What is MOSFET Amplifier : Working & Its Applications

An amplifier is an electrical device, used to enhance the amplitude of the input signal. It is an essential part of audio sources like a record player or CD player and also other devices, like equalizers, pre-amps & speakers. The subcategory of the amplifier is the MOSFET amplifier that uses MOSFET technology for processing digital signals by using less power. At present, MOSFET amplifiers are a design choice in 99% of the microchips around the world.


The MOSFET amplifier was invented and fabricated in 1959 by Dawon Kahng & Mohamed Atalla. After that, they launched it as the “silicon-silicon dioxide field-induced surface device” at the Solid-State Device meeting held at “The University of Carnegie Mellon” in Pittsburgh, Pennsylvania in early 1960. This article discusses an overview of a MOSFET amplifier and its working with applications.

What is MOSFET Amplifier?

An amplifier that uses Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) technology is known as a MOSFET amplifier. MOSFET is also called the MOS (metal-oxide-silicon) transistor and it is one kind of insulated-gate field-effect transistor. So this transistor is fabricated through silicon material.

This amplifier is the most commonly used FET amplifier. The main benefit of a field-effect transistor used for amplification purposes is that it has less o/p impedance & maximum i/p impedance.

MOSFET Amplifier Circuit & Its Working

A MOSFET amplifier circuit is shown below. A MOSFET amplifier simple circuit diagram is shown below. In this circuit, the drain voltage (VD), the drain current (ID), the gate-source voltage (VGS) & the locations of gate, source & drain are mentioned through the letters “G”, “S”, and “D”.

Generally, MOSFETs work in three regions like Linear/Ohmic or Cut-off & Saturation. Among these three regions, when MOSFETs are used as amplifiers, they should operate in an ohmic region where the current flow throughout the device increases when the applied voltage is increased.

MOSFET Amplifier Simple Circuit
MOSFET Amplifier Simple Circuit

MOSFET can be used as a small-signal linear amplifier within many applications. Usually, in the amplifier circuits, field-effect transistors work within the saturation region. So in this region, the flow of current does not depend on drain voltage (VD) but the current is the main function of the Gate voltage (VG) simply. In these amplifiers, normally the operatingg point is within the saturation region.

In the MOSFET amplifier, a small change within gate voltage will generate a large change within drain current like in JFET. So, MOSFET will increase a weak signal’s strength; consequently, it acts as an amplifier.

MOSFET Amplifier Working

A complete MOSFET amplifier circuit can be designed by including a source, drain, load resistor & coupling capacities to the above circuit. The biasing circuit of the MOSFET amplifier is shown below.

Biasing Circuit of MOSFET Amplifier
Biasing Circuit of MOSFET Amplifier

The above biasing circuit includes a voltage divider, and the main function of this is to bias a transistor in one way. So, this is the most frequently used biasing method in transistors. It uses two resistors to confirm that voltage is separated and & distributed into the MOSFET at the right levels. It is realized through two R1 & R2 parallel resistors. The C1 & C2 coupling capacitors in the circuit protect the biasing DC voltage from the AC signal to be amplified. At last, the output is given to a load, formed by the RL resistor. The bias or gate voltage can be given by

VG = Vsupply x (R2/R1+R2)

Here, the R1 & R2 values are usually large to enhance the amplifier’s input impedance & also to reduce the ohmic power losses.

Input & Output Voltages (Vin & Vout)

To make it simpler, we need to consider that there is no load is connected with the drain branch in parallel. The input voltage (Vin) can be given through the gate (G) to source (S) voltage like VGS. The voltage drop across the RS resistor can be given by RS×ID.

According to the transconductance (gm) definition,  the ratio of ID (drain current) to VGS (gate-source voltage) once a constant drain-source voltage is applied.

(gm) = ID/VGS

So, ID = gm×VGS & the input voltage (Vin) can be factored by VGS like the following.

Vin = VGS x (1+gmRs)

The o/p voltage (Vout) is simply given through the voltage drop across the drain resistor (RD)

Vout = – RD x ID = -gmVGS RD

Voltage Gain

The voltage gain (AV) is the ratio of input voltage and output voltage. After that simplification, the equation will become

Av = – RD/Rs=1/gm

In the above equation, sign “-” comes from the fact that the MOSFET amplifier inverts the o/p signal in equivalence with the BJT CE Amplifier. So, the phase shift is 180 ° or π rad.

