Submitted to: Dr. B.K. Kanaujia

Submitted By: Ashwini Kumar Naresh RF I year Roll No. 00410100613

Microwave amplifier designEarlier microwave devices relied on tubes,

such as klystrons and travelling wave tubes.But due to dramatic improvements and

innovations in solid state technology that have occurred since 1970s, most RF and microwave amplifiers today use transistor devices.

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

Field Effect Transistor (FET)Microwave field effect transistors can be

used well into millimeter range with high gain and low noise figure.

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

Ri is series gate resistanceRds is drain to source resistanceCgs is gate to source capacitanceCds is drain to source capacitanceCgd is gate to drain capacitancegm is transconduactance

Two port power gainsConsider an arbitrary two port network as shown

below, connected to source Zs and ZL,

respectively as shown below.We will derive expressions for three types of

power gain in terms of S parameters and reflection coefficients.

Power gain = G = PL/ Pin is the ratio of power dissipated in load ZL to power delivered to input of two port network.

Available gain = GA = Pavn/ Pavs is the ratio of power available from two port network to power available from source.

Transducer Power gain = GT = PL/ Pavs is the ratio of the power delivered to the load to power available from the source.

Transistor amplifier circuitA single stage microwave transistor circuit

can be modeled as shown below where a matching circuit is used on both sides of transistor.

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

Then overall transducer gain is GT = GSGOGL.

StabilityTwo types of stability are defined: 1. Unconditional Stability: The network is unconditionally stable if and for all passive sources and load impedances. 2. Conditional Stability: The network is

conditionally stable if and only for certain range of passive source and

load impedances.

Conditions for unconditional stability :

Test for unconditional Stability : K- test, where it can be shown that a device is unconditionally stable if Rollet’s condition ,defined as

Along with

Amplifier Design1. Single Stage Transistor amplifier

design2. Broadband Transistor amplifier

design3. Power Amplifiers

Single Stage Transistor amplifier design (a) Design for Maximum Gain:After stability of transistor has been determined, stable regions for reflection coefficients of load and source have been located, input and output matching sections can be designed. Since Go is fixed , overall gain is controlled by gains, Gs and GL.

Because most transistors appear as significant mismatch, resulting frequency is narrowband.

For maximum power transfer at input network

For maximum power transfer at output network

Maximum transducer gain

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

frequency response

(b) Design for Specified Gain• In many cases it is preferable to design for less than maximum gain to improve bandwidth or to obtain specified values of amplifier gain.• The design is facilitated by plotting constant gain circles on smith chart, to represent loci of relection coefficients of source and load that give fixed values of gain (GS and GL).•The expression for GS and GL for unilateral case

Maximum gain values are

Normalized gain factors

The results for constant gain circles are

Eg: Circuit design and frequency response for a transistor amplifier with 11 dB gain at 4 GHz are shown below

Fig: transducer gain and return loss

(c) Low- Noise Amplifier Design• In receiver applications, it is especially required to have preamplifier with as low noise figure as possible.• Generally low noise and maximum gain can’t be achieved together. Some sort of compromise has to be made. •This is done by using constant gain circles and circles of constant noise figures.•The noise figure of a two port amplifier is expressed as

Using reflection coefficients instead of admittance

And the noise figure is achieved as

First we define noise figure parameter, N

The constant noise circles with centers at

And radius at

are obtained.Eg: Circuit design for LNA at 4 GHZ and 2 dB

nise figure with maximum gain

Constant gain and noise figure circles

2. Broadband Transistor amplifier design(a)Balanced amplifiers: The balanced amplifier uses two 90

degree couplers to cancel input and output reflections from two identical amplifiers.

Because of phasing properties of hybrid amplifiers, reflections from amplifier inputs cancel at the input to hybrid, resulting in improved impedance match.

Fig: A balanced amplifier using 90 degree hybrid couplers

Performance and optimization for a balanced amplifier

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

(b) Distributed AmplifierA cascade of N identical FETs have their gates connected to a transmission line having a characteristic impedance Zg, with a spacing of lg, while the drains are connected to a transmission line of characteristic impedance Zd, with spacing ld Procedure of amplification: The input signal travels down the gate line, with each FET tapping off some input power. The amplified output signals from FETs form a travelling wave on drain line.

Fig: Configuration of an N-stage distributed amplifier.

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

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

3. Power AmplifiersPower amplifiers are used in final stages of radar

and radio transmitters to increase radiated power level.

Typical output powers maybe on the order of 100-500 mW for mobile or data communication systems, or in 1-100 W for radar or fixed point radio systems.

Important considerations for RF and microwave power amplifiers are efficiency, gain, intermodulation products and thermal effects.

Various power combining techniques in conjunction with multiple transistors are used if higher output powers are required.

Characteristics of Power Amplifiers and Amplifier Classes:The power amplifier is usually the main consumer of DC power in most handheld power devices. Its efficiency is important and given by ratio of RF output power to DC input power n= Pout/ PDC

Power amplifiers are usually designed to provide best efficiency, even if it means resulting gain is less than maximum possible.

Classes of amplifiersClass AClass BClass CHigher classes, such as D, E, F and S

Class A Class A amplifiers are inherently linear circuits, where transistor is biased to conduct over entire range of input signal cycle. Because of this class A amplifiers have theoretical maximum efficiency of 50%. Most small signal and low noise amplifiers operate as class A circuits.

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

Usually two complementary transistors are operated in class B push pull amplifier to provide amplification over entire cycle.

The theoretical efficiency of class B is 78%.

Class C:Class C amplifiers are operated with the transistor

near cut off for more than half of the input signal cycle, and generally use a resonant circuit in the output stage to recover the fundamental.

Class C amplifiers obtain efficiency near 100%, but can only be used with constant envelop modulations

Higher Classes:Classes such as D, E, F and S use transistor as a switch to pump a highly resonant tank circuit, and may achieve very high efficiencies.The majority of communication transmitters at UHF frequencies or above use class A, AB, Or B power amplifiers because of need for low distortion products

RefrencesMicrowave Engineering By David M. Pozar,

Second and Third editionNaval University of Americawww.ncbi.nlm.nih.gov