This document covers microwave amplifiers and oscillators, focusing on amplifier designs, stability considerations, gain, and noise factors essential for RF applications. Key topics include the importance of matching networks, stability analysis, and the trade-offs between gain, noise figure, and power output. It also discusses various amplifier parameters and methods to ensure stability and minimize noise in designs.

1. 18ECE304T-MICROWAVE INTEGRATED CIRCUITS
UNIT III- Microwave Amplifiers and Oscillators
S1- Introduction to amplifiers, Stability considerations in active networks
S2- Gain Consideration in Amplifiers,
S3- Noise Consideration in active networks
Dr.S. Bashyam
Assistant Professor
Department of ECE, SRM IST

2. S1- Introduction to amplifiers
• Amplifier designs at RF differ significantly from the conventional low-frequency
circuit approaches and consequently require special considerations.
• Appropriate matching is needed to reduce the VSWR and avoid undesirable
oscillations.
• stability analysis in conjunction with gain and noise figure circles, is a basic
ingredient needed to meet the often competing requirements of gain, gain
flatness, output power, bandwidth, and bias conditions

3. S1- Introduction to amplifiers
Generic Amplifier System:
• A generic single-stage amplifier configuration embedded between input and output
matching networks is shown below.
• Input and output matching networks, are needed to reduce undesired reflections and thus
improve the power flow capabilities.
• Amplifier is characterized through its S-matrix at a particular DC bias point

4. S1- Introduction to amplifiers
Key Parameters:
• Gain and gain flatness (in dB)
• Operating frequency and bandwidth (in Hz)
• Output power (in dBm)
• Power supply requirements (in V and A)
• Input and output reflection coefficients (VSWR)
• Noise figure (in dB)
• Also parameters such as intermodular distortion (IMD) products, harmonics,
feedback, and heating effects, all of which can seriously affect the amplifier
performance.

5. S1- Introduction to amplifiers
Amplifier Power Relations:
• Simplified schematics of a single-stage amplifier

6. S1- Introduction to amplifiers
Problem:

7. S1- Introduction to amplifiers

8. S1- Stability considerations in active networks
Stability Circles:
• The phenomenon of oscillations can be understood in the context of a voltage
wave along a transmission line.
• If , then the return voltage increases in magnitude (positive feedback)
causing instability.
• Conversely, causes a diminished return voltage wave (negative feedback).
• Let us regard the amplifier as a two-port network characterized through its S
parameters and external terminations described by .
• Stability then implies that the magnitudes of the reflection coefficients are less
than unity. Namely,

9. S1- Stability considerations in active networks
Stability Circles:

10. S1- Stability considerations in active networks
Unconditional Stability Circles:

11. S1- Stability considerations in active networks
Stability Factor:
• It is always prudent to determine that both the conditions are
fulfilled to ensure an unconditionally stable design.

12. S1- Stability considerations in active networks
Stabilization Methods(Input):

13. S1- Stability considerations in active networks
Stabilization Methods (Output):

14. S2- Gain Consideration in Amplifiers
Unilateral Design:
• Unilateral power gain: or
• where Gs and GL are gains associated with input and output matching networks and Go is the
insertion gain of the transistor

15. S2- Gain Consideration in Amplifiers
Problem:

16. S2- Gain Consideration in Amplifiers
Unilateral Figure of Merit Error Fluctuation
Problem 2:

17. S2- Gain Consideration in Amplifiers
Bilateral Design:
• For many practical situations the unilateral approach may not be appropriate
because the error committed by setting S12 = 0 could result in an intolerably
imprecise design.
• The bilateral design takes into account this feedback

18. S3- Noise Consideration in active networks
Noise Figure Circles:
• In many RF amplifiers, the need for signal amplification at low noise level
becomes an essential system requirement.
• Unfortunately, designing a low-noise amplifier competes with such factors as
stability and gain.
• For instance, a minimum noise performance at maximum gain cannot be
obtained.
• It is therefore important to develop a method that allows us to display the
influence of noise as part of the Smith Chart to conduct comparisons and observe
trade-offs between gain and stability.

19. S3- Noise Consideration in active networks
Noise Figure Circles:
• From a practical perspective, the key ingredient of a noise analysis is the noise
figure of a two-port amplifier in the admittance form
or in the equivalent impedance representation
where Zs = l/Ys is the source impedance

20. S3- Noise Consideration in active networks
• When using transistors, typically four noise parameters are known either through
datasheets from the FET or BJT manufacturers or through direct measurements.
They are:

21. References
1. RF circuit design Theory and applications, Reinhold Ludwig, Pavel Bretchko