Types of MOSFET Amplifiers

MOSFET amplifiers are available in three types like common source (CS), common gate (CG), and common drain (CD), where each type along with its configuration is discussed below.

Common Source MOSFET Amplifier

Common source amplifier can be defined as when the i/p signal is given at both the terminals of the gate (G) & source (S), the o/p voltage can be amplified & attained across the resistor at the load within the drain (D) terminal. In this configuration, the source terminal acts as a common terminal in between the i/p and o/p.

The common-source MOSFET amplifier is related to the CE (common-emitter) amplifier of BJT. This is very popular due to high gain and larger signal amplification can be achieved. The small-signal and hybrid π model of a common source MOSFET amplifier is shown below.

Common Source (CS) Mosfet Amplifier
Common Source (CS) Mosfet Amplifier

In the following small-signal CS MOSFET amplifier, the ‘RD’ resistor measures the resistance in between the drain (D) & the ground (G). This small-signal circuit can be replaced by the hybrid-π model which is shown in the following figure. So, the current induced within the o/p port is i = −gmvgs as specified through the current source. Therefore,

vo = −gmvgsRD

By examination, one observes that

Rin = ∞, vi = vsig, vgs = vi

So the voltage gain of open-circuit is

Avo = vo/vi = −gmRD

One can replace a linear circuit driven by a source by its Th´evenin equivalence.

From the small-signal circuit, one can change the output fraction in the circuit by a Norton’s or Thevenin’s equivalence. In this case, using the Norton equivalence is more convenient. To verify the Norton equivalence resistance, set vi = 0, so that the circuit will be an open circuit, so there is no current flow. And through the test-current technique, the o/p resistance is

R0 = RD

The load resistor (RL) is connected to the o/p across RD, then the terminal voltage gain through the voltage divider formula can be expressed as;

Av = Avo (RL/RL + Ro) = −gm (RDRL/RL + RD) = −gm(RD||RL)

From the information that Rin = ∞, after that vi = vsig. So, the voltage gain (Gv) is the similar as the voltage gain accurate (Av),

Gv = vo/vsig = −gm(RD|| RL)

Thus, the CS MOSFET amplifiers have infinite i/p impedance, high o/p resistance & high voltage gain. The output resistance can be reduced by decreasing the RD but also the voltage gain can also be decreased. A CS MOSFET amplifier suffers from a poor high-frequency performance like most of the transistor amplifiers do.

Common-Gate (CG) Amplifier

A common-gate (CG) amplifier is normally used as a voltage amplifier or current buffer. In the CG configuration, the source terminal (S) of the transistor works like the input whereas the drain terminal works like the output & the gate terminal is connected to the ground (G). The common gate amplifier configuration is mainly used to provide high isolation in between i/p & o/p to prevent oscillation or less input impedance.

The small-signal model and T model of a common-gate amplifier equivalent circuit are shown below. Here in the ‘T’ model, the gate current is always zero.

CG MOSFET Amplifier
CG MOSFET Amplifier

If, the applied voltage is ‘Vgs’  & the current at the source is ‘Vgs*gm’, then:

R=V/I=>R=Vgs/Vgsgm=1/gm

Here, the common gate amplifier has less input resistance, which can be given as Rin = 1/gm.

 

The input resistance is typically a few hundred ohms. The o/p voltage can be given as

vo = −iRD

where i = −vi/1/gm = −gmvi

Therefore, the open-circuit voltage can be given as

Avo = vo/vi = gmRD

The circuit’s output resistance is Ro = RD

The small i/p impedance is harmful to the amplifier gain. So by the formula of the voltage divider, we can get

Vi/vsig = Rin/ Rin + Rsig = 1/gm/1/gm + Rsig

The ‘vi’ is attenuated as compared to vsig, because ‘Rsig’ is normally superior to 1/gm.

Once a load resistor ‘RL’ is connected to the o/p, then the right voltage gain is then

Av = gmRD||

Therefore, the voltage gain is expressed as

Gv = (1/gm/Rsig + 1/gm) gm(RD||RL) = RD||RL/Rsig + 1/gm

When the i/p impedance is less, it is excellent for matching sources through a less i/p impedance because of the maximum power theorem; however, it draws additional current, involving high power utilization from the source of the signal.

