What Is a Logarithmic Amplifier?

A logarithmic amplifier refers to an amplifier circuit whose output signal amplitude and input signal amplitude have a logarithmic function. Converting a signal to its equivalent logarithm involves a non-linear operational amplifier.

The actual log amplifier always has both linear and log amplification functions. When the input signal is weak, it is a linear amplifier with a large gain. When the input signal is strong, it becomes a log amplifier. Decrease. Logarithmic amplifiers play a particularly important role in radar equipment. It can not only ensure a wide dynamic range of the radar receiver, but also limit the level of clutter interference output by the receiver to achieve the effect of constant false alarm. For monopulse radars (see tracking radars), the angular error signal can also be normalized; for moving target display radars, fixed target fluctuations can also be suppressed.

In many literatures, the classification of logarithmic amplifiers is also quite confusing. Depending on the basis for implementing the logarithmic function, some divide them into diodes, triode logarithmic amplifiers and cascaded logarithmic amplifiers, and some divide them into true ones. Logarithmic and log-like amplifiers, etc. But for decades, with the development of semiconductor theory, technology, and analog integrated circuits, many methods of logarithmic amplifier implementation have been eliminated, and their classification methods have not been scientific. At present, logarithmic amplifiers can be classified into three categories according to the structure and application field of the logarithmic amplifiers on the market: basic logarithmic amplifiers, baseband logarithmic amplifiers, and demodulated logarithmic amplifiers.

Basic logarithmic amplifier is also called translinear logarithmic amplifier. It is based on the logarithmic characteristics of the bipolar triode (BJT) to realize the logarithmic transformation of the signal. This type of logarithmic amplifier can respond to slowly changing input signals. It is characterized by excellent DC accuracy and very wide dynamic range (up to 180dB). The disadvantage is poor AC characteristics.

The baseband logarithmic amplifier is also called a video logarithmic amplifier (although it is rarely used in video display related applications). It overcomes the shortcomings of the basic logarithmic amplifier and can respond to rapidly changing inputs. The principle is to use a "stage-by-stage compression" technology, which has good AC characteristics but small dynamic range.

The demodulated logarithmic amplifