MOSFET vs BJT amplifier output stages
MOSFET vs BJT amplifier output stages
While many debate the merits of Tube vs Solid State designs, Within the solid-state camp there is another important distinction: MOSFET vs BJT amplifier output stages. Both are transistors and can produce exceptional results, but they behave differently — both electrically and sonically. And as is always the case with audio, it is not a matter of which is best, but about which you prefer and why.
MOSFETs are voltage-controlled devices
At the electrical level, there are some key differences between MOSFET and BJT transistors. They have high input impedances and lower transconductance than BJTs. Like tubes, they tend to exhibit softer transfer characteristics as they approach clipping. They are thermally stable, especially Lateral MOSFETs, and these characteristics tend to produce a more relaxed and fluid sound.
BJT transistors are current-controlled devices
They have higher transconductance than MOSFETs and stronger current gain. Circuits built around BJTs typically achieve lower output impedance and greater current capability. These characteristics can translate into sound that feels tightly controlled with strong dynamic authority.
These are the electronic characteristics of MOSFETs and BJTs and their sonic tendencies, but circuit design matters more than device type.
Subjective listening results in MOSFET designs being described as having a slightly warmer presentation, with midrange dimensionality and smooth highs. Because of this, many who like tube amplification will find MOSFET sound more familiar.
BJTs are often chosen for their ability to deliver high current with authority
In the same way, BJT designs are often described as having a tight bass grip, strong transient attack with a perception of greater resolution and immediate presentation. In high power designs.
MOSFET designs offer an ideal balance of control and musicality
When a design calls for moderate power and speakers of average efficiency, MOSFET designs offer an ideal balance of control and musicality. In larger rooms with low impedance loads, or with systems requiring extreme dynamic headroom, BJT based designs often provide the necessary current reserves.
Because these devices behave differently, we choose the output technology that best suits the intended application.
We use Exicon lateral MOSFETs in the KWA 99 monoblocks because of the tonal balance, musical density and control that serves most speakers well. They are designed for listeners who value musical flow, rich tone and the resolution, control and reliability of solid-state designs.
The KWA 300 amplifier is designed for demanding speakers and uncompromising systems
The KWA 300 Reference Stereo amp design is based on high-current BJT transistors with a massive high current power supply, to provide the current reserves needed for authority and control.
As in all things audio related, design is key. A poorly designed MOSFET amp can sound weak and lifeless. A poorly designed BJT amp can sound sterile and aggressive. Implementation is key, and knowing which technology and designs suits each system.
Similarly to the tube vs solid-state debate, MOSFET and BJT designs both offer exceptional results in the right application, the right system and good design.
Liquidity or authority, texture or grip – different devices with one goal: Musical Truth.
Elegance.Simplicity.Truth
More Truth
Readers interested in learning more about amplifier design can also explore our other articles:
Tube – Solid-State Hybrid System Design: The ModWright Way
ModBlog: Tube vs Solid-Stage Power