Note how beta is in there again !īy adding Rb2 and making it much lower value than beta/gm Rb2 "takes over" and allows us to have more control and also making the influence of beta smaller. Without Rb2 there will only be the internal small signal input resistance of the NPN, it has value beta/gm. base-biased emitter feedback voltage-divider emitter feedback collector-feedback diode feedback active-feedback bias All five are found in Class A and AB operation, while Class B and C amplifiers can implement other methods. That would not actually happen of course, it would settle somewhere in the middle.īut notice how by "wasting" current through Rb1, Rb2 I can basically ignore what happens to the base current and therefore beta as well.įor small signals adding Rb2 also has an advantage as Rb2 with Rb1 forms a voltage divider controlling how much of the output signal is fed-back. Biasing circuit: The biasing circuit consists of action of 3 resistances, R 1, R 2 and R E. When the batteryis attached to the base-emitter junction of the npn transistor as indicated below, current will flow as thebase-emitter junction is in the forward direction. DEVICE FOR REMEMBERING: The arrow on the npn transistor is Not Pointed iN. The negative feedback creates an offsetting effect that tends to keep the Q-point. Bipolar transistors have three terminals: base, emitter, and collector. ![]() Now if beta halves there would be zero current left for Rb2. The collector voltage provides the bias for the base-emitter junction. Thatś not much of a difference now is it ?Ĭompare that to the situation where Ib = 1 uA but Rb1 provides only 2 uA so for Rb2 thereś only 1 uA left. If now for some reason beta is halved, the base current would become 2 uA. Now if the base current is 1 uA and we make 100 uA flow through Rb1 that leaves 99 uA for Rb2. Adding Rb2 would "steal" some current from the base of the NPN, so to prevent that we decrease the value of Rb1 such that it provides extra current. Why does adding Rb2 increase stability with respect to variations in Beta One of the advantages of this type of self biasing configuration is that the resistors provide both automatic biasing and Rf feedback at the same time." Obviously it must also be greater than the base current required for the minimum value of Beta, β. The current flowing through RB1 is generally set at a value equal to about 10% of collector current, IC. "Adding an additional resistor to the base bias network of the previous configuration improves stability even more with respect to variations in Beta, ( β ) by increasing the current flowing through the base biasing resistors. We able to build and test the common emitter amplifier circuit. ![]() ![]() Why does adding Rb2 increase stability with respect to variations in Beta, ( β ) by increasing the current flowing through the base biasing resistors? The unclear (to me) original text is below. CONCLUSION From the experiment 3, Common Emitter Amplifier(Fixed Bias Circuit). The circuit with the text is shown below (in quotes). Resistor Rb2 was added to what was previously Configuration #3. We take as an example the Common Emitter Amplifier (CEA) as the configuration. I was reviewing/following a tutorial describing the pros/cons of various transistor base biasing configurations. This article presents how to achieve a proper biasing of bipolar transistors.
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