Clive Poole

Microwave amplifier design

Whether characterized as low noise, high power, wideband, or otherwise, all electronic amplifiers have in common the defining characteristic of providing finite positive Power Gain at the frequency, or range of frequencies, of interest. The superficially simple concept of power gain, that is, that the signal power delivered to the load exceeds the signal power supplied to the input of the amplifier, is actually a little tricky to define in practice, as there is more than one way to define “power in” and “power out.” This results in a number of different definitions of power gain as explained in Chapter 7.

A simplified approach to predicting negative resistance in a microwave transistor with series feedback

This paper addresses the question of how to predict the series feedback termination required to generate negative resistance in a microwave transistor in different circuit configurations. Although this question is usually approached through computer modelling and simulation, we present here a simple closed form equation that predicts the value of feedback reactance most likely to generate negative resistance at the input of a 2-port sub-circuit comprising a transistor plus series feedback, based solely on the 2-port S-parameters of the transistor. This simple equation provides new insight into the effect of series feedback on a transistor and leads us to propose a general rule, namely that the series feedback reactance required to generate negative resistance in a given transistor in a given configuration depends on the sign of the imaginary part of the sum of the 2-port S-parameters for the device in that configuration.

Chapter 6 – S-parameters

The use of linear immittance parameters to represent active andpassive circuit elements was introduced in Chapter 5. As we increase the frequency of operation, the limitations of immittance parameters, which are ratios of measured voltage and current, become increasingly apparent. The most obvious limitation relates to the fact that immittance parameters are measured with open- and short-circuit terminations, which become increasingly difficult to implement accurately as the frequency range increases. In Section 2.2, we introduced the concept of reflection coefficient, being the ratio of reflected voltage wave to incident voltage wave at any discontinuity on a transmission line. It turns out that reflection coefficients are much easier to measure at microwave frequencies than static voltages and currents, so would not it be better if we had a method of characterizing multi-port networks based on some reflection coefficient-like parameters?

Chapter 17 – Microwave mixers

The ability to translate a band of frequencies from one part of the frequency spectrum to another is an essential function within almost all radio frequency (RF) and microwave systems, whether it be radio transmitters, receivers, or test equipment. This frequency translation function, also known as “mixing,” or “heterodyning,” is carried out by a circuit called a mixer.

Chapter 5 – Immittance parameters

One widely used approach to network analysis and design is to treat the particular component or subcircuit of interest as a two-port network. Under this regime, the component or subcircuit is treated as a black box where only the input and output ports are accessible (hence the name “two-port”). In the absence of any information about the internal composition of the black box, we can fully characterize it in terms of relationships between measured signal currents and voltages at the external terminals, assuming the circuitry inside the box is linear with no independent current or voltage sources.

Low-noise oscillator design

When we covered oscillators in Chapter 15 we postulated that, at power on, oscillations originate from noise perturbations inherent in the various components, that then get amplified by the gain element, resulting in a signal that builds up in amplitude and is pulled toward the oscillator design frequency by a frequency selective element (the resonator or tank circuit). Aside from the consequent, profound observation that noise is essential to initiating oscillation, this description also implies that the gain element is inherently nonlinear. The reasoning is as follows: Although the Barkhausen amplitude criterion of equation (15.2.4) implies unity loop gain in steady-state conditions, if the oscillator did have unity loop gain for all signal levels then the initial noise perturbations would never get amplified sufficiently to build up to the final signal amplitude. Notwithstanding the Barkhausen criterion, the effective loop gain must be significantly greater than unity at very low signal levels, reducing to exactly unity as the signal amplitude approaches the steady-state level. Likewise, under steady-state conditions, if the signal amplitude should drop for any reason, the higher loop gain at lower amplitudes, due to this loop gain nonlinearity, will drive the amplitude back up to a steady-state level, which is limited, at its upper bound, by the power supply voltage. In short, it is the inherent nonlinearity of the gain element (i.e., high gain at low signal levels and low gain at high signal levels) that primarily acts to maintain the oscillator output signal at a constant amplitude. This fundamental amplitude stabilization mechanism in oscillators was first described by Lord Rayleigh as early as 1896.

Three-port analysis techniques

We established in Chapters 5 and 6 that we can use S -parameters to represent any linear network with an arbitrary number of ports, n . It may seem odd, therefore, to devote an entire chapter to the specific case of three-port networks. The reason for this chapter is that the three-port representation has a particular significance in microwave active circuit design since many designs involve applying feedback to transistors, which are three terminal devices. A body of specialist design techniques has been developed, based on the three-port S -parameter representation of a transistor, that, due to their elegance and power, are worth covering in some detail in a chapter of their own.

Transmission line theory

To someone with a superficial knowledge of electrical technology it seems obvious that when two physically separated points are connected using a length of conducting wire, and assuming the resistance of the wire can be ignored, the voltage at the remote end of the wire will be the same as that at the source. In other words, the physical length of the wire and physical location of the source and destination, again ignoring resistive effects, are immaterial to understanding of how the circuit works.

Microwave semiconductor materials and diodes

The reader familiar with low-frequency active devices, fabricated in silicon (Si) or germanium (Ge) material, will be struck by the much wider range of semiconductor materials and exotic semiconductor devices that are encountered at RF and microwave frequencies.

Low-noise amplifier design

So far we have largely ignored the presence and effects of electrical noise in the circuits we have been designing. Noise is always present in any electronic or microwave system, however, and can often be the deciding factor in assessing system performance. In this chapter and Chapter 16, we will specifically address the problem of how to design amplifiers (this chapter) and oscillators (Chapter 16) for optimum noise performance.