A JFET has high input impedance because-
input in reverse biased
Understanding JFET Input Impedance
A Junction Field-Effect Transistor (JFET) is a type of field-effect transistor. One of the key characteristics of a JFET is its high input impedance. Input impedance refers to the opposition to the flow of current into the input terminals of a device. A high input impedance means that very little current is drawn from the signal source connected to the input.
Why JFETs Have High Input Impedance
The primary reason for the high input impedance of a JFET lies in how its input terminal, the gate, is biased relative to the source terminal during normal operation. In a JFET, the control voltage is applied between the gate and the source terminals. For the JFET to operate correctly in its active or saturation region (where it's typically used as an amplifier), the gate-source junction must be reverse biased.
Let's consider the P-channel and N-channel JFETs:
- For an N-channel JFET, the gate (P-type material) is made negative with respect to the source (N-type channel). This applies a reverse bias across the PN junction formed between the gate and the channel.
- For a P-channel JFET, the gate (N-type material) is made positive with respect to the source (P-type channel). This also applies a reverse bias across the PN junction.
When a PN junction is reverse biased, only a very small leakage current flows. This leakage current is typically in the picoampere (pA) to nanoampere (nA) range. Since the input current to the JFET (the gate current, $I_G$) is the current flowing through this reverse-biased gate-source junction, $I_G$ is extremely small.
Input impedance ($Z_{in}$) is calculated as the ratio of the input voltage ($V_{in}$) to the input current ($I_{in}$). In the case of a JFET, the input voltage is the gate-source voltage ($V_{GS}$), and the input current is the gate current ($I_G$).
Mathematically, input impedance is:
$$Z_{in} = \frac{V_{GS}}{I_G}$$
Since the gate current ($I_G$) under reverse-biased conditions is very small, dividing the gate-source voltage ($V_{GS}$) by this tiny current results in a very large value for the input impedance. This is why JFETs are known for their high input impedance, often in the range of hundreds of megaohms ($\text{M}\Omega$) or even gigaohms ($\text{G}\Omega$).
Analyzing the Options
Let's look at the given options:
| Option | Explanation |
|---|---|
| 1. it is made of semiconductor material | While JFETs are made of semiconductor material, this alone doesn't explain high input impedance. Other semiconductor devices like Bipolar Junction Transistors (BJTs) can have relatively low input impedance because their input junction (base-emitter) is typically forward-biased. |
| 2. input in reverse biased | This is the correct explanation. The gate-source junction, which is the input terminal, is reverse-biased during normal operation, leading to a very small gate current and thus very high input impedance. |
| 3. of impurity atoms | Impurity atoms (dopants) are necessary to create the P-type and N-type semiconductor regions that form the JFET structure, including the PN junction. However, the presence of impurity atoms itself doesn't directly cause high input impedance; it's how the resulting junction is biased. |
| 4. none of these | This option is incorrect because option 2 correctly explains the reason for the high input impedance. |
Based on the analysis, the high input impedance of a JFET is a direct consequence of its input junction (gate-source) being operated under reverse bias conditions, resulting in a minimal input (gate) current.
Important Questions from Types of FET
The transconductance g mof a JFET is equal to:
\(\frac{ I _{ DSS }}{ V _{ P }}\left(1-\frac{ V _{ GS }}{ V _{ P }}\right)\)
The CMOS inverter can be used as an amplifier when:
In JFET, the Pinch‐off Voltage can be defined as:
In JFET, the current density in the x-direction is:
A. σ(x)E x
B. qN DμE x
C. \(\rm \frac{q}{2 \in_s}N_D\mu\)
D. \(\rm \frac{N_D\mu}{2 \in_s}\)
Choose the correct answer from the options given below:
Match List I with List II
List – I
List – II
JFET - Bias
Characteristic Equation
A.
Self - bias
I.
\(\rm I_D = \frac{V_{SS}-V_{GS}}{R_S}\)
B.
Voltage – divider bias
II.
\(\rm I_D = \frac{V_{EE}-V_{BE}}{R_E}\)
C.
Source bias
III.
V GS = -I DR S
D.
Current – source bias
IV.
\(\rm I_D = \frac{V_{G}-V_{GS}}{R_S}\)
Choose the correct answer from the options given below:
