ModBlog – MosFet vs BJT Transistors

Solid state amplifiers, all use BJT or Mosfet power transistors at their outputs, including Class D designs. The purpose of the transistor in this case is to buffer current into low impedance speaker loads. So, which transistor should we use and how do they differ in sound? The two basic choices are Mosfets (Metal-Oxide Semiconductor Field-Effect Transistors) and BJT (Bipolar Junction Transistors). And to further split the hair, there are also ‘vertical’ and ‘lateral’ Mosfets. Each have their strengths and sonic traits that make them preferrable in different applications. Much like tube and solid stage, Mosfet and BJT transistors have their own sound. At ModWright we use them both to their unique strengths and I will explain why.

First of all, Mosfets and BJT transistors operate differently, with regards to current and voltage. Much like tubes, Mosfets require voltage at their input or ‘Base’ to turn on, while BJT’s require current at their input or ‘Collector’. Also like tubes, Mosfets have a more gradual turn on and clipping behavior and lower transconductance (gain) than BJT’s. BJT’s have very high transconductance and provide exceptional current delivery into difficult speaker loads. Mosfets offer excellent thermal stability and rugged reliability, while BJT’s can be prone to thermal runaway if not managed properly.

OK, so Mosfets sound like tubes and BJT transistors sound like Solid State? Well, not exactly, but the comparison is relevant in terms of how they operate. At ModWright, all of our solid state amps have excellent control, stability, tonal balance and organic presentation that makes customers question if our amps are tube or Solid State. A customer once told me that our KWA 99 Mosfet Monoblock amps were the ‘Holy Grail’ of amplifiers because they offered resolution without glare and warmth without being soft or slow. He found them to be ideal driving his Magico speakers.

We have designed power amps with both Mosfet and BJT transistors. Our first amp, the KWA 150SE was BJT based and our second amp, the KWA 100SE, was Mosfet based. Both amps exemplified the strengths of the two transistor types. Many people told me that our KWA 100SE sounded like a 100W SET (Single Ended Triode) tube amp, providing the 3-dimensionality and midrange beauty that only very low power SET amps could. The KWA 150SE provided 150W into 8 ohms and when bridged in mono, could provide up to 600W into 4 ohms. People loved the KWA 150SE for its speed, resolution and special midrange and balanced presentation.

One important area of consideration when choosing BJT or Mosfet transistors, is heat and thermal reliability. Because Mosfets are very stable at high operating temperature, it is possible to bias Mosfet amps into Pure Class A or very-high Class AB operation. BJT amps can go into thermal runaway if not properly designed. As a result, you will find BJT designs to be Class A/B, with great attention paid to thermal stability and reliability. Because of this, you will typically find higher power amplifier designs are BJT based.

To further split the hair a bit, there are two types of Mosfet power transistors: Vertical and Lateral. Lateral Mosfets were originally designed and optimized for audio (see Hitachi, early Threshold and Pass designs), in terms of linearity and stable biasing. Vertical Mosfets were later designed for switch-mode power supplies, rather than audio.

To my ears, Lateral Mosfets are much more musical, organic and natural sounding. Vertical Mosfets used in the same design sound strained and lack liquidity. This may be why many people today say that BJT amplifier designs sound better than Mosfet. I argue that both can sound great if designed properly and why we have chosen both BJT and Mosfet in different amplifier designs.

Another important difference between Lateral and Vertical Mosfets has to do with their thermal stability and reliability. Lateral Mosfets have a negative temperature coefficient near the bias regions. This means that within their nominal bias region, the lateral Mosfet will ‘self-regulate’ or protect itself at high operating temperatures. Vertical Mosfets on the other hand, like BJT’s, can tend towards thermal runaway if not carefully controlled.

The KWA 99 Mosfet Monoblock power amps are 100W into 8 ohms and 200W into 4 ohms. They are a very high bias Class AB design, and operate at temperatures between Class A and AB. They run hotter than BJT designs and will not heat your room. This allows us to offer a 100W/200W amp that sounds like Pure Class A in the range of power used most of the time. They weigh 30lb amps, and are attractively priced.

The new KWA 300 Reference Power amp uses BJT transistors. It too is a high bias Class AB design, but does not run as hot as the KWA 99’s. The KWA 300 is a BIG amp. To produce 300W into 8 ohms and 550W into 4 ohms, it needs A LOT of heatsink area and a BIG power transistor. I chose BJT transistors for this amp because I expect it will be used with very high-end loudspeakers, many of which will provide punishing loads. I want it to handle all such loads with ease and provide fast, incisive sound with impact and control. The KWA 300 weighs 100lb

Importantly however, all of our amps are based on the solid-state Music Stage input circuit, which is comprised of both BJT and Mosfet transistors. This design uses zero global feedback and gives all of our amps a natural and organic presentation, possessing the strengths of the best tube and solid-state designs.

Much like our approach to the use of tube and solid-state in our component and system designs, both Mosfet and BJT transistors are used to their strengths. All of our amps will provide power with control, detail, resolution and clarity without glare or edge. The goal is always to provide a design that allows the music to stand on its own.