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 eg.
Figure (a) Triode valve as an amplifier.
The straight part ABC of dynamic characteristics is taken where load resistance is RL and plate supply voltage is Ep. 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 Ip 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 Ip + ip where ip 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 rp is plate resistance (internal) of the valve and μ is amplification factor of the valve.
Potential drop across rp = iprp and Potential across RL = ipRL
In a triode valve, voltage eg across the grid circuit appears as μeg in the plate circuit. So it can be written as:
-μeg= iprp + ipRL ......................(1)
To get higher voltage amplification, RL should be large. The maximum value of voltage gain is μ for RL = ∞. The significance of negative sign is that the output and input voltages are out of phase, i.e., by 1800.
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