ACTIVE MW CKTS

Oscillators, Amplifiers and Mixers

Design is tough

Bias network is required,

Devices are nonlinear,

Unintended oscillations produced by circuit instability

AC > 1

PC < 1, S parameters with magnitudes greater than unity are physically impossible

MWA

Heart of RF and MW systems

MWA

Tubes Klystrons and TWT

Solid-state Tunnel or varactor diodes

Today Transistor devices such as Si BJTs, GaAs or SiGe HBTs, Si MOSFETs, GaAs MESFETs, or GaAs or GaN HEMTs

Adv Rugged, low-cost, and reliable and can be easily integrated in ICs

Req. small size, low noise figure, broad bandwidth, and medium to high power capacity

IMP G, Stability, Noise figure (NF), Noise Factor (F)

Degradation of the SNR

MICROWAVE AMPLIFIER - GAINS

Two port gains, by definitions

Transducer power gain

Power gain

Available power gain

PL PAVS PIN PAVN

These three definitions of power gain are used to

design different types of amplifiers:

Operating Power Gain, G. Maximum linear output power

amplifiers

Available Power Gain, GA Low Noise Amplifiers (LNAs)

Transducer Power Gain, GT, Simultaneously conjugate

matched input and output ports (leads to maximum linear

gain)

1. 3 factors how power G red. By S & L ckts

2. G not dep , 3. both 4. If S & L are both

conjugate matched

Passive Unit: Dealt with the

matching properties at input Passive Unit: Dealt with the

matching properties at output

Gain of Transistor

STABILITY

The quality of being stable

Ability of a substance to remain unchanged over time

An amplifier circuit has gain G and if its not properly designed, it may oscillate

It is referred to as circuit stability

A brief period of oscillation could permanently

damage a circuit

Since large V and P levels that might be generated

TYPES OF STABILITY

Unconditionally stable:

in

STABILITY CONDITIONS

Here if transistor is unilateral:

S12= 0, then in , out are just S11 and S22.

STABILITY CIRCLES

It can be very helpful to generate a graphical

depiction of the range of and values that may lead to instability

Stability circles define the boundary between

stable and potentially unstable and

1. Center

2. Radius

STABILITY TESTS

K- test: For unconditionally stable

test:

If >1, the device is unconditionally stable This is the mu stability factor. Larger values of imply greater stability.

STABILITY REGIONS

Boundary of Stability circle is in=1.

Stable region For S11 < 1 For S11 >1

Unconditionally stable