Biasing BJT – quick/approximate

Following up the exact analysis earlier, this CE bjt amplifier can be analysed with the approximation quickly. Still starting from this:

BJT amplifier
BJT amplifier

Removing all AC components such as AC signal input and capacitors to have this circuit:

BJT amplifier DC equivalent
BJT amplifier DC equivalent

Now the circuit is ready to be analysed by without Thevenizing using voltage divider rule for the base voltage and then obtain the amplfied current IE.

VB=VCC*R2/(R1+R2)=15V*30k/(130k+30k)=2.8125V approximately without considering the drop due to the base current loading effect. Following the diode BE drop of 0.7V,


Since IB is small or insignificant, IE=VRE/RE=2.1125V/1.2K=1.7604mA

Compare the IE=1.4mA using the exact analysis, there is a slight difference. Can you tolerate this error?

Everything else can be solved after this: VRC=IC*RC, VRE=IE*RE and IE=IC approximately and using KVL to find VCE=VCC-VRC-VRE. VCE found to be 7.07V which is a little different from the exact analysis.