Field-Effect Transistors
FET amplifiers are preferred for:
High input resistance
Low noise
Low power consumption
Widely used in:
Communication receivers
Switching circuits
Digital and analog systems
Voltage-controlled
High input impedance
Lower gain
Low noise
Current-controlled
Moderate input impedance
Higher gain
Better linearity
Common-Source (CS): High voltage gain, inverted output
Common-Drain (CD): Unity gain, no inversion (buffer)
Common-Gate (CG): Gain with no inversion, low input impedance
Class A: Linear operation, low distortion
Class B and AB: Improved efficiency
Class C: High efficiency, used in RF
Class D: Switching amplifier, very efficient
Note: FETs dominate in Class D due to superior switching performance
FETs are ideal for:
Analog/digital switches
CMOS logic circuits
Multiplexers, switched-capacitor circuits
Enhancement-mode MOSFETs are widely used in CMOS
Voltage-controlled: Gate voltage controls drain current
Transconductance \(g_m = \frac{\Delta I_D}{\Delta V_{GS}}\)
AC Modeling: Simpler than BJT models
Impedance:
Input: Very high
Output: Comparable to BJT
Low-noise amplifiers
RF and high-frequency designs
Buffering and signal interfacing
Power switching and control
FET AC Model
\(r_{gs}^{\prime}\) and \(r_{ds}^{\prime}\) are very high so neglected in the simplified circuit.
A common-source JFET amplifier is characterized by:
AC input at the gate.
AC output from the drain.
The source terminal is common to both.
Source is at AC ground:
No source resistor, or a bypassed one.
Ensures the amplifier gain is maximized.
Circuit Overview :
Input signal coupled via capacitor to the gate.
\(R_G\) ensures:
Gate is held near 0 V (since \(I_{GSS} \approx 0\)).
Very high input resistance \(\rightarrow\) avoids loading the signal source.
Bias voltage appears across \(R_S\), and bypass capacitor \(C_2\) ensures:
Source is at AC ground.
Gate-source voltage swings about its Q-point \(V_{GSQ}\).
As \(V_{GS}\) becomes less negative, \(I_D\) increases.
As \(I_D\) increases, \(V_D\) drops (since \(V_D = V_{DD} - I_D R_D\)).
Result:
\(V_{DS}\) is 180° out of phase with \(V_{GS}\).
This is characteristic of a common-source amplifier.
Transfer Characteristic Curve :
Sinusoidal \(V_{GS}\) causes sinusoidal \(I_D\).
Drain Curve : Signal causes Q-point to shift along the load line.
Nonlinear nature of transfer curve causes distortion.
Minimize distortion: Keep signal swing within a limited range on the load line.
Objective: Find Q-point \((I_D, V_{GS})\)
Graphical Method:
Use transconductance curve.
Plot load line: Intersection gives Q-point.
Mathematical Method:
This results in a quadratic equation in \(I_D\).
Replace capacitors with shorts (AC coupling).
Replace \(V_{DD}\) with AC ground.
Resulting AC equivalent circuit:
Drain and source both tied to AC ground.
Very high input resistance:
Due to reverse-biased gate–source junction.
Important Note:
\(V_{\text{out}}\) is 180° out of phase with \(V_{\text{in}}\)
Hence, \(A_v\) is negative.
Advantages:
High input resistance.
Voltage amplification with phase inversion.
Design Consideration:
Ensure Q-point lies in the linear region for undistorted amplification.
