Power Amplifier Oscillation: Parasitics and Effects on Amplifiers
Oscillation is universally ambiguous regardless of the stage. The term itself means, in essence, back and forth and is also referred to as vibrations or transitioning between two states. Overall, in each instance, this generally equates to repetitive variations concerning a measure of time, and it usually correlates to a point of equilibrium.
For example, if you are watching your favorite college football team and they score a touchdown, you experience an emotional high. However, if that same team’s quarterback throws a pick-six on their next offensive possession, you experience an immediate emotional low followed by yelling expletives at your TV screen. That is the equivalent of oscillation because you went from one emotional state to another, and the completion of this cycle of oscillation would occur when you experienced the next emotional high.
Like in life, oscillations happen in the field of electronics. Some oscillations are a natural part of a device's functionality, whereas with other devices it is an unwanted phenomenon. For example, in RF power amplifiers, there is a type of oscillation called parasitic oscillation. This particular type of oscillation falls into the category of unwanted phenomena.
What is Oscillation?
In summary, oscillation is the repetitive variation, ordinarily within a measurable amount of time, with reference to a central point or value equilibrium. Also, this includes the transitioning between two or more different states, usually opposites. As I alluded to earlier, the term vibration is another word that precisely describes mechanical oscillation. Furthermore, two of the most widely seen examples of oscillation are alternating current (AC) and a swinging pendulum.
However, oscillations occur all around us, not just in mechanical systems but also in dynamic systems and nearly every field of science. Other desired examples of oscillation include the human heartbeat, geothermal geysers, the vibration of strings in string instruments, the firing of nerve cells in the human brain, and Cepheid variable stars that essentially experience fluctuations in their brightness.
As listed above, there are circumstances in which oscillations are not only welcome but also a requirement for proper functionality, as in the human heart. However, like in most things, there is an equal but opposite that exists. In regards to oscillation, this opposite I speak of is called parasitic oscillation. In terms of RF power amplifiers, parasitic oscillation is persona non grata.
Undesired Oscillations in Power Amplifiers
An RF power amplifier, for example, is a type of electronic amplifier that converts a low-power RF signal into a higher power signal. In general, an RF power amplifier drives the antenna of a transmitter. Overall, the design goal here includes power output, gain, power efficiency, bandwidth, linearity (production of accurate copies of the input), input and output impedance matching, and proper heat dissipation.
However, as in the case with all electronic components and devices, amplifiers will perform and function as intended, until there is a disruption in their functionality. One such disruption can occur when there is parasitic oscillation present in the circuits or components of the power amplifier. Furthermore, these types of disruptions (parasitic oscillation) are more critical and noticeable at higher frequencies. So, what causes these oscillations? Well, there are multiple answers to this single question, and I will discuss this in more detail in the following paragraphs.
What Causes Oscillations in Power Amplifiers?
The occurrence of oscillations in power amplifiers, in many cases, occurs at higher frequencies, high gains, and poor design layout. Also, one of the first steps in assessing oscillations is to measure the amplitude and frequency of the oscillation. If you are using an oscilloscope, you can stabilize the waveform using the oscilloscope’s internal trigger.
In terms of causes, usually oscillations around the 3dB bandwidth are a product of input-output feedback. Keep in mind that when dealing with higher frequency oscillations, you will need to use a spectrum analyzer to observe them. Furthermore, oscillations of this type generally cause waveform distortion and are usually in the range of hundreds of Megahertz to Gigahertz.
Moreover, this is frequently and, ultimately, the byproduct of parasitic oscillations by the device's output or input.
Potential layout of an amplifier circuit can vary greatly.
Parasitic Oscillations in Power Amplifiers
As discussed previously, parasitic oscillation is an unfavorable electronic oscillation, or cyclical variations in output current or voltage, in an electronic power amplifier or digital device. Also, you can usually attribute its cause to feedback in an amplification device. Furthermore, as stated earlier, the issue occurs more notably in RF power amplifiers, audio amplifiers, and various other types of electronic amplifiers. However, this is not limited to just amplifiers; oscillations of this type can also occur in digital signal processors as well.
Moreover, parasitic oscillation is one of the central issues that control theory addresses. As you may know, control theory deals with the behavior of dynamical systems within the fields of mathematics and engineering. Furthermore, control theory outlines the necessary conditions for a system to maintain a controlled output while undergoing the effects of various inputs. Although its use was initially in automation and robotics, it is now serving in fields such as genomics, sociology, and physiology for regulatory processes.
There are numerous reasons why parasitic oscillations are undesired. One of those reasons is, it can cause EMI for other devices in the circuit. This is because parasitic oscillations can also radiate as radio waves that affect other circuits. If you have ever heard annoying pitch sound coming from say your car speakers or headphones, you were witnessing parasitic oscillation in an audio system.
Reasons Why Parasitic Oscillations are Unwanted
In power amplifiers, one of the critical reasons why parasitic oscillations are unwanted is because it wastes power, which in turn, generates heat. Furthermore, a circuit experiencing parasitic oscillation will not amplify linearly, which causes distortions in the desired signal.
Concerning digital circuits, parasitic oscillations may only occur on specific logic transitions, and the result is the erratic operation of subsequent stages. An example of this would be a counter stage that will possibly see many counterfeit pulses and count erratically.
Overall, parasitic oscillations are a disruptive force that deters circuit functionality and inhibits performance. In many cases, it halts functionality entirely, and thus you should take measures to mitigate its effects.
Mitigating Parasitic Oscillations
There are several measures one can take to reduce or even prevent parasitic oscillation. In reference to power amplifiers, it starts with the layout of the amplifier’s circuits, i.e. design. Their design should include a plan that features input and output wiring that is not adjacent. This, in turn, prevents capacitive or inductive coupling.
In regards to circuits themselves, you can use metal shielding over the sensitive portions of the circuit. Also, the use of bypass capacitors at power supply connections is favorable. This will prevent interstage coupling through the power supply and provide a low-impedance path for AC signals.
Concerning PCBs, you should separate the high-power and low-power stages. Furthermore, you should arrange the ground return traces in a manner that prevents substantial current flows in the shared portions of the ground trace. However, in certain circumstances, the issue may only be rectified by the introduction of another feedback neutralization network. Moreover, this feedback neutralization network needs calculations and adjustments that eliminate the negative feedback within the passband of the amplifying device.
Ensuring proper trace routing will prove helpful for any amplifier design.
When attempting to solve issues with oscillation, you must first locate the source. I know this sounds like an easy home run hit out of the park in underhand pitch softball, but there are some devious curveballs at play. In regards to power amplifiers, the source could just as easily be the amplifier itself or components and leads not within the amplifier.
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