Cascode amplifier

Cascode amplifier is a two stage circuit consisting of a transconductance amplifier followed by a buffer amplifier. The word “cascode” was originated from the phrase “cascade to cathode”. This circuit have a lot of advantages over the single stage amplifier like, better  input output isolation, better gain, improved bandwidth, higher input impedance, higher output impedance, better stability, higher slew rate etc. The reason behind the increase in bandwidth is the reduction of Miller effect.  Cascode amplifier is generally constructed using FET ( field effect transistor) or BJT ( bipolar junction transistor). One stage will be usually wired in common source/common emitter mode and the other stage will be wired in common base/ common emitter mode.

Miller effect.

Miller effect is actually the multiplication of the drain to source stray capacitance by the voltage gain. The drain to source stray capacitance always reduces the bandwidth and when it gets multiplied by the voltage gain the situation is made further worse. Mulitiplication of stray capacitance increases the effective input capacitance and as we know, for an amplifier, the increase in input capacitance increases the lower cut of frequency and that means reduced bandwidth. Miller effect can be reduced by adding a current buffer stage at the output of the amplifier or by adding a voltage buffer stage before the input.

FET Cascode amplifier.

FET cascode amplifier

The circuit diagram of a typical Cascode amplifier using FET is shown above. The input stage of the circuit is an FET common source amplifier and the input voltage (Vin) is applied to its gate. The output stage is an FET common gate amplifier which is driven by the input stage. Rd is the drain resistance of the output stage. Output voltage (Vout)  is taken from the drain terminal of Q2. Since the gate of Q2 is grounded, FET Q2′s source voltage and the FET Q1′s drain voltage are held almost constant. That means the upper FET Q2 offers a low input resistance to the lower FET Q1. This reduces the gain of lower FET Q1 and as a result the Miller effect also gets reduced which results in increased bandwidth. The reduction in gain of the lower FET Q1 does not affect the overall gain because the upper FET Q2 compensates it. The upper FET Q2  is not affected by the Miller effect because the charging and discharging of the drain to source stray capacitance is carried out through the drain resistor and the load and the frequency response if affected only for high frequencies (well over the audio range).

In Cascode configuration, the output is well isolated from the input. Q1 has almost constant voltage at the drain and source terminals while Q2 has almost constant voltage at its source and gate terminals and practically there is nothing to feed back from the output to input. The only points with importance in terms of voltage are the input and output terminals and they are well isolated by a central connection of constant voltage.

Practical Cascode amplifier circuit.

Practical cascode amplifier circuit

A practical Cascode amplifier circuit based on FET is shown above. Resistors R4 and R5 form a voltage divider biasing network for the FET Q2. R3 is the drain resistor for Q2 and it limits the drain current. R2 is the source resistor of Q1 and C1 is its by-pass capacitor. R1 ensures zero voltage at the gate of Q1 during zero signal condition


source:circuitstoday.com

Google Project Glass : Way To New Evolution

Google (Nasdaq: GOOG) has revealed its long-rumored plans to create augmented reality (AR) eyeglasses.

Google's Project Glass

Augmented reality is a live, direct or indirect view of a physical environment that's augmented by computer-generated input such as sound, video, graphics or GPS data.

Work on the AR glasses is proceeding as Project Glass, which is part of the Google X Lab, a facility run by Google said to be somewhere in California's Bay Area where about 100 leading-edge projects are reportedly being undertaken.

The scientists behind Project Glass are Babak Parviz, Steve Lee and Sebastian Thrun. They asked for viewer feedback on a video they posted on YouTube.

Through the Looking Glass

The video is shot from the point of view of someone wearing a pair of AR glasses. The wearer can apparently take a phone call while wearing the glasses, listening and speaking with the caller, whose avatar or photo pops up on one lens of the glasses.

 

 

The wearer can also transmit what he or she sees during the conversation. For example, looking out of the window triggers data about the weather that's shown on the lens.

Looking at objects while walking pulls up information on the lens. For example, when the wearer passes a subway station, a notice that service had been suspended is called up a Google Map showing a walking route.

A Siri-like voice application puts up information in response to questions. For example, when the user walked into a bookstore and asked where the music section was, a map indicating that section came up on the lens.

The user can also instruct the glasses to take a photo.

Project Glass seems to incorporate features from Google Maps, Android's voice response system, Google's geolocation service and other features.

"We plan to share a lot more details as we continue to work on this project," Google spokesperson Katelin Todhunter-Gerberg told TechNewsWorld. However, "our only on-the-record statement is in the Google+ post." 

Who's Making the Glasses

Babak Parviz, from the Project Glass Team, is an associate professor of electrical engineering at the University of Washington who has been working on bionic contact lenses that could wirelessly stream information across their surface, much like the AR glasses.

He led a team that created a prototype contact lens that contained a single pixel of information as a proof of concept. The lens has an antenna to draw power from an external source. An integrated circuit in the lens stores this energy and transfers it to a transparent sapphire chip containing one blue LED.

Another member of the Project Glass team, Sebastian Thrun, is a Google Fellow and a part-time research professor of computer science at Stanford University. He was formerly the director of the Stanford Artificial Intelligence Laboratory and led the development of the Google self-driving car.

My AR Glasses Cup Runneth Over

"The question is whether [AR glasses] will initially be more helpful or more annoying," Rob Enderle, principal analyst at the Enderle Group, told TechNewsWorld.

"Let's say you are driving, and just before the guy in front of you hits his brakes a note pops up warning you of traffic ahead so you don't actually see his brake lights in time," he said. "In that instance, the glasses will become a huge liability."

However, if the glasses work, Enderle will use them. "They would alert me to things I want to know and likely otherwise would miss," he explained. "And they'd be very handy in navigation, particularly while walking or on a bike."

Google will have to work out some knots in its idea first in order to have the glasses accepted by the general public.

"Putting something over someone's eyes to create a new human/machine interface is fascinating, but given how much we use our eyes, it doesn't come without risk," Enderle pointed out.

"This is like the 3D TV thing all over again," Maribel Lopez, principal analyst at Lopez Research, told TechNewsWorld. "Nobody wants to buy 3D TV because nobody wants to wear the dorky glasses."

Further, it's not likely that people whose vision doesn't require correcting would want to wear AR glasses, Lopez pointed out. "It's got to be something that's integrated into your normal routine, like cycling glasses for when you're going to ride your bike, and not a stretch." 

 

 

Source:http://www.technewsworld.com/rsstory/74784.html