Thus, the common gate MOSFET amplifier has less i/p resistance ‘1/gm’. So, this is undesirable because it will draw a huge current once it is driven through an input voltage. The CG amplifier’s voltage gain can be made related in magnitude to that of the common source amplifier once RD||RL can be made large as compared to Rsig + 1/gm. The o/p resistance can be made high as Ro = RD. The frequency performance of this amplifier is high.

Common Drain Amplifier or Source MOSFET Follower

A common-drain (CD) amplifier is one where the input signal is given to the gate terminal & the output is obtained from the source terminal, making the drain (D) terminal common to both. The CD amplifier is frequently used as a voltage buffer to drive small o/p loads. This configuration provides extremely high i/p impedance & low o/p impedance.

This common drain amplifier circuit is similar to the emitter follower circuit of the BJT. So, it is used as a voltage buffer. This amplifier is a unit-gain amplifier including very huge input impedance although a smaller o/p impedance. So it is excellent for high-impedance circuit matching to a less -impedance circuit otherwise to a circuit that works with a larger supply current.

The small-signal & T-model equivalent circuit of the common drain amplifier is shown below. In this circuit, the i/p input source can be signified through an equivalent voltage of Thevenin (vsig) & resistor (Rsig). A load resistor (RL) can be connected to the o/p in between the source (S) & ground (G).

Common Drain (CD) MOSFET Amplifier
Common Drain (CD) MOSFET Amplifier

Since the gate current (IG) is zero for the above circuit,

Rin = ∞

By using the formula of the voltage divider, it is noticed that voltage gain correct or gain of terminal voltage is

Av = vo/vi = RL/RL + 1/gm

The voltage gain of an open-circuit (RL = ∞) & Avo = 1

The o/p resistance can be obtained by changing the correct element of the MOSFET amplifier through Thevenin’s equivalence. By using the test current technique at this end, one sets the Vi value to 0, and therefore

     Ro = 1/gm

Because of the endless input impedance (Rin), vi = vsig, & the overall voltage gain, Gv is similar when the voltage gain proper Av

Gv = Av = RL/RL + 1/gm

Since Ro = 1/gm is normally small through large load resistor ‘RL’, the gain is low than unity, however is near to unity. Therefore, this is a source follower, as the source voltage tracks the i/p voltage, however, it can supply a larger current toward the o/p than the i/p current.

MOSFET Amplifier Solved Problems

The mosfet amplifier example problems are discussed below.

Example1:

A mosfet amplifier with a common source is designed with an n-channel MOSFET. Its threshold voltage (Vth) is 1.5 volts and conduction parameter (K) is 40mA/V2. If the voltage supply is +20 volts & the load resistor (RL) is 450 Ohms. Find out the values of the required resistors to bias the MOSFET amplifier at 1/4(VDD). For an undistorted & symmetrical o/p waveform, fix the DC biasing voltage for the drain terminal of the MOSFET to half the voltage supply.

Solution:

The given values are VDD = +20v, Vth= +1.5v, k = 40mA/V2 & RD = 450Ω.

1). Drain current (ID)

VD = VDD/2 = 20/2 = 10V

ID = VD/RD = 10/450 = 22mA

2). Gate-source Voltage (VGS)

ID =  k(VGS-VTH)^2

VGS = √ID/K + Vth = √0.022/0.04 + 1.5 = 2.241V

3). Gate voltage (VG)

VG = 1/3 VDD => 20/3 = 6.6V

VG = VGS + VS =>

Vs = VG – VGS = 6.6 – 2.24 = 4.36V

Once KVL is applied across the MOSFET, then drain source voltage & VDS can be given as

VDD = VD +VDS +VS

VDS = VDD – VD – VS

VDS = 20 – 10 – 4.36 = 5.64 V

4). Source Resistance (RS)

RS = VS/ID = 4.36/0.022 = 198.18 Ohms

The voltage divider resistors ratio like R1 & R2 are necessary to provide 1/3VDD is measured as;

VG = VDD (R2/R1+R2)

If we use R1 = 100kΩ & R2 = 50kΩ, this will satisfy the VG = 1/3VDD condition. In addition, the bias resistors combination will provide an i/p resistance to the MOSFET amplifier 67kΩ.

