13 Jul 2026

Drain and Transfer Characteristics of a JFET

practical pg-ii fet transistor characteristics

Aim

To study the drain and transfer characteristics of a junction field-effect transistor and determine its transconductance.

Apparatus

JFET, regulated DC supplies, milliammeter, voltmeters, bias resistors, and connecting leads.

Figure

Separate gate-bias and drain supplies connected to the JFET under test.

Theory

Current flows through the channel between drain and source. A reverse-biased gate junction controls the channel width, so the drain current is controlled by gate voltage rather than gate current. In the pinch-off region, the transfer characteristic is approximated by Shockley’s equation,

\[I_D=I_{DSS}\left(1-\frac{V_{GS}}{V_P}\right)^2.\]

Observations

$V_{DS}$ (V)	$I_D$ at $V_{GS}=0$ (mA)	$I_D$ at $V_{GS}=-1$ V (mA)
2	4.1	2.8
4	5.0	3.1
6	5.2	3.2
8	5.3	3.2

Graph

Drain current plotted against gate-source voltage.

Result

The drain current becomes nearly constant after pinch-off. From the transfer data, the transconductance near $V_{GS}=-1\,\text{V}$ is approximately $2.8\,\text{mS}$.

Viva Questions

1. Why is gate current small? The gate junction is reverse biased.
2. What is pinch-off? The condition in which the channel narrows enough for drain current to become nearly constant.
3. What is transconductance? $g_m=\Delta I_D/\Delta V_{GS}$ at constant $V_{DS}$.

Maxima Code

Download the Maxima calculation file.