Discussion in 'General Electronics Chat' started by bhuvanesh, Aug 29, 2013.
what is the use of fixed bias circuit
To keep the bias voltage stable.
IIRC - Fixed bias dates back to the days of valves (tubes) a battery or stabilised supply from the mains transformer provided a fixed negative voltage that the grid leak resistors were tied to.
Later on, designers realised that the cathode current could develop a small voltage across a resistor - as the cathode is allowed to rise above chassis, the grid leak resistor pulls the grid negative wrt the cathode, so fixed bias was no longer required.
Transistors are a little different as the base doesn't have to be pulled negative to set the operating point - it needs a small forward current to set the collector operating point.
Bearing in mind fixed bias was applied to germanium transistors with their huge temperature coefficient - it was soon found to be a *REALLY* bad idea!
Even with modern silicon transistors, fixed bias in the form of a single resistor to base from Vcc or even a potential divider, thermal runaway would still be a problem - and gain spread means each resistor would be select on test for the production line.
The first incarnation of automatic bias was to feed the base resistor from the collector - as collector current rises, its voltage falls and reduces the base current. That's basically negative feedback, but its AC nfb as well as DC nfb - which clobbers the stage gain a bit.
The more usual automatic bias these days is to feed the base from a voltage divider and include an emitter resistor - as emitter current increases; so does the voltage across the emitter resistor - which cancels out some of the voltage applied to the base.