The field effect transistor is a component that uses only one junction instead of two as in bipolar transistors. Even though it is only one junction that also functions like a diode, the actual layout of the materials make it have some properties that allow a single junction device function like a transistor.
The layout of the FET is a bar of semiconductor material that has a ring of an oppositely doped semiconductor material around it. This Transistor is called the junction field effect transistor or JFET.
There are two types of JFET, called N-channel and P-channel. The name comes from the type of material that makes up the bar of material, for example the N-channel is a bar of N material with a ring of P material around it.
The explanations here are given for P-channel JFETs, as with bipolar transistors, just reverse polarities for N-channel JFETs.
Similar to the Bipolar transistors, FETs have three terminals, Source, Gate and Sink that correspond in function to the Collector, Base and Emitter of the BJT, respectively
Sunday, November 7, 2010
Junction FET operation (JFET)
Junction FETs work with the diode junction in reverse bias, that is, a more positive voltage is applied to the cathode instead of the anode, the cathode being the gate terminal.
When a gate voltage is applied, the junction depletion region widens by action of the reverse bias of the PN junction. With enough voltage applied, the depletion region widens enough as to completely divide the P material bar, effectively preventing current from flowing. When the gate voltage is lowered, the depletion region shrinks again and current can flow again.
Even in the absence of a control voltage at the gate, the transistor is able to conduct current through its P material body, and works like a semiconductor resistor. When a gate voltage is present, it effectively increases the resistance of the JFET's body, thus controlling the amount of current flowing through it.
Since the PN junction of the JFET is in reverse bias mode, very little current flows (only leakage current caused by heat), so it is useful in applications where loading of a previous stage can affect its behavior or there's a need to limit the amount of consumed current, as in low power applications.
When a gate voltage is applied, the junction depletion region widens by action of the reverse bias of the PN junction. With enough voltage applied, the depletion region widens enough as to completely divide the P material bar, effectively preventing current from flowing. When the gate voltage is lowered, the depletion region shrinks again and current can flow again.
Even in the absence of a control voltage at the gate, the transistor is able to conduct current through its P material body, and works like a semiconductor resistor. When a gate voltage is present, it effectively increases the resistance of the JFET's body, thus controlling the amount of current flowing through it.
Since the PN junction of the JFET is in reverse bias mode, very little current flows (only leakage current caused by heat), so it is useful in applications where loading of a previous stage can affect its behavior or there's a need to limit the amount of consumed current, as in low power applications.
Capacitive Coupling: Isolating AC from DC
In order to connect an alternating signal into the transistor amplifier in a way that the circuitry that generates the signal doesn't interfere with the operation of the amplifier, and also that the biasing and operation of the transistor amplifier doesn't change the way the circuitry of the source signal operates, we need a way to isolate them from each other.
Since the only component of interest that needs to be shared by both circuits is the alternating signal (AC signal), we need to use a component that will let the ac component pass while blocking the any DC of the bias circuitry or the signal generator.
As you learned in a previous lesson, a capacitor is a component that can store energy in the form of an electric field created by lumping charges close to each other but still isolated. Current cannot directly cross the insulating layer inside the capacitor, effectively blocking any direct current flow.
But something interesting happens when a capacitor is affected by an alternating current. On the positive half of an AC wave, one side of the capacitor is filled with an inrush of electrons, while on the other side, electrons are pushed out to be replaced with holes, until the capacitor is fully charged and no more charges move.
For the moment when the capacitor is charging, the amount of electrons entering one plate of the capacitor is the same as the electrons being pushed out from the other side, almost as if the electrons had just crossed the insulating layer.
When the polarity is reversed the effect happens once again, the electrons are now drawn towards the voltage source, leaving holes in the plate of the capacitor. These holes draw the electrons that were previously pushed away, into the plate of the capacitor. The net effect is again as if the electrons crossed the insulating layer to get to the voltage source.
In practice, it is not the actual crossing of the electrons that is of use, but the movement of them on both sides of the capacitor that can be used as a current in the circuit.
Summarizing, the capacitor blocks any current that tries to directly cross the insulating layer, but it can't stop the electrons from being drawn to or away from the plates, effectively letting alternating voltages get through.
This effect is used to isolate the DC component from both sides while allowing the ac to flow, and is called capacitive coupling.
Since the only component of interest that needs to be shared by both circuits is the alternating signal (AC signal), we need to use a component that will let the ac component pass while blocking the any DC of the bias circuitry or the signal generator.
As you learned in a previous lesson, a capacitor is a component that can store energy in the form of an electric field created by lumping charges close to each other but still isolated. Current cannot directly cross the insulating layer inside the capacitor, effectively blocking any direct current flow.
But something interesting happens when a capacitor is affected by an alternating current. On the positive half of an AC wave, one side of the capacitor is filled with an inrush of electrons, while on the other side, electrons are pushed out to be replaced with holes, until the capacitor is fully charged and no more charges move.
For the moment when the capacitor is charging, the amount of electrons entering one plate of the capacitor is the same as the electrons being pushed out from the other side, almost as if the electrons had just crossed the insulating layer.
When the polarity is reversed the effect happens once again, the electrons are now drawn towards the voltage source, leaving holes in the plate of the capacitor. These holes draw the electrons that were previously pushed away, into the plate of the capacitor. The net effect is again as if the electrons crossed the insulating layer to get to the voltage source.
In practice, it is not the actual crossing of the electrons that is of use, but the movement of them on both sides of the capacitor that can be used as a current in the circuit.
Summarizing, the capacitor blocks any current that tries to directly cross the insulating layer, but it can't stop the electrons from being drawn to or away from the plates, effectively letting alternating voltages get through.
This effect is used to isolate the DC component from both sides while allowing the ac to flow, and is called capacitive coupling.
Tuesday, September 7, 2010
Post job requirements for free
You can post your job requirements in EDA, Analog & Digital Design, Verification and Embedded for free on http://jobs.digitalelectronics.co.in . For more info please head to the site itself.
Monday, September 6, 2010
IIT Video Lectures Available for all on YouTube
The Indian Institute of Science (IISc) and IITs, the premier engineering colleges of India, have earlier joined hands to produce the world's largest repository of technical video lectures and web-based courses geared towards science and engineering students. The IIT and IISc faculty have recorded around 4,750 videos for this project till date and anyone, anywhere can watch the lectures online through YouTube at http://youtube.com/iit. [via] Digital Inspiration Technology Blog
Thursday, September 2, 2010
Intel chef Paul Otellini
What is interesting about the title of this post? Check out the article on Channel News Australia. These guys have messed up big time!
Wednesday, September 1, 2010
Join the YES campaign of "Just One Light" initiative
Say YES for changing ONE light to LED at home. Get your organization to say YES to change ONE light per employee to LED and to encourage each employee to adopt ONE light for home.To know more and to say YES, see the brochure at http://bit.ly/aJyQHp
If you can reach your organization to implement LED lights and to encourage employees to adopt LED lights through this campaign, leave a comment indicating name of the organization /department and approx. number of employees.
Do share the brochure with friends and colleagues and help make the campaign a success.
If you can reach your organization to implement LED lights and to encourage employees to adopt LED lights through this campaign, leave a comment indicating name of the organization /department and approx. number of employees.
Do share the brochure with friends and colleagues and help make the campaign a success.
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