How to design Self Biased BJT amplifier

Self bias is one of the method used for bias a BJT(Bipolar Junction Transistor). Self bias is also known as collector feedback bias. In this method of biasing the fraction of collector voltage is fed back into the base to neutralize and collector changes. This is done in order to stabilize the Q-point of the amplifier. Q-point means Quiescent point which means quiet(still and resting). 


For construction and real time testing see Testing of self biased BJT amplifier on breadboard with PC oscilloscope.

The self bias or collector feedback bias circuit diagram for signal amplification purpose.

self biased BJT amplifier circuit diagram
 In the above circuit, V1 is ac signal source which has magnitude of 10mV and frequency of 1KHz which is to be amplified. VCC is the DC voltage of 5V. The resistor Rb is the feedback resistor. The resistor Rc is the collector resistor. The following shows the DC biased circuit.

self biased DC circuit for BJT


The capacitor C1 and C2 are coupling capacitors. The C1 capacitor is used to couple the input ac signal into the dc biased circuit. The C2 capacitor is used to couple the amplified signal at the collector to the load resistor RL which is 10KOhm.

To calculate the resistor Rb, Rc values and the coupling capacitor C1 and C2 values we can use the online BJT amplifier design calculator. The following picture shows the values obtained.


The following picture shows the circuit diagram with calculated component values.
Self Biased BJT amplifier circuit diagram

The following graph shows the input signal vin and output amplified signal vout simulated in multisim electronics circuit simulation software.

The following are some other circuit calculation example for other types of biasing method.

- How to bias a BJT using voltage divider biasing

- How to Design Collector-Emitter Feedback biased BJT Amplifier

- How to bias a BJT using base bias

- How to design emitter biased BJT amplifier

- How to plot BJT characteristic curves in Multisim 

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