Transistor common-collector circuit

Common-base, common-emitter, and common-collector circuits for a bipolar npn transistor (A, B, C, respectively), and the equivalent grounded-grid, grounded-cathode, and grounded-plate circuits for vacuum-tube triodes (A corresponds to A, B to B, and C to C). Adapted from Terman [5]. 

A totally different way of looking at transistor action is by using a common-emitter circuit (Fig. 9.19B) Now the input current is (a relatively small) iBr and the output is the relatively large) collector current zc this collector current is still controlled by the Ebers-Moll equation, but the current gain is now explicit ... 

Eigure 11 shows a schematic and collector characteristics for a common emitter n—p—n transistor circuit. The load line crossing these characteristics shows the allowed operation of the transistor with a supply voltage, = 12 V a load resistor, 7 = 2 and a bias resistor, 7 g = 20 kQ. The load line corresponds to the equation = 7 7 -H. Plotting the load line on the collector characteristics defines BJT behavior 0.6 V is required... 

Transistors are always operated so that amplification is obtained for a signal whose voltage is small on the input side (relative to the bias on the emitter) and small on the output side (relative to the collector voltage). Therefore small-signal theory applies. The designer obviously wants all signals to be amplified linearly, that is, by a common factor this makes the circuit behave more reasonably. 

The transformer inductance plus the parallel capacitance is called a "tank circuit." It can be attached to any of the three transistor wires, but in this case (position C), it is being used on the collector, which is the most commonly used position. (If the inductor on the base has no capacitor, it is not a tank circuit.)... 

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