A typical FET Debye length is approximately 5 nm at room temperature. To meet the requirement of the Debye length, biorecognition events must occur within 5 nm. As the height of an aptamer is generally less than 5 nm, aptamers are well-suited for use as biorecognition probes in FET biosensors. FET (Field-Effect Transistor) Basics Field-Effect Transistors (FETs) are unipolar devices, and have two big advantages over bipolar transistors: one is that they have a near-infinite input resistance and thus offer near-infinite current and power gain; the other is that their switching action is not marred by charge-storage problems, and they thus outperform most bipolars in terms of digital switching speeds.
In electronic circuits, amplifiers are used to increase the strength or amplitude of the input signal without any phase change and frequency. Amplifier circuits are made up of either FET (Fied Effect Transistor) or normal bipolar junction transistor-based on their 3 terminals. The advantage of amplifier circuit using FET over BJTs is used as small-signal amplifiers because they produce high input impedance, high voltage gain, and low noise in the input signal. FET is a voltage-controlled device with three terminals -source, drain, and gate. Based on these terminals, FET is divided into 3 amplifier configuration that corresponding to 3 configurations of Bipolar transistors. They are common-source, common drain (source-follower), and common-gate amplifier circuits. The common – source amplifier circuit is most widely used than any other amplifier circuits because it can produce high input and output impedance, and also its performance is high. Here is a complete description of the common-source amplifier using FET.
What is a Common Source Amplifier
When the input signal is applied at the gate terminal and source terminal, then the output voltage is amplified and obtained across the resistor at the load in the drain terminal. This is called a common source amplifier. Here source acts as a common terminal between the input and output. It is also known as a voltage amplifier or a transconductance amplifier. It produces current gain and voltage gain according to the input impedance and output Impedance. To produce voltage gain along with high input impedances FET’s are used in these circuits.
Common Source Amplifier Circuit
The circuit diagram of the common source amplifier with N-channel FET along with the coupling and biasing capability is shown below. This circuit will be similar to the common-emitter follower of Bipolar Junction transistor. If we use P-channel FET, the polarity of the input voltage will be reversed.
Common Source Amplifier Circuit
Design
The design of a common source amplifier with FET is the same as the Class A amplifier using BJT (Bipolar Junction transistor). The design configuration of the common source amplifier using N-channel and P-channel FET is shown below.
P-channel (Common Source)
The common source FET amplifier consists of 3 terminals. They are, source, gate, drain.
- Source: The majority of the source of carriers required for the device enters through this terminal. Through a source terminal, current enters the channel, which is denoted by IS
- Drain: The majority of the carriers in the channel leaves through this terminal. That is draining. Hence, conventional current enters the channel, which is denoted by ID.
- Gate: This terminal always controls the conductivity of the channel. Hence, the flow of current in the output is controlled with the help of a voltage level across the gate.
This amplifier can provide medium input Impedance, medium output impedance, medium current gain, medium voltage gain, reverse output with respect to input which means output signal will be in 180 degrees phase change. From these characteristics, we can conclude that this amplifier can give high-level performance over other amplifier circuits like a common drain (source follower) and common gate. Hence it is most widely used than other amplifier circuits.
Frequency Response
The frequency response of this amplifier is the most important factor. The circuit designs of low frequency and audio transistor circuits are different from RF applications. The capacitors and the type of FET used in the operation may affect the frequency response of the amplifier.
The frequency response of this amplifier is limited. This is the main drawback. The amplified output voltage can be applied to either a common-drain circuit ( voltage follower) or a common-gate circuit (current follower). To obtain better frequency response, common-drain and common-gate circuits are combined to form a cascade amplifier circuit.
If the biasing arrangement is improper, then some form of distortion may appear in the amplified output signal. That is amplitude distortion, which can occur due to the effect of phase shift, clipping of signal, and also frequency distortion. The frequency response of this amplifier with active load is shown below
Frequency Response of Common Source Amplifier
Common Source Amplifier Working
This amplifier can work as either a transconductance amplifier or a voltage amplifier. If the amplifier is working as a transconductance amplifier, then the input signals are amplified and modulate the current flowing to the load. If the amplifier working as a voltage amplifier, then the input signal is amplified and modulates the current passing through the FET and changes the voltage across the load resistor according to the Ohm’s law.
The common source amplifier working can be explained from the above circuit diagram. Its working is similar to the working of a common-emitter follower of the BJT circuit.
When the input signal is applied at the gate terminal through the capacitor C1. The use of this capacitor is to check whether the gate terminal is affected by any DC voltage of the previous stage. The resistor R2 of around 1Mega ohms is between the gate and the ground holds the potential. The voltage is developed across the resistor R2 that can hold the source above the ground. The bypass capacitor C2 provides the additional gain for the AC signal. The amplified output voltage is obtained across the resistor R3 at the load at the drain terminal of the circuit. This amplified output voltage is coupled to the AC signal of the next stage by the capacitor C3 by blocking or eliminating the DC components. The amplified output signal of this amplifier is 180 degrees out of phase with respect to the input signal and produces high power gain.
The operation of the P-channel common source amplifier using FET is also similar to the N-channel common source amplifier FET except the voltage polarities will be reversed. In the reverse-biased state, there will be no current flowing between gate and source. Hence the gate current is zero. Then the gate voltage (DC) is given as,
Vg = Ig x Rg
The DC voltage at source is given by,
Vs = Id x Rs
Then the gate to source voltage is given by,
Vgs = Vg – Vs = 0- Id x Rs
Since Ig = 0
Vgs = – Id x Rs
Where
Id = drain current
Vgs = gate-source voltage
Vg = gate voltage
Rs = resistor at the source
Rg = resistor at Gate
Ig = gate current
The flow of DC components in the drain current can make the resistor Rs to provide self-biasing and feedback to the input from the output.
