Name: Vacuum Tubes and Semiconductors
Uploaded: 2019-07-19
Duration: 8 min 14 s
Description: Vacuum tubes were the first devices where the electronic discharge was realised. Further work on them gave birth to semiconductor electronics. Let's discuss...

Question

With a circuit diagram, explain how the static characteristic curves of a triode are drawn?

Show how the tube constants are estimated with the help of these curves.

Accepted Answer

Circuit diagram of the triode-

The above circuit diagram is used to study the characteristics of the triode. $P$ represents the plate, $K$ represents the cathode and $G$ represents the control grid. The plate is connected to battery $E_{p p}$ from where it is given positive potential whereas the cathode is heated with the help of fialment. The grid is connected to the negative potential. Here, $I_{p}$ denotes the plate current, $E_{p}$ denotes the plate voltage and $E_{g}$ represents the grid voltage.

Plate characteristics-

The graph between plate voltage and plate current at fixed values of grid voltage is called plate characteristics of a triode.

First, the grid voltage $E_{g}$ is set to $0 V$ . Now, the plate voltage $E_{p}$ is increased and the corresponding values of $E_{p}$ and $I_{p}$ are noted. Now, the corresponding values of $E_{p}$ and $I_{p}$ are noted for different values of grid voltage $E_{g}$ . Plotting the respective values will obtain a plate characteristic graph as shown.

Mutual characteristics-

The graph between the grid voltage and plate current at fixed values of plate voltage is called mutual characteristics of a triode.

First, the plate voltage is set to a value of $50 V$ . Now, the grid voltage $E_{g}$ is increased till the current drops to zero and the corresponding values of $E_{g}$ and $I_{p}$ are noted. This process is repeated for different values of $E_{p}$ and the corresponding values of $E_{g}$ and $I_{p}$ are noted. Plotting the respective values will obtain a mutual characteristic graph as shown.

Determination of tube constants with the help of plate characteristics-

The above graph shows the plate characteristics of a triode.

Plate resistance is defined as the rate of change of plate voltage with respect to plate current.

$r_{p} = \frac{∆ E_{p}}{∆ I_{p}} - - - (i)$

From the graph,

$∆ E_{p} = O F - O G ∆ E_{p} = G F$

Similarly, $∆ I_{p} = E D$

Putting these values in $(i)$ , we get,

$r_{p} = \frac{G F}{E D}$

Transconductance or mutual conductance is defined as the rate of change of plate current with respect to the grid voltage.

$g_{m} = \frac{∆ I_{p}}{∆ E_{g}}$

From the graph,

$∆ I_{p} = A C, ∆ E_{g} = E_{g 2} - E_{g 1}$

$∴ g_{m} = \frac{A C}{E_{g 2} - E_{g 1}}$

Also, we know that the amplification factor, $μ = r_{p} \times g_{m}$

Hence, by obtaining values of $G F, E D, A C, E_{g 2}$ and $E_{g 1}$ , we can estimate the values of $r_{p}$ , $g_{m}$ and $μ$ .

Determination of tube constants with the help of mutual characteristics-

The above graph shows mutual characteristics of a triode.

We know that, $r_{p} = \frac{∆ E_{p}}{∆ I_{p}}$

From the graph,

$∆ E_{p} = E_{p 2} - E_{p 1}, ∆ I_{p} = D E ∴ r_{p} = \frac{E_{p 2} - E_{p 1}}{D E}$

We know that, $g_{m} = \frac{∆ I_{p}}{∆ E_{g}}$

From the graph,

$∆ I_{p} = D E, ∆ E_{g} = F G$

$∴ g_{m} = \frac{D E}{F G}$

We know that, $μ = r_{p} \times g_{m}$ .

Therefore, we can estimate the values of $r_{p}, g_{m}$ and $μ$ by obtaining the values of $E_{p 2}, E_{p 1}, D E$ and $F G$ from the graph.

With a circuit diagram, explain how the static characteristic curves of a triode are drawn?

Show how the tube constants are estimated with the help of these curves.

Important Questions on Electron Emission

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