Methods Of Biasing

Base Bias

 

The simplest form of biasing is to use a single resistor.  A single source provides the biasing requirements of both PN junctions.  The base of the transistor is returned to  via and the value of  determines the level of

This form of biasing is not as accurate or stable as the aforementioned potential divider bias. 


Feedback base bias

A better more stable form of biasing arrangement is shown below, is to connect the bias resistor  between the collector and base terminals of the transistor.

Now any increase in  increases the potential difference across , lowers the quiescent collector potential set by the biasing arrangement, decreasing  as before.  However, this time the fall is fed back to the base via , decreasing the forward bias on the base , decreasing  and ultimately decreasing , thus correcting the original rise owing to thermal runway.

This NEGATIVE FEEDBACK also occurs when an ac signal is applied to the base and causes a severe reduction in amplification.  As the example above shows, the value of the biasing resistor is dependent upon the gain of the transistor and this circuit does NOT stabilise the quiescent conditions against changes in transistor parameters (production spread).