A glossary of signal amplifier design terms and related theory

Amplifier. A device that takes in an electrical signal and outputs a magnified facsimile, broadly speaking there are two types:
  1. Small signal amplifier. Generally referred to as "voltage" amplifier as it converts a small input voltage into a much larger output voltage. The SPA series of external amplifier modules available from DataQuest Solutions provide are a good example of this and are often used as pre-amplifiers to magnify voltage before the main amplifier stage, such as that found on PC-based high frequency signal capture cards, e.g. the Spectrum M2i, M3i & M4i series.

  2. Large signal amplifier. Often referred to as a "power" amplifier, this can deliver heavy load currents at their output. A small voltage may first be amplified using a small signal amplifier, or the power amplifier can used alone where less voltage magnification is required but where high current is still paramount. Typical output loads include loudspeakers and motors.
3dB bandwith graph Bandwidth. The range of signal frequencies an amplifier can handle within specified bounds of gain, distortion, efficiency, etc. For signal amplifiers boundaries are frequently defined as the upper and lower frequency -3dB point, also called the half-power point, which is -3dB down from maximum (with 0dB as the maximum output value). In voltage amplitude terms it is 70.7% of the maximum amplitude gain.

BNC (Bayonet Neill-Concelman) connector. A common type of RF connector used for coaxial cable. Panel mounting versions also available as utilised on the SPA series of amplifier modules. Made to match the characteristic impedance of shielded cable at either 50 ohms or 75 ohms so as to avoid signal reflections being generated. Typically specified for use at frequencies up to 4 and 2 GHz respectively.

Coupling (AC). Where a capacitor is used to block the DC bias on a signal. Placed in series in the signal path a value is chosen to allow all frequencies of interest to pass with minimal attenuation. Often utilised in the connection of two amplifier circuits with differing bias levels. Also know as a high-pass filter.

Coupling (DC). Where a capacitor is used to block higher frequencies. Placed between the signal line and ground, it can be used to attenuate unwanted interference picked up along signal cables from radio stations etc. Also known as a low pass filter.

Decibel. A logarithmic measure of two signals as a ratio, either in terms of voltage current or power. One decibel (dB) is one tenth (1/10th) of a Bel, the Bel being too large a unit of practical measurement; it is hence prefixed with deci. One advantage of using decibel over arithmetic ratios or percentages is that the total decibel gain of a series of components (such as amplifiers and attenuators) can be calculated simply by summing the decibel gains of the individual components. (See also term: Gain)

Distortion. Where (aside from intended amplification) the output signal may not be an exact reproduction of the original input signal waveform. This may be due to a number of reasons including:
  1. The input signal may be too large, causing the amplifier to exceed it range limit.
  2. Amplification may not be taking place over the whole signal cycle due to the signal exceeding the output range of the amplifier due to incorrect biasing /DC offset at the input.
  3. The amplification may not be linear over the entire frequency range of inputs.
Distortion can be due any of the following elements or their combination.

clipping distortion plot Amplitude distortion. Typified by "clipping" where the peaks on the amplified signal are seen to flatten, this may due either due to incorrect DC biasing and/or over driving of the input.

A DC blocking capacitor can be applied in the former case (keeping in the mind the attenuation effect it will have on the frequency components of the signal). Commercial amplifiers units, such as those of our SPA series may have such a coupling capacitor selectable by switch, carefully selected to match the bandwidth specification of the amplifier. In the case of over driving of the input the simple fix is by a reduction of input signal amplitude.

Frequency distortion. This occurs when the level of amplification varies with frequency, most notably due to the amplifiers bandwidth. However, the output waveform can become distorted if harmonic frequencies significantly increase in amplitude and combine with the fundamental frequency. Normally, the amplitude of these harmonics is a fraction of the fundamental amplitude and therefore has very little or no effect on the amplified output waveform.

Phase distortion. The time delay between the signals fundamental and harmonics can change depending on the amplifiers capacitive and inductive reactance. The resulting combination of these delays i.e. phase shifts will change the appearance of output signal. This distortion will depend on the construction of the amplifier and will increase progressively within its frequency bandwidth.

