Common Circuit Application(Voltage-Controlled Resistor)

V_GS must be between zero and V_GS.off .

Output Impedance of a Transistor for the Emitter Follower (Common Collector)

The output impedance seen by the load (R_E in this example) is defined as:

Z_out = - dV_out / dI_out

The minus sign in the derivative comes from the fact the output impedance has the effect of decreasing V_out. The output current I_E, which is related to the base current.

V_in = I_BR_S + V_BE +V_E [V_out = V_E]

V_out = V_E = V_in – I_BR_S – V_BE [I_out = I_E and I_B = I_E/(β + 1)]

V_out = V_in – {I_E/(β + 1)}R_S – V_BE = V_in – {I_out/( β + 1)}R_S – V_BE

= - I_out{R_S/(β + 1)} + (V_in – V_BE)

So, Z_out = - dV_out/dI_out = -d/dI_out [ - I_out{R_S/(β + 1)} + (V_in – V_BE) ]

Z_out = R_S/(β + 1)

Thus we obtain the result that the impedance of the source, as viewed by the load, is reduced by the factor ~ 1/ β.

Zout = {RS/( β + 1) ≈ RS/β

Types of Amplifiers

The transistor is a three terminal device, thus the input and the output must share one terminal in collector, common emitter, and common base.

Common Nomenclature (npn Example)

BJT Schematic Symbols

(Mnemonics for remembering the directions of the arrows are in parenthesis.)