Tektronix Encore

Gain Some Guidance On Amplifier Measurements

March 15, 2017

Amplifiers are widely used in electronic circuits and systems. They raise signal amplitudes as needed, whether to drive low-impedance audio amplifiers or higher-impedance RF/microwave circuits. They are characterized by various sets of measurements, with some for audio amplifiers and some for higher-frequency amplifiers. Evaluating the performance of an amplifier calls for the right tools, and finding the right test equipment is a matter of understanding which measurements must be performed.


Amplifier measurements differ by frequency, with some measurements for audio amplifiers, and different measurements for higher-frequency amplifiers. Audio amplifiers, which are typically specified from 20 Hz to 20 kHz, are typically tested from at least 20 Hz to 50 kHz to account for any harmonics that might be generated by an amplifier. Audio amplifier measurements include frequency response, gain, output power, total harmonic distortion (THD), intermodulation distortion (IMD), signal-to-noise ratio (SNR), crosstalk, and damping factor.


Testing Audio Amplifiers

Whether for measurements at audio or microwave frequencies, a basic amplifier test setup requires a signal generator and an analyzer, with frequency and amplitude performance levels that exceed those of the amplifiers they will be testing. Sometimes, as with the 2700 series of audio analyzers from Audio Precision, the test signal generator and analyzer are in the same package. Each Audio Precision SYS-2722 series audio analyzer contains an analog (sine wave) signal generator with frequency range of 10 Hz to 204 kHz and 2 ppm frequency accuracy and an analog signal analyzer with frequency range of 10 Hz to 500 kHz.


The generator provides more than 26 V signal power to the input of an amplifier under test. The analyzer’s wide frequency range enables measurements of meaningful harmonic and intermodulation signal products generated by an audio amplifier under test. The 2700 series audio analyzers feature amplitude accuracy of ±0.06 dB and amplitude flatness of ±0.008 dB from 20 Hz to 20 kHz. Analysis bandwidths can be measured as wide as 500 kHz and the analyzer maintains low noise level, with residual total harmonic distortion + noise (THD + N) conservatively rated at -112 dB for a 1-kHz bandwidth. The 2700 series audio analyzers provide a small library of IMD test signals, including CCIF, DFD, DIM, DIN, SMPTE, and TIM waveforms, to simplify distortion testing according to different standards.


Testing RF/Microwave Amplifiers

Testing RF/microwave amplifiers also requires a signal generator and an analyzer, although at much higher frequencies and wider bandwidths to cover the frequency ranges of amplifiers used for such applications as wireless communications networks. Output power levels to be tested tend to be much lower at higher frequencies, usually a few watts compared to the hundreds or thousands of watts of power from audio amplifiers. Although many different technologies are used for amplifiers in RF/microwave applications, most can be categorized as low-noise amplifiers (LNAs), which are typically used in communications receivers to boost very low-level signals from an antenna, and power amplifiers (PAs), as used to drive antennas in communications transmitters. The parameters to measure these two types of amplifiers differ somewhat, with noise figure (NF) an important parameter for LNAs which is not normally considered for PAs. Parameters in common for the two amplifier types are those related to output levels, such as gain, output power at 1-dB compression (P1dB), and third-order intercept point (IP3). Some parameters, such as input and output return loss, also known as voltage standing wave ratio (VSWR), are also important for both types of amplifiers.


Generating Test Signals

For versatility, a test signal source for RF/microwave amplifiers should provide suitable bandwidth and frequency range for testing a wide range of amplifiers, such as a frequency range as wide as 100 kHz to 40 GHz in the Keysight Technologies N5183A MXG series of analog signal generators from Keysight Technologies. These signal generators provide +11 dBm standard output power and +19 dBm optional output power to 20 GHz, which is generally enough test signal power to drive an amplifier under test to its rated output power. Amplitude flatness is an important parameter for an amplifier test signal source, and these signal generators offer amplitude that is flat within ±1 dB to 40 GHz for output levels from -10 to +10 dBm.


Analyzing Amp Outputs

For analyzing RF/microwave amplifier performance, a suitable spectrum analyzer can provide the frequency range and dynamic range required to measure most amplifier characteristics at lower power levels (typically less than 1 W). As an example, the wideband Keysight Technologies PSA Series E4440A spectrum analyzer from Keysight Technologies covers a frequency range of 3 Hz to 26.5 GHz with ±0.19 dB amplitude accuracy. It handles input levels as high as +30 dBm (1 W) and can make one-button power measurements. This versatile analyzer also has low noise levels and is available with an optional noise “measurement personality” test module so that it can test LNAs as well as PAs at microwave frequencies.


Don’t Forget Accessories

For cases where high-power amplifiers must be tested at RF/microwave frequencies, additional hardware will be needed, such as an attenuator or directional coupler with suitable bandwidth and power-handling capability, to reduce the relative test signal power presented to the analyzer. Of course, since such components are added to the test signal path, they should have amplitude accuracy/flatness that matches or exceeds that of the test signal source to ensure accurate amplifier gain and amplitude measurements, with low VSWR for optimum impedance matching.


Analyzer Options

As an alternative to a spectrum analyzer for testing power, a power meter such as the Keysight/Agilent 437B, when teamed with a suitable power sensor, such as the Keysight/Agilent 8482B single-channel power sensor from Keysight (1 mW to 25 W, 100 kHz to 4.2 GHz), can measure power levels as high as +44 dBm (25 W) from 100 kHz to 110 GHz.


Meeting Your Requirements

These are a few examples of the test gear required for testing amplifiers, at audio and at microwave frequencies. Browse our website for a wide selection of analyzers and signal generators, keeping in mind the specifics of their own amplifier test applications.

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