Amplifier Design Using FET

8/12/2019 Amplifier Design Using FET

1/39

Submitted to: Dr. B.K. Kanaujia

Submitted By: Ashwini Kumar NareshRF I yearRoll No. 00410100613


2/39

Microwave amplifier designEarlier microwave devices relied on tubes, such asklystrons and travelling wave tubes.

But due to dramatic improvements and innovations insolid state technology that have occurred since 1970s,most RF and microwave amplifiers today use transistordevices.

Microwave transistor amplifiers are rugged, low cost,reliable, and can be easily integrated with integratedcircuits.


3/39

Field Effect Transistor (FET)Microwave field effect transistors can be used well intomillimeter range with high gain and low noise figure.

Typically used are GaAs MESFET due to desirable gainand noise features because of high electron mobility ofGaAs compared to Si.


4/39


5/39


6/39

Two port power gainsConsider an arbitrary two port network as shown below,connected to source Zs and ZL, respectively as shownbelow. We will derive expressions for three types of power gain interms of S parameters and reflection coefficients.


7/39

Power gain = G = P L/ Pin is the ratio of power dissipatedin load Z L to power delivered to input of two port network.

Available gain = G A = Pavn/ Pavs is the ratio of poweravailable from two port network to power available from

source.

Transducer Power gain = G T = PL/ Pavs is the ratio of thepower delivered to the load to power available from the

source.


8/39

Transistor amplifier circuit A single stage microwave transistor circuit can bemodeled as shown below where a matching circuit isused on both sides of transistor.


9/39

We can define separate effective gain factors for theinput matching network, the transistor and the outputmatching network as follows:

Then overall transducer gain is G T = GSGOGL.


10/39

StabilityTwo types of stability are defined:

1. Unconditional Stability: The network is

unconditionally stable if andfor all passive sources and load impedances.2. Conditional Stability: The network is conditionallystable if andonly for certain range of passive source and loadimpedances.


11/39

Conditions for unconditional stability :

Test for unconditional Stability : K- test, where itcan be shown that a device is unconditionally stable ifRollets condition ,defined as

Along with


12/39

Amplifier Design1. Single Stage Transistor amplifier design2. Broadband Transistor amplifier design3. Power Amplifiers


13/39

Single Stage Transistor amplifier

design(a) Design for Maximum Gain:

After stability of transistor has been determined,stable regions for reflection coefficients of load andsource have been located, input and outputmatching sections can be designed. Since Go is fixed, overall gain is controlled by gains, Gs and G L.Because most transistors appear as significantmismatch, resulting frequency is narrowband.


14/39

For maximum power transfer at input networkFor maximum power transfer at output networkMaximum transducer gain

Eg: circuit design and frequency response for anamplifier for maximum gain at 4 GHz is shown below


15/39


16/39

frequency response


17/39

(b) Design for Specified Gain In many cases it is preferable to design for less

than maximum gain to improve bandwidth or toobtain specified values of amplifier gain.

The design is facilitated by plotting constant gaincircles on smith chart, to represent loci of relectioncoefficients of source and load that give fixed values of gain (G S and G L).

The expression for G S and G L for unilateral case


18/39

Maximum gain values are

Normalized gain factors


19/39

The results for constant gain circles are

Eg: Circuit design and frequency response for atransistor amplifier with 11 dB gain at 4 GHz are shownbelow


20/39


21/39

Fig: transducer gain and return loss


22/39

(c) Low- Noise Amplifier Design In receiver applications, it is especially required

to have preamplifier with as low noise figure aspossible.

Generally low noise and maximum gain cant beachieved together. Some sort of compromise has tobe made.

This is done by using constant gain circles andcircles of constant noise figures.

The noise figure of a two port amplifier isexpressed as


23/39

Using reflection coefficients instead of admittance

And the noise figure is achieved as

First we define noise figure parameter, N


24/39


25/39

Constant gain and noise figure circles


26/39

2. Broadband Transistor amplifier

design(a) Balanced amplifiers:

The balanced amplifier uses two 90 degree couplers

to cancel input and output reflections from twoidentical amplifiers. Because of phasing properties of hybrid amplifiers,reflections from amplifier inputs cancel at the input to

hybrid, resulting in improved impedance match.


27/39

Fig: A balanced amplifier using 90 degree hybridcouplers


28/39

Performance and optimization for a balanced

amplifier

Fig: Gain and return loss, before and after optimizationfor a balanced receiver


29/39

(b) Distributed Amplifier A cascade of N identical FETs have their gatesconnected to a transmission line having acharacteristic impedance Z g, with a spacing of l g, while the drains are connected to a transmission lineof characteristic impedance Z d, with spacing l d

Procedure of amplification:

The input signal travels down the gate line, witheach FET tapping off some input power. Theamplified output signals from FETs form a travelling wave on drain line .


30/39

Fig: Configuration of an N-stage distributed amplifier.

The first step in analysis of distributed amplifer is toemploy unilateral version of FET equivalent circuit todecompose the circuit as shown on next slide.


31/39

Fig: Gain versus frequency for a distributed ampliferfrom 1 to 18 GHz for N= 2, 4, 8, 16stages


32/39

3. Power AmplifiersPower amplifiers are used in final stages of radar andradio transmitters to increase radiated power level.Typical output powers maybe on the order of 100-500mW for mobile or data communication systems, or in1-100 W for radar or fixed point radio systems.Important considerations for RF and microwavepower amplifiers are efficiency, gain, intermodulation

products and thermal effects. Various power combining techniques in conjunction with multiple transistors are used if higher outputpowers are required.


33/39

Characteristics of Power Amplifiers and Amplifier Classes:

The power amplifier is usually the main consumerof DC power in most handheld power devices. Its

efficiency is important and given by ratio of RFoutput power to DC input powern= Pout/ P DC

Power amplifiers are usually designed to providebest efficiency, even if it means resulting gain isless than maximum possible.


34/39

Classes of amplifiers

Class AClass B

Class CHigher classes, such as D, E, F and S


35/39

Class A

Class A amplifiers are inherently linear circuits, wheretransistor is biased to conduct over entire range ofinput signal cycle.Because of this class A amplifiers have theoretical

maximum efficiency of 50%.Most small signal and low noise amplifiers operate asclass A circuits.

Class BThese are biased to conduct only during one half ofthe input signal cycle


36/39

Usually two complementary transistors are operated inclass B push pull amplifier to provide amplificationover entire cycle. The theoretical efficiency of class B is 78%.

Class C:Class C amplifiers are operated with the transistor nearcut off for more than half of the input signal cycle, and

generally use a resonant circuit in the output stage torecover the fundamental.Class C amplifiers obtain efficiency near 100%, but canonly be used with constant envelop modulations


37/39

Higher Classes:Classes such as D, E, F and S use transistor as a switchto pump a highly resonant tank circuit, and mayachieve very high efficiencies.The majority of communication transmitters at UHFfrequencies or above use class A, AB, Or B poweramplifiers because of need for low distortion products


38/39

RefrencesMicrowave Engineering By David M. Pozar, Secondand Third editionNaval University of America www.ncbi.nlm.nih.gov


39/39

Amplifier Design Using FET

Documents

Transcript of Amplifier Design Using FET