# Discussion reply bias voltage | Electronic Engineering homework help

• Why is emitter bias more stable than base bias?

An emitter bias is bias voltage applied to the emitter electrode of a transistor. Aside of having an emitter bias being independent of the BDC, an emitter bias has a gain of 1 or less than 1 means that it can never oscillate because oscillation requires positive feedback and a gain of greater than 1 to sustain oscillation.

• Explain why the base bias Q-point changes with temperature.

Base biasing circuit fixes the value of base current and as a result both IC and VCE(Q-point) are directly proportional to voltage gain. A biasing method should be designed to make the Q-point (IC and VCE) independent of transistor and insensitive to temperature problems. Using negative feedback alleviates this. Example would be let say you have a BJT (base bias) that has parameters that IC increases by 9% per degree Celsius for a fixed VBE. That circuit works perfectly at 25 degrees Celsius but then at a temperature of 35 degrees Celsius IC will be roughly doubled which could cause the BJT to being saturation. How does emitter-feedback bias improve on base bias? (Utilizing Thevenin equivalency of a voltage-divider biased transistor circuit) Using an emitter feedback bias utilizes RE (emitter resistor) as tricky feedback. Suppose IC becomes larger than the designed value due to temperature then VE = (RE)(IE) will increase. Since VBB and RB do not change, (KVL) the BE loop shows that IB should decrease which will reduce IC back to its design value. If IC becomes smaller than its design value opposite happens, IB has to increase and will increase and stabilize IC. This feedback analysis shows that the Q-point is independent of BJT parameters: IE _ IC= _

• How does emitter-feedback bias improve on base bias?

A configuration in which a bipolar junction transistor or a BJT is  reinforced with an emitter resistor for enhancing its stability with  regards to changing ambient temperatures, is called an emitter  stabilized bias circuit for BJT.

References: Why Is Emitter Bias More Stable Than Base Bias | PDF (scribd.com)