Onkyo A-SV610PRO, Service Manual

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NPN Transistor working with Negative supply and Negative bias.
[[Rectangular box shows DVM]]
NPN transistor response to a negative supply and negative bias is not different than PNP with Positive bias
and Positive supply. Inherently NPN transistor will fully turn off with negative supply at its collector and
biased with any voltage
≤ 580mv positive. Both the inverted and non-inverted outputs of the transistor will
stop at the lowest 600mv DC level even when the vb is near zero. Even though NPN transistor may not be
configured as such often times it is still possible to find a circuit design employing such technique for the
purpose of switching and sensory (compare) device buffer.
Fig
1 and Fig2 are just to show some of the results that may be found when the value of R3 is changed.
However Q 1 can be made to output ≥ 0vdc by biasing it with a positive voltage.
In such instance the value of R6 becomes a
critical factor on how Q 1 shall function. Q 1 can be said to have turned off from conducting negative
voltage when biased by positive voltage except now it is responding to the base voltage by allowing current
to flow through to the Emitter as well as Collector. One can assume that the transistor has gone in to a
breakdown mode at this point. As long as the current is low this may not be considered critical operating
circuit.
Fig 1
Fig2
Fig3