DC Analysis and DC Load Line – Bipolar Junction Transistor – Analog Electronics


Students in case of amplifier it has two inputs AC input as well as DC input so while analyzing if we analyze the circuit only for DC input then it is called as DC analysis if we have only AC input then it is called as AC analysis but how to do that yes we can do that because we know that there is something called as superposition theorem it states that if there are multiple source available in the circuit you can treat one source at a time and when there are AC as well as DC applied what I am going to do is while performing DC analysis we will consider only DC source where AC is absent hence circuit diagram for DC circuit becomes which has VC C which is DC battery RC RB and again DC battery called as VB B as shown in the circuit diagram a DC battery is applied at the base and at the collector the voltage between collector to emitter is called as VC and voltage between base to emitter is called as VB in order to get DC load line we need to apply kvl to the output before that what is exactly DC load line DC load line is a graph which has all possible values of IC and VCE I repeat DC load line is a graph which has all possible values of output current IC and output voltage VC so apply KVL to the output we’ll get we will get equation as VCC minus current flowing from collector that is IC multiplied by RC minus VC is equal to zero so equation becomes VCC minus IC into RC minus VC e is equal to zero hence we can write IC into RC is equal to minus VC plus VC C hence IC is equal to minus 1 by RC into VC plus VCC divided by RC so we have got equation of IC in order to draw a line we required minimum two points how to get these two points that is a question in our mind so what we can do is to get value of IC I can put VCE is equal to 0 and to get value of VCE we can put IC is equal to 0 thus we can have two points to draw a load line case one put IC is equal to 0 when IC is equal to 0 VCE becomes equal to VCC this is point a if we put VC E is equal to 0 then we get IC is equal to VCC by RC that is point B so we have got point A and point B so we can easily draw a DC load line in output characteristics this is very interesting the DC load line is graph of all values of IC and VCE for common emitter configuration IC and VC will be on output characteristics that is why DC load line is drawn at output characteristics you so we will plot DC load line for which a point and B point we have already calculated where as this is called as IC and this is VC if we plot point a and B we will get this as DC load line this is VCE is equal to VCC that is point a and this is VCC divided by RC point a and this is called as DC load line now what we can do is we can get something called as operating point Q operating point Q is defined as it is a point for a particular value of IC and for corresponding value of VC what we can do is we can select Q point at exactly at the middle of the load line when output characteristics intersect DC load line we can get different cue points but for faithful amplification we are going to select Q point at exactly middle of the load line as shown in the diagram this is how we can plot DC load line and we can get operating point at the center of the load line thank you

22 thoughts on “DC Analysis and DC Load Line – Bipolar Junction Transistor – Analog Electronics

  1. Sir please explain the concept of stiff and not stiff voltage divider bais?
    Describe how dc input resistance at the transistor base affects the bias?

  2. it feels like you are memorizing the stuff just before you are shooting a video. very unclear due to your English. it should have been better if you explained in Hindi

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