Crossover distortion. Where an amplified signal travels above and below the zero volts point a transition point occurs where one part of the amplifier circuit takes over form the other resulting in "flat spot" or "dead band".

crossover distortion oscilloscope trace

This is typical of simple push-pull Class B amplifiers and can produce high levels of crossover distortion due to their cut-off point biasing. Most commercially developed amplifiers will have more sophisticated design to eliminate such distortion through input stage pre-biasing etc.

See also the term "oscillation" below.


Gain. This is the degree of "magnification " that an amplifier provides its input signal. It can be expressed as a ratio, hence: -
 
Voltage gain: Av = Voltage out / Voltage in
Current gain: Ai = Current out / Current in
Power gain: Ap = Power out / Power in (or alternatively Av * Ai)

Then converted to decibels (dB) by applying these ratios into the appropriate formula: -
 
Voltage Gain in dB: = 20 log (Av)
Current Gain in dB: = 20 log (Ai)
Power Gain in dB: = 10 log (Ap)

Note that the DC power gain of an amplifier is equal to ten times the common log of the output to input ratio.

Oscillation. Typified be the uncontrolled swinging of signal amplitude at certain frequency, this may be caused by the amplifier itself, the cables or by external electrical components and produces distortion in the output signal. Within the 3dB bandwidth of the amplifier it is most commonly caused by output feedback of the signal to the input and parasitic oscillation is a term frequently used in such a situation. Since the ground and supply rail of an amplifier is common to both the input and output stages of an amplifier, output current flowing through the impedance of these connections can also couple signals back to the input, so careful circuit design is required. Output cable(s) running close to the input can introduce feedback via capacitive or inductive coupling. External cables should have good shielding to help avoid this. Connector and cable impedances should match to avoid reflectance of the output signal back along the conductor.

Roll-off. Loss of gain, usually expressed as decibels per octave (dB/octave) or decade (dB/decade) where an octave is a doubling of frequency and a decade is ten times. Note that roll-off can occur with decreasing as well as increasing frequency, although most amplifiers frequency response resembles that of a low-pass filter, that is the amplifier gain attenuates with increasing frequency. This is usually shown graphically as a logarithmic bode plot.

amplifier roll_off logarithmic bode plot Voltage offset. It is frequently advantageous to be able to shift signal voltage levels by the aid of trimmer potentiometer so that the input signal best fits within the amplitude range of the amplifier, or in the case of the output best fits the next instrument or amplifier range. Our SPA series being primarily pre-amplifiers, have a trimmer for such a purpose on the output. It is also a measure of the ideal amplifier as when the input connection is shorted the output should be as close as possible to virtual ground (Vout = 0V) and an input offset adjustment may help compensate for this.

Voltage drift. The undesirable effect of changes in temperature and long period time. In the former case the drift is stated in microvolts per centigrade (uV/C). All amplifiers exhibit drift, however a good amplifier only exhibits only microvolt change. These measurements are referred to the amplifier inputs, thus removing the need to account for amplifier gain.

Voltage noise. Amplifiers generate random voltage at the output even when there is no signal applied. This can be due to thermal noise and flicker noise of the devices. For applications with high gain or high bandwidth, noise becomes a very important consideration. Although noise generated by the amplifier appears at the output its measured value for datasheets is described as input voltage noise, as it is considered as equivalent noise at the input as though it is being generated at that point. Noise in any system is dependent on bandwidth of the system and this must be specified when defining noise performance. It is common practice to define noise for a 1Hz bandwidth and specified in nanovolts per square root of Hertz (nV/√Hz). Over a limited bandwidth, noise power can be considered to be proportional to bandwidth and noise voltage proportional to the square root of bandwidth. The level of noise generated by an amplifier system generally varies over a wide spectrum of input frequencies.

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© Dataquest Solutions Ltd. 31.01.2012
Last modified 18.06.2015