An amplifier can provide a wide variety of functions such as driving ADCs, drive multiple video loads, operate as video or other types of filter, drive high speed instrumentation signals, and much more. They can also act as oscillators, but that can be actually be a problem since an amplifier should oscillate only when the designer wants it to.
However, an amplifier can have a mind of its own and oscillate at will if the board is designed incorrectly. So what should a designer do to protect against unwanted oscillation? The main thing is to recall the lessons of early electronics classes which taught that oscillation a function of capacitance, inductance, and feedback.
So, the key is to make any extraneous capacitive and inductive feedback paths are reduced or eliminated by designing the board carefully. This is especially important for higher speed amplifiers (greater than 50MHz).
Invisible sources of capacitance and inductance can come from the board, the load (especially if the load is capacitive), and/or the layout. Furthermore, currents flowing to bypass capacitors on different places on the board can take different paths which can lead to distortion.
So, ironically, some techniques for reducing distortion are the opposite of what is recommended to guard against oscillation. (A designer’s job is never easy, is it?) So what are some of the layout considerations to keep in mind to keep everything in balance and fight distortion and oscillation when designing in an amplifier or video filter?
Looking first at oscillation, when directly driving a capacitive load with an amplifier, the load together with amplifier's output impedance creates phase lag which can cause peaking or oscillation.
Some amplifiers have the ability to drive capacitive loads, but for those that cannot, a small series resistance (Rs) placed at the amplifier’s output can enhance stability and settling time (Figure 1 below).
Figure 1: A small series resistance (Rs) placed at the amplifier’s output can enhance stability and settling time.
When driving a transmission cable with an inherent impedance (such as a coax cable) as shown in Figure 2 below, resistors Rs and RL should be set equal to the cable’s impedance (Zo), and the capacitor C should be set to match the cable impedance over a wide frequency range to compensate for the amplifier's increasing output impedance (with increasing frequency).
Figure 2: Driving cable or transmission line.
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