Microwave Amplifier Design: Full Notes for Microwave-Engineering

The Microwave Amplifier Design notes include a complete introduction, power gains, stability, and the types of the design. Go through the important learning about Microwave Amplifier Design below.

MICROWAVE AMPLIFIER DESIGN

Two-Port Power Gains

• The gain and stability of a general two-port amplifier in terms of parameters of the transistor will be investigated for an amplifier and oscillator design. Three types of power gain can be derived as

• where G is independent of Zs. GA is defined with an assumption that conjugate matching of both source and load depends on ZS but not ZL. GT depends on both ZS and ZL. Whenever input and output are both conjugately matched, the gain is maximized and G = GA = GT.
• The average power delivered to the network.

• The power delivered to load

.

• Then, the power gain

• Where

• More generally, most useful power definition is Transducer Power Gain account for both source and load mismatch

• Where

• A similar relation can be obtained by the Equivalent circuit parameter.

Stability

• There are necessary conditions for a transistor amplifier to be stable based on the possible oscillation for input and output impedance has a negative real part of a two-sub group:
  • Conditional Stability: If and , the network is stable for a range of passive source and load impedance.
  • Unconditional Stability: If and for all passive sources and loads, the network is unconditionally stable.
• The stability condition is usually frequency dependent since matchings generally depend on frequency (stability may be possible for a frequency but not possible for others). A rigorous treatment of stability requires S parameters of the network have no poles in the right-half complex plane in addition to and . If device is unilateral S21 = 0, more simply results |S11| < 1 and |S22| < 1 are enough for stability.
• Stability Circles: Applying the above requirement for unconditional stability, the following conditions have to be satisfied

• These conditions define a range for and where amplifier will be stable. Finding this range by using Smith chart, plotting the input and output Stability Circles are defined as loci in the (or ) plane for which (or ), then define boundaries between stable and unstable regions. The equations for input and output stability conditions can be extracted as

• If the device is unconditionally stable, the stability circles must be completely outside (or totally enclose) the Smith chart. This can be stated mathematically as

Single Stage Amplifier Design

• Maximum gain with stability can be realized when input and output sections provide a conjugate match between source and load impedance, but generally as a narrowband. To perform this:

• conditions simultaneously have to be satisfied means that also by maximizing the transducer gain, first of all , then should be solved by considering stability conditions. It is also preferable to design for less than the maximum obtainable gain, to improve bandwidth (or to obtain a specific amplifier gain). To do that, Constant Gain Circles on the Smith chart to represent loci of and that give fixed values of gain are used. Besides stability and gain, the Noise Figure of the amplifier should be minimized by using Constant Noise Figure Circles.

Broadband Amplifier Design

• The bandwidth can be improved by designing for less than maximum gain will improve bandwidth, but the input and output ports will be poorly matched. Common approaches to solving this problem are listed below
  • Compensated Matching Network,
  • Resistive Matching Network,
  • Negative Feedback,
  • Balanced Amplifiers,
  • Distributed Amplifiers.

Power Amplifiers

• This is used to increase power level with consideration of efficiency, gain, intermodulation, and thermal effect. Amplifier Efficiency is defined as

• with the effect of input power, Power Added Efficiency, PAE

• where G is the power gain. PAE drops quickly with frequency. Another parameter is Compressed Gain defined as the gain of the amplifier at 1 dB compression gain as

• where G0 is the small-signal (linear) power gain. Class A amplifiers with theoretically maximum efficiency %50 are inherently linear that the transistor is biased to conduct over an entire range of input signal cycles (low-noise amplifier). Class B amplifiers with theoretically maximum efficiency % 78 are biased to conduct only during one-half of the input signal cycle (Push-Pull amplifier). Class C amplifiers with efficiency near % 100 are operated with transistor near cut-off for more than half of the input signal cycle (in a resonant circuit, constant envelope modulation). Higher classes such as D, E, F, and S are also used with high efficiency.

Large Signal Characterization

• If the input power is small enough, the S parameter is independent of input power and the linear small-signal model is suitable for modeling. But for high input powers, transistor behaves as a nonlinear device (Large-Signal Characterization) and more difficult to design. The following methods are possible for Large Signal Characterization
  • Measure the output power as a function of source and load impedances and produce tables, then determine large signal source and load reflection coefficients to maximize power gain for particular output power.
  • Plot contours (Load-Pull Contours) of constant power output on a Smith chart as a function of load reflection coefficient with conjugately matching at input and design for a specified gain.
  • Use the nonlinear equivalent of the transistor circuit.

• Especially for designing Class A amplifiers, the stability can be checked by using the small-signal model because instabilities begin at low signal levels.

 

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