Configuration: Zero-biased common-source n-channel D-MOSFET
DC Conditions:
Gate at \(0 \, \text{V dc}\)
Source grounded \(\rightarrow\) \(V_{GS} = 0 \, \text{V} \Rightarrow I_D = I_{DSS}\)
Signal Behavior:
\(V_{gs}\) swings above and below 0 V
Negative swing \(\rightarrow\) Depletion mode \(\rightarrow\) \(I_D \downarrow\)
Positive swing \(\rightarrow\) Enhancement mode \(\rightarrow\) \(I_D \uparrow\)
AC Analysis: Same as in JFET amplifier
Configuration: Common-source n-channel E-MOSFET with voltage-divider bias
Signal Swing :
\(V_{gs}\) varies around \(V_{GSQ}\)
\(I_D\) swings around \(I_{DQ}\)
Operation stays in enhancement mode
| Characteristic | JFET | D-MOSFET | E-MOSFET |
|---|---|---|---|
| Full Form | Junction Field Effect Transistor | Depletion-mode Metal-Oxide-Semiconductor FET | Enhancement-mode Metal-Oxide-Semiconductor FET |
| Channel Type | Physically present (n or p) | Physically present (n or p) | Not present initially; formed by VGS |
| Operation Mode | Only depletion mode | Depletion and enhancement modes | Only enhancement mode |
| Gate-Source Voltage (VGS) | Negative (n-channel), Positive (p-channel) | 0, negative, or positive | Must be positive (n-channel) or negative (p-channel) |
| Gate Current | Small (reverse-biased pn junction) | Very small (insulated gate) | Very small (insulated gate) |
| Input Impedance | High (less than MOSFETs) | Very high | Very high |
| Control Mechanism | Gate voltage controls channel width | Modulates conductivity of existing channel | Gate voltage induces channel |
| Turn-on Behavior | Conducts at VGS = 0 V | Conducts at VGS = 0 V | Requires threshold VGS to turn ON |
| Symbol Difference | Gate arrow touches channel | Arrow does not touch (bar in symbol) | Arrow does not touch channel |
| Manufacturing Complexity | Simpler | More complex | Most complex |
| Applications | Low-noise amplifiers, analog circuits | Analog and RF circuits | Digital circuits, power switching |
Also known as a Source-Follower (analogous to the Emitter-Follower in BJT)
Circuit Example:
Self-biased JFET
\(\mathbf{C_1}/\mathbf{C_2}\): Input/Output coupling capacitor
Key Characteristics:
Input: Gate and Output: Source
Drain is common to both input and output \(\rightarrow\) No drain resistor needed
Output voltage follows input: \(V_{\text{source}} \approx V_{\text{gate}}\) (in phase)
Voltage Gain of Source-Follower
Observation:
\(A_v < 1\), always slightly less
If \(g_m R_S \gg 1\), then \(A_v \approx 1\)
No phase inversion: Output is in-phase with input
As in common-source configuration:
Gate is reverse-biased \(\rightarrow\) Very high input resistance
Conclusion:
\(R_{\text{in}}\) is typically very large, dominated by \(R_G\)
Excellent for buffering applications due to high input, low output impedance
Analogous to: Common-Base (CB) BJT amplifier
Key Features:
Input: Source terminal
Output: Drain terminal
Gate is common \(\rightarrow\) connected to ground
Low input resistance (unlike CS and CD configurations)
Circuit Reference:
Self-biased configuration
\(\mathbf{C_1}/\mathbf{C_2}\): Input/Output coupling capacitor
Where: \(R_d = R_D \parallel R_L\)
Observation:
Same gain expression as common-source amplifier
No phase inversion (output in phase with input)
Unlike CS and CD (which use gate as input), CG uses source as input:
Conclusion:
Low input resistance
Suitable for low-impedance signal sources
Uses: Primarily MOSFETs, not BJTs or linear-mode FETs
Fundamental difference:
Operates with switching transistors (ON/OFF)
Not linear amplification like Class A, B, or AB
Key advantage:
Theoretical Efficiency:\(100\%\)
Practical Efficiency: \(> 90\%\)
Compared to:
Class A: \(\sim 25\%\)
Class B/AB: \(\sim 79\%\)
Block Diagram:
Pulse-width modulator → MOSFET switch stage → Low-pass filter → Speaker
PWM: Converts analog input to pulses of varying width
Wider pulse = Higher input amplitude
Narrower pulse = Lower input amplitude
Zero input = Square wave output
Generated using: Comparator
Comparator Behavior:
Outputs high or low depending on input polarity
Operates with a sine wave (input) and triangular wave (carrier)
Output swings rail-to-rail (e.g., \(\pm 12\) V to \(\pm 24\) V)
PWM spectrum includes:
Input signal frequency (\(f_{in}\))
Modulator frequency (\(f_m\))
Harmonics above and below \(f_m\)
Requirement:
\(f_m\) must be much higher than max \(f_{in}\)
Why?
To easily filter out harmonics
Pass only the original input signal through the low-pass filter
MOSFET Configuration:
Complementary push-pull
Operate in ON/OFF mode, not linear
Switching logic:
When \(Q_1\) is ON \(\rightarrow\) \(Q_2\) is OFF and vice versa
ON: High current, low voltage drop \(\rightarrow\) Low power dissipation
OFF: No current \(\rightarrow\) No power dissipation
Power dissipation occurs only during switching transitions
Ideal Power Dissipation:
\(P_{DQ1} = 0 \, \text{W}\), \(P_{DQ2} = 0 \, \text{W}\)
Practical Limitations:
Small \(V_{DS(on)}\)
Switching losses
Losses in comparators & wave generator
Stages:
Small analog audio input
PWM modulator (Comparator + triangle wave)
MOSFET switching stage (with gain)
Low-pass filter \(\rightarrow\) Removes PWM carrier & harmonics
Amplified analog output to speaker
