would then be only 200 mA.

Bob

[Wavebourn]

Bob, I used 6 of them for 500W on 2 Ohm before, now I use only one + one FET in parallel for the same purposes, and with the same sonic qualities. 50% less power gain is no good, compensating it with feedback we introduce high order harmonics.

-Anatoliy

[john curl]

That is a good idea, Wavebourn. I was tempted to use something like that for a PA power amp. It has the problem of a mismatch in impedance, at both input and output, which could cause some real problems. Still, it can be an interesting approach.
Personally, I use FET's when they will allow the power output that I need to achieve. Most vertical FET's won't handle difficult loads or a short circuit, for more than an instant.

[Wavebourn]

Originally posted by john curl
That is a good idea, Wavebourn. I was tempted to use something like that for a PA power amp. It has the problem of a mismatch in impedance, at both input and output, which could cause some real problems. Still, it can be an interesting approach.
Personally, I use FET's when they will allow the power output that I need to achieve. Most vertical FET's won't handle difficult loads or a short circuit, for more than an instant.

John, if you use real transistors you should not be afraid of impedance mismatches. Real problems happen when impedance mismatch happens when output current is crossing zero, and resulting voltage changes fine bias of output transistors. In my case it is nothing against 4V I have already, so guitar strings and reverberation fade out naturally, pleasing ears and soul. Yes, there are some crossover distortions, but they are not audible since power level when they happen is high enough. Also, input capacitances introduce lower impedance on higher frequencies, but suppose they are 200 pf effectively on background of 30 mA of base current... Nothing at all!

[Wavebourn]

Originally posted by traderbam

One of the most unfortunate drawbacks for audio amps is the need for higher bias voltages and higher bias currents...combined these tend to make FET amps less power efficient.

...now let's compare it to filament and anode power losses of class A tube amps that sound better...

Russian joke: "What do you want really from taxi, painted chess-matt, or comfortable riding?"

[traderbam]

My editing has gone awry.

My other comment is that comparing "ft" is not really valid as it does not take account of charge. Comparing the speed of bipolars and FETs of equal power rating shows bipolars are nearly as fast; same order of magnitude.