We can get this design a single step extra by measuring the input & output coupling capacitors values. If a lower cut-off frequency of mosfet amplifier is 20Hz, then the two capacitors values are used to calculate the gate biasing network’s input impedance as:

Rin = R1XR2/R1+R2 = 100X50/100+50 = 33 Kilo Ohms

f(-3DB) = 20 Hz = 1/2πRinC

C = 1/2πfRin => 1/2πx20x33000 => 1/4144800 => 0.24 uF

So, the final circuit of the single-stage MOSFET amplifier is given as;

Single Stage Amplifier Circuit
Single Stage Amplifier Circuit

Example2:

the following CD MOSFET amplifier circuit includes voltage divider bias, the two resistors like R1 = 2.5 M Ohm & R2 = 1.5 M Ohm respectively, then what is the Rin value?

CD MOSFET Amplifier
CD MOSFET Amplifier

The given data is; R1 = 2.5 M Ohm, R2 = 1.5 M Ohm

So, Rin = R1||R2 => R1xR2/R1+R2

Substitute the values in the above equation then we can get the Rin value.

Rin = 2.5×1.5/2.5+1.5

Rin = 2.5 M Ohm x1.5M Ohm/4M Ohm

Rin = 3.75/4 = 937.5 K Ohms

Difference between BJT and MOSFET Amplifier

The difference between the Mosfet amplifier vs transistor amplifier is listed below.

BJT Amplifier

MOSFET Amplifier

BJT includes three terminals like emitter, base, and collector. MOSFET includes three terminals like source, drain, and gate.
This transistor uses three configurations like common emitter, common base, and common collector. This transistor uses three configurations like common source, common drain, and common gate.
BJT have less input impedance In MOSFET amplifiers, except CG amplifier, CS & CD have a high input impedance
BJT amplifiers have higher transconductance It has less transconductance
BJT’s are common due to their wider commercial accessibility & longer history. These are Discrete circuit amplifiers

 

BJT amplifier is used where less input impedance is necessary. The CB amplifier is used in preamplifiers, moving coil microphones, UGHF & VHF RF amplifiers. MOSFET amplifiers are applicable in RF-based applications and also used in sound systems. The MOSFETs switching action can be used to make chopper circuits

MOSFET Amplifier vs Regular Amp

The difference between mosfet amp vs regular amp is, amplifier is an electronic circuit that is used to amplify the signal amplitude which is given to its i/p terminals and generates a high amplitude signal as an output.

A mosfet amplifier is a subcategory in amplifiers that use MOSFET or metal–oxide–semiconductor field-effect transistor technology to process digital signals by fairly less power consumption.

Advantages

The advantages of the MOSFET amplifier include the following.

  • Mosfet amplifier has low losses.
  • The communication speed of this amplifier is high.
  • It is better as compared to other devices like Thyristor, IGBT, etc.
  • Mosfet amplifiers occupy less space and fast.
  • Consumes less power as compared to BJTs.
  • FET amplifiers have very high i/p impedance & low o/p impedance.

The disadvantages of the MOSFET amplifier include the following.

  • Its design is expensive as compared to normal designs
  • Gain is very small

Applications

The applications of a MOSFET amplifier include the following.

  • The MOSFET amplifier is used for signal amplification.
  • These are used in small-signal linear amplifiers due to their high input impedance which makes the biasing of these amplifiers is easy.
  • MOSFET amplifiers are extensively used in radio frequency applications.
  • The MOSFET amplifier is the most frequently used FET amplifier.

Why use a MOSFET instead of a transistor?

There are many reasons to use a MOSFET in place of a transistor-like Mosfet is faster, has very high input impedance, and is less noisy.

Which power amplifier has the highest efficiency?

The class D power amplifier has the highest efficiency as compared to other amplifiers like class A, class B, class AB, and class C. The class D amplifier uses non-linear switching technology & the o/p devices can be either turned on or turned off.

Which amplifier has the highest gain?

Common emitter (CE) transistor amplifier has the highest voltage gain, current gain, and power gain.

Does a MOSFET increase the voltage?

The maximum input voltage can be increased by adding additional P-MOSFETs in series.

How does a MOSFET act as an amplifier?

A small change within gate voltage generates a huge change within drain current as in JFET. So this fact will make the MOSFET amplify a weak signal, as a result, it acts as an amplifier.

Thus, this is all about an overview of mosfet amplifier, types, working, example problems, advantages, disadvantages, and its applications. In this amplifier, the command signal is a gate signal that controls the flow of current in between the Source (S) & the Drain (D). Here is a question for you, what is a BJT amplifier?

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