Question

In a diode, when there is a saturation current, the plate resistance $\left({\mathrm{r}}_{\mathrm{p}}\right)$, is

• A. zero
• B. infinite quantity
• C. data insufficient
• D. some finite quantity

Answer 1

The correct option is B

infinite quantity

Step 1: Formula of Plate resistance:

Plate resistance is defined as the ratio of the potential difference between the plate and cathode of a vacuum tube to the resulting current.

The formula for plate resistance can be given as:

$\mathrm{Plate}\mathrm{resistance}\left({\mathrm{r}}_{\mathrm{p}}\right)=\frac{\mathrm{\delta V}}{\mathrm{\delta I}}$

Where;

$\mathrm{\delta V}$= Change in voltage

$\mathrm{\delta I}=$ Change in current.

Step 2: Condition for a saturation current:

At saturation, the current is known to be constant, and thus the change in current can be taken as zero, which can be given as:

$\mathrm{\delta I}=0$

Step 3: Calculation of the Plate resistance when there is a saturation current:

Putting the value of saturation current in the given equation of plate resistance, we can get:

At saturation,

$$\begin{gathered} \left({\mathrm{r}}_{\mathrm{p}}\right)=\frac{\mathrm{\delta V}}{0} \\ \Rightarrow \left({\mathrm{r}}_{\mathrm{p}}\right)=\mathrm{infinity}. \end{gathered}$$

Hence, in a diode, when there is a saturation current, the plate resistance $\left({\mathrm{r}}_{\mathrm{p}}\right)$, is an infinite quantity.

So, option B is correct.