When gate to source voltage of a FET changes from –5.2 volt to –5 volts, the corresponding drain current changes from 1 mA to 1.3 mA, the transconductance is
1500 μ mho
What transconductance means. For a FET, gm measures how effectively the gate voltage controls the drain current — it is the slope of the transfer characteristic at the operating point:
\(g_m=\left.\dfrac{\Delta I_D}{\Delta V_{GS}}\right|_{V_{DS}\ \text{constant}}\)
Step 1 — find the change in gate voltage.
\(\Delta V_{GS}=(-5)-(-5.2)=0.2\ \text{V}\)
Note that the gate becomes less negative, which is the direction that opens the channel further.
Step 2 — find the change in drain current.
\(\Delta I_D=1.3-1.0=0.3\ \text{mA}=0.3\times10^{-3}\ \text{A}\)
Step 3 — divide.
\(g_m=\dfrac{0.3\times10^{-3}}{0.2}=1.5\times10^{-3}\ \text{S}\)
\(g_m=1.5\ \text{mA/V}=1500\ \mu\text{mho}\)
Units and sanity check. Transconductance has the units of conductance — siemens, historically written as "mho" (ohm spelt backwards). A typical small-signal JFET has gm of 1–5 m℧, i.e. 1000–5000 μ℧, so 1500 μ℧ is exactly the expected order. The distractors 29 μ℧ and 510 μ℧ are far too small for a device whose current moved 0.3 mA for a 0.2 V change, and 1620 μ℧ would need ΔID = 0.324 mA.
Why gm matters. It sets the voltage gain of a FET amplifier, \(A_v=-g_mR_D\), so this single number links the device to the circuit. It is not a constant: differentiating \(I_D=I_{DSS}\left(1-\dfrac{V_{GS}}{V_P}\right)^{2}\) gives
\(g_m=g_{m0}\left(1-\dfrac{V_{GS}}{V_P}\right), \qquad g_{m0}=\dfrac{2I_{DSS}}{|V_P|}\)
so gm falls as the gate is driven more negative — which is why the value quoted always belongs to a stated bias point.
Hence, the transconductance is 1500 μ mho.
Similar Questions
In an FET, following characteristics are given :
1. JFET is called as variable current resistor.
2. JFET is called Voltage variable resistor.
3. \(g_{d}=g_{d0}\left(1-\dfrac{V_{GS}}{V_{P}}\right)\), where gd0 is value of drain to source conductance for zero bias.
4. \(g_{d}=g_{d0}\left(1+\dfrac{V_{GS}}{V_{P}}\right)^{2}\)Which one of the following is true ?
For MOSFET, consider the following statements
A. In triode region, FET works as voltage variable resistor
B. For p-channel MOSFET, if VSG \(\gt\) |VTp|, then transistor will be in off mode
C. Amplification factor is inversly proportional to the value of VGS/Vp.
D. In saturation region, FET is used as amplifier.
E. For depletion MOSFET, channel is diffused and current flows even if VGS is zero
Choose the correct answer from the options given below :
Transfer characteristics for n-channel FET as a function of temperature T is shown in the figure
Choose the correct answer from the options given below :
In a JFET the gate to source voltage is given as
In a JFET self-bias configuration shown below :
The drain current is
(a) \(K\dfrac{W}{L}(V_{gs}-V_{th})V_{ds}\)
(b) \(I_{DSS}\left(1-\dfrac{I_DR_S}{V_P}\right)^{2}\)
(c) \(I_{DSS}\left(1+\dfrac{I_DR_S}{V_P}\right)^{2}\)
(d) \(K\dfrac{W}{L}(V_{gs}-V_{th})^{2}\)
Out of these which are correct ?
Assertion (A) : FET is a device, which depends for its operation on the control of field or voltage due to applied current.
Reason (R) : FET has a very high input resistance of the order of mega-ohms. It is also immune to radiations.
Match the following :
List - I List - II (circuit in the original question) (a) JFET VGS = 0V (i) 
(b) JFET self-bias (ii) 
(c) JFET Fixed bias (iii) 
(d) FET Low Frequencies circuit (iv) 
Codes :
Which of the following statements are valid for JFET ?
(a) \(I_{DS}=I_{DSS}\left(1-\dfrac{V_{GS}}{V_P}\right)\)
(b) \(I_{DS}=I_{DSS}\left(1+\dfrac{V_{GS}}{V_P}\right)\)
(c) \(\mu=\dfrac{g_m}{r_d}\)
(d) \(\mu=g_m\cdot r_d\)
Options :
The JFET in a circuit shown in Figure, has an IDSS = 10 mA and Vp = − 5V. The value of resistance RS for a drain current IDS = 6.4 mA is (Choose the nearest value)
In a certain FET circuit, VGS = 0V, VDD = 15V, IDSS = 15 mA, and RD = 470Ω. If RD is decreased to 330 Ω, IDSS is :
Important Questions from Field Effect Transistors
Which of the following is the characteristic of Field-effect transistor?
- In junction field effect transistor, the drain current can be approximated as:
The expression for the transconductance (g m) of a JFET is:
Field Effect transistor is:
A FET has
