7.6 Differential Amplifers

Chapter 7.6 Differential Amplifers

Radio Frequency Integrated Circuit Design Second Edition Book
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Radio Frequency Integrated Circuit Design Second Edition Book

  • 7.6 Differential Amplifiers 215load impedance is not fully known for all frequencies, and hence making the circuit unconditionally stable would be recommended. For example, for a power ampli-fier or LNA connected to a 50W antenna, it can happen that a potential stability problem is identified away from the operating frequency, but only for an impedance away from 50W. Even though the antenna may be 50W at the operating frequency, instability is a real possibility since the antenna impedance at the other frequencies is expected to be quite different. Techniques to improve stability will depend on the frequency, impedance, and location of stability problems as identified by the stability factors over frequency, input and output stability circles, observation of time-domain oscillations, and dis-continuities in the maximum gain plots. Techniques to improve stability can include adding small series resistance or large parallel resistance at the input or output. We note that real series inductors with finite Q can improve stability because of their series resistance, while the use of ideal inductors can make stability problems worse. Also, frequency selective networks may work if the problem frequencies are away from the operating frequency. For example, highpass or lowpass matching networks may be used. The use of a cascode structure can improve reverse isolation (feedback) and this can help with stability; however, it is important to have the base of the cascode transistor (or the gate of a CMOS cascode transistor) at low imped-ance, for example, bypassed with a decoupling capacitor; otherwise, the cascode structure can degrade stability.7.6  Differential AmplifiersAny of the amplifiers that have already been discussed can be made differential by adding a mirrored copy of the original and connecting them together at the points of symmetry so that voltages are no longer referenced to ground, but rather swing plus or minus relative to each other. This will be illustrated in the following sec-tions. The advantage of differential amplifiers and differential circuits in general is that the positive and negative signals are referenced to each other instead of to ground. As a result, the connection to the off-chip ground is less critical and as a result packaging is less important. As well, noise coupled from the substrate or from external sources often appears roughly equally on positive and negative nodes; hence, for signals that are processed differentially, such noise is reduced. Finally, the signal swing can be increased in differential circuits since either side can swing positive and negative from the nominal operating point. On the negative side, dif-ferential circuits typically require more transistors and more current as there are two signal paths. As well, since the positive signal is referenced only to the nega-tive signal, in a high-gain circuit additional circuits called common-mode feedback circuits are used to ensure that the operating point or average value of the signals is correct. 7.6.1  Bipolar Differential PairWhile this is hard to describe, it is easy to show an example of a differential common- emitter amplifier (more commonly called a differential pair or emitter-coupled pair)