Amplifier Selection Criteria for PCB Designers

Everyone is likely familiar with the classic 741 op-amp, especially if you recall your early electronics classes. However, when it comes to specialized applications, the range of available amplifiers is enough to make any designer’s head spin. Once you have an understanding of how different amplifiers quote different specifications, it becomes easier to determine the best amplifier for your application. We’ve compiled a list of important amplifier selection criteria for PCB designers.

Amplifier Classes

All amplifiers are divided into different classes, which determines their usefulness in different applications. Here are 5 common classes of amplifiers:

• Class A. These amplifiers are intended to be highly linear and are always biased on. Therefore, they are not suitable for high power applications as they will consume more power than amplifiers in other classes.
• Class B. These amplifiers were designed as a more efficient alternative to Class A amplifiers. However, because they use FETs, which require some minimum input to switch on the transistors, they do not perfectly reproduce the input waveform and produce some distortion at lower input signal strength. This is known as crossover distortion.
• Class AB. These amplifiers are arguably the most commonly used amplifiers for a wide range of applications. They provide higher efficiency than a Class A amplifier without crossover distortion. They also have comparable linear range.
• Class C. These amplifiers are more often used in RF applications. They can be designed with broad bandwidth thanks to the use of an internal LC tank circuit or other circuitry to provide strong gain at high frequencies. However, they have lower linearity than the aforementioned amplifier classes.
• Class D. These amplifiers use some form of PWM to control the output. The output is converted back to an analog signal with a low-pass filter at the output. These are often used in motor control applications by converting the output to a much higher frequency PWM signal.

Example Class D audio amplifier

Note that there are many other classes of amplifiers with various levels of specialization. No matter which class of amplifier you choose to use, you’ll need to weigh some different specifications for different amplifiers.

Important Specifications for Amplifier Selection Criteria

When selecting an amplifier for working with analog signals, pay attention to the following specifications:

• Open loop and closed loop voltage gain. The open loop gain effectively tells you the maximum gain you can produce with your amplifier. In reality, you will measure the closed loop gain once feedback is applied. Note that this is a function of frequency; a Bode plot of the gain spectrum will resemble that of a low pass filter.
• Linear range. There are several ways to quote this value. The relationship between the input and output signal is never perfectly linear, but it can come close in many applications. This can be specified as a range of input signal levels (usually in dBm) or as a maximum input value with some associated distortion value.
• Dynamic range. This is simply the difference between the smallest and largest possible output values. The lowest value is limited by the noise floor, while the highest is limited by the linear input range. In general, the dynamic range is DR = SNR + 1.
• Bandwidth. For generic amplifiers, this is actually related to the rise time, which is the time required for the circuit to switch (10% to 90%). This will limit the range of useful frequencies in the amplifier (see the note below this list).
• Slew rate. This is the rate of change in the output, usually in V/us or V/ns.
• Common-mode rejection ratio. This is the ability of the amplifier to reject common-mode noise present at both inputs of the amplifier.
• Efficiency. This number is really a statement regarding the amount of power dissipated as heat. A more efficient amplifier dissipates a lower fraction of power as heat.
• Input. Amplifiers can be fully single-ended or fully differential (i.e., differential input and differential output).

All of the above parameters will be a function of the input frequency. Specialized amplifiers will have bandwidth that is specified in certain frequency ranges. Make sure the bandwidth overlaps with the frequency range of interest. There are other important specifications for amplifiers used in specific applications.

Power Amplifiers

All power amplifiers (normally Class B, C, or AB) are designed to run near their nonlinear compression point and will dissipate a significant amount of power during operation. In general, the power output from an amplifier will decrease as temperature increases; high quality stable amplifiers should provide less than 1 dB decrease in power output across the entire range of operating temperatures. Other specifications should exhibit similar stability.

When selecting a power amplifier, whether intended for specific applications or in general applications, the previously listed points should still be considered. However, power amplifiers have evolved for different applications, and the specifications listed for different amplifiers are adapted to designers working with these specialized applications. One excellent example is in RF power amplifiers, where amplifiers for different frequency bands are based on different semiconductor processes.

The inherent nonlinearity in these amplifiers will lead to some unintended effects during operation. Designers from the audio community are likely familiar with total harmonic distortion (THD) or total harmonic distortion plus noise (THD+N). Harmonic distortion is a nonlinear effect, where higher order harmonics of the desired signal are present at the output. Your power amplifier should have the lowest possible THD or THD+N level (normally expressed as a percentage).

Power amplifiers for working with frequency modulated signals usually specify distortion in terms of the third-order intercept point (3OIP). The nonlinear nature of power amplifiers will generate higher order harmonics and intermodulation products, which arise due to nonlinear frequency mixing among different frequencies in a frequency modulated signal. These intermodulation products appear as sidebands in the output spectrum of the amplifier. This distortion level due to nonlinearity is also quoted as intermodulation distortion (IMD) outside the RF community.

Example OIP3 extrapolation in a power amplifier for frequency modulated signals.

Although there are many possible intermodulation products, the odd-order products are the most important as they lie closest to the frequency range you are working with. The third-order intermodulation products lie closest to the desired frequencies, followed by the fifth, seventh, and so on. 3OIP is normally quoted as an input power value at which the intensity of the third-order intermodulation products will have the same output intensity as the desired signal.

Octopart is here to give you access to a huge range of general-purpose and specialty amplifiers components for your next system. If you are unsure which amplifier you need, try using our Part Selector guide to determine the best option for your next product.

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