Among semiconductors, the muting transistor and its audio application is perhaps the most obscure and poorly understood of all. There is extremely little information on this subject available on the internet and what is available is sketchy. This discussion is an attempt to tie the pieces together and to add some insight –I have been working on this off and on for some time and for me it has been a learning experience. While I do not claim expertise in this subject, I question if such experts actually exist.
Applications for muting circuits
Applications for muting circuits abound. Included are synthesizers, audio amplifiers, pre-amplifiers, audio mixers and audio line & level equipment (both consumer and professional). In amplifiers and pre-amplifiers, the muting circuit is intended to prevent the popping of loudspeakers when power is applied. In mixers and audio line & level equipment, the muting circuit may also be employed to select /deselect various input channels –such must be done cleanly or quietly.
Types of muting circuits
Obviously, the function of muting circuits is to mute (kill) the audio signal. This can be done in a number of ways including switches, relay contacts, junction FETs, analog switches and bipolar muting transistors. Switch and relay contacts can be noisy and intermittent due to contact bounce. Furthermore, properly applied contacts must be dry circuit rated due to both the low current and potential across the contacts –this forces the use of bifurcated dry circuit contacts that are special and expensive. While a junction FET is sometimes applied as a variable attenuator, its use as a muting device is a misapplication due to the lack of symmetry because it cannot support more than about 600mV due to its intrinsic body diode. Analog switches can do the job if applied correctly, but lack the audio range required in professional line and level applications, may be subject to crosstalk between sections and are also very sensitive to ESD. On the other hand, the bipolar muting transistor does everything well and inexpensively.
What is unique about the bipolar muting transistor?
The standard NPN transistors is symmetrical in that it consists of 3 layers, 2 layers of “N” material (collector & emitter) separated by a layer of “P” material (base). However, that is as far as the symmetry goes as the layer thicknesses and doping varies greatly. The standard transistor still functions with the collector and emitter terminals reversed, but the hFE (current gain) in this mode is reduced by perhaps two orders of magnitude and the reversed voltage rating (Vebo) generally limited to about 5V.
In comparison, the muting transistor is a good deal more symmetrical –it has high reverse hFE and some types offer a 15 to 40V base to emitter reverse voltage rating (Vbeo). In effect, it is rated for AC operation and this suits it well for audio voltage signals that are AC in nature. Whichever terminal (emitter or collector) is the most negative becomes the effective emitter terminal. If sufficient base drive is applied, it will saturate either polarity, and this is exactly what is required to implement the mute function.
Two categories of muting transistors
The two categories are graded by the base to emitter reverse voltage rating (Vbeo).
The 1st category Vbeo is limited to 5V and is suited for consumer electronics that has an average program signal level of -7.8dbu (0.315Vrms or 0.445Vpeak). A 5V rated device provides 21db headroom to handle program material transients.
The 2nd category Vbeo starts at 15V and is suited for professional studio line and level devices that have an average program signal level of +4dbu (1.228Vrms or 1.736Vpeak). A 15V rated device provides 18.7db headroom to handle program material transients.