Applications
The applications of common source amplifier are as follows
- Used in amplification of sensor signals
- Used in low noise amplification of RF signals
- Used in communication systems like TV and FM receivers
- Used as voltage-controlled devices in op-amps
- Used as cascade amplifiers and RF amplifier circuits.
Thus, this is all about the common source amplifier of single-stage N-channel FET – definition, circuit, design, working, frequency response, and applications. The purpose of this amplifier is, it can be used as either a transconductance amplifier or a voltage amplifier. It can provide high power gain, medium current, and voltage gains according to the input and output impedances. Here is a question to you, ” What are the V-I characteristics of common source amplifier?”
FET, Field Effect Transistor Circuit Design Includes:
FET circuit design basicsCircuit configurationsCommon sourceCommon drain / source followerCommon gate
Field effect transistors are used in circuit design as they are able to provide very high input impedance levels along with significant levels of voltage gain.
Unlike the bipolar transistor which is a current controlled device, the field effect transistor is voltage controlled. This makes the way FET circuits are designed rather different to the way bipolar transistor circuits are designed.
However, circuits with current and voltage gain can still be designed and similar circuit formats are adopted.
FET circuit basics
When considering the use of a FET circuit, it is necessary to consider FET technology and the type of field effect transistor will be the most applicable.
Note on Field Effect Transistor Technology:
The field effect transistor, FET, is a three terminal device which provides voltage gain. Having a high input impedance the electric field in the vicinity of the input terminal called the gate modifies the current flowing in what is called the channel between terminals called the source and drain.
Read more about the Field Effect Transistor Device & How it Works
The FET has three electrodes:
- Source: The Source is the electrode on the FET through which the majority carriers enter the channel, i.e. at acts as the source of carriers for the device. Current entering the channel through the source is designated by IS.
- Drain: The Drain is the FET electrode through which the majority carriers leave the channel, i.e. they are drained from the channel. Conventional current entering the channel via the drain is designated by the letters ID. Also Drain to Source voltage is often designated by the letters VDS
- Gate: The Gate is the terminal that controls the channel conductivity, hence the level of voltage on the gate controls the current flowing in the output of the device.
FET circuit design parameters
When starting out on the design of a FET circuit, it is necessary to determine the basic requirements for the circuit. These will govern many of the decisions regarding the type of circuit topology to use and also the type of FET to use.
There can be a number of parameters required in the requirements for the transistor circuit design:
- Voltage gain: The voltage gain is often a key requirement. It is the output signal voltage divided by the input signal voltage.
- Current gain: This is the gain of the FET circuit in terms of current. It may be necessary to drive a high level of current into the load.
- Input impedance: This is the impedance that the previous stage will see when it is providing a signal to this FET circuit in question. FETs inherently have a high input impedance to the gate and therefore FETs are often used where this is of paramount importance.
- Output impedance: The output impedance is also important. If the FET circuit is driving a low impedance circuit, then its output must have a low impedance, otherwise a large voltage drop will occur in the transistor output stage.
- Frequency response: Frequency response is another important factor that will affect the FET circuit design. Low frequency or audio transistor circuit designs may be different to those used for RF applications. Also the choice of the FET and capacitor values in the circuit design will be greatly affected by the required frequency response.
- Supply voltage and current: In many circuits the supply voltage is determined by what is available. Also the current may be limited, especially if the finished FET circuit design is to be battery powered.
Use Of Fetal Parts
FET types for circuit design
As there are several different types of field effect transistor that can be used, it is necessary to define at least some of the FETs that can be used within the circuit design process.
The table below defines some of the different types and characteristics that can be encountered.
Use Of Fetal Pillow
| FETs for Use in Circuit Design | |
|---|---|
| Characteristic | Details |
| N-channel | An N channel FET has a channel made from N-type semiconductor in which the majority carriers are electrons. |
| P-channel | An P channel FET has a channel made from P-type semiconductor in which the majority carriers are holes. |
| J-FET | The J-FET or junction FET is a form of FET where the gate is formed by using a diode junction onto the channel. The isolation is maintained by ensuring that the diode junction remains reverse biased when operated within the circuit. It is a key requirement of the FET circuit design to ensure the junction remains reverse biased for satisfactory operation. |
| MOSFET | This type of field effect transistor relies on a metal oxide later between the gate and channel. It offers a very high input resistance. |
| Dual-gate MOSFET | As the name implies, this form of MOSFET has two gates. In FET circuit design, this gives additional options. |
| Enhancement mode | Enhancement mode FETs are OFF at zero gate-source voltage. They are turned on by pulling the gate voltage in the direction of the drain voltage, i.e. towards the supply rail, which is positive for N-channel devices and negative for P-channel devices. In other words by pulling the gate voltage towards the drain voltage, the number of carriers in the active layer of the channel is enhanced. |
| Depletion mode | In a depletion-mode MOSFET, the device is normally ON at zero gate-source voltage. Any gate voltage in the direction of the drain voltage will tend to deplete the active area of channel of carriers and reduce the current flowing. |
Use Of Fetal Tissue In Research Back In The Spotlight
When designing an FET circuit, it is first necessary to select the required type of FET. Factors including the basic type of FET including whether it is a junction FET or MOSFET or another type as well as the mode type and other factors all need to be determined to before it is possible to proceed with the circuit design.
More Circuits & Circuit Design:
Op Amp basicsOp Amp circuitsPower supply circuitsTransistor designTransistor DarlingtonTransistor circuitsFET circuitsCircuit symbols
Return to Circuit Design menu . . .