Electrical Engineering ⇒ Topic : Triode as an Amplifier

TRIODE AS AN AMPLIFIER

In a triode valve, the required change of plate current needs a small change in grid potential whereas it needs a large change in plate potential. The action of a triode valve as an amplifier is based on this principle

The basic circuit

The circuit for voltage amplifier using a vacuum triode is shown in Figure (a). The signal voltage, which is to be amplified is applied between the cathode and grid in series with e_g.

                                Figure (a) Triode valve as an amplifier.

The straight part ABC of dynamic characteristics is taken where load resistance is R_L and plate supply voltage is E_p. The point B is taken in the linear part of the characteristics. At this point, grid is at negative potential OH. So no signal is applied to grid. The steady current I_p flows corresponding to point B. This current is known as quiescent current. Now, if AC signal is applied to the grid, the quiescent point will vary and the total plate current is given by I_p + i_p where i_p is the AC component.

From Figure (b), the positive half of the AC signal wave increases the grid voltage from OH to OG. The plate current changes from OE to OF. During negative half of AC signal, the grid voltage decreases from OH to OK and the plate current decreases from OE to OD.

In Figure (b), Q represents the potential variations applied to the grid and P represents the current variations in the plate circuit. Let r_p is plate resistance (internal) of the valve and μ is amplification factor of the valve.

                                                              Figure (b)

Therefore,

Potential drop across r_p = i_pr_p and Potential across R_L = i_pR_L

In a triode valve, voltage e_g across the grid circuit appears as μe_g in the plate circuit. So it can be written as:

                                                    -μe_g= i_pr_p + i_pR_{L                             ......................(1)}

                                                           .................. (2)

_Hence,

                                                                           ....... (3)

                                                ............. (4)

To get higher voltage amplification, R_L should be large. The maximum value of voltage gain is μ for R_L = ∞. The significance of negative sign is that the output and input voltages are out of phase, i.e., by 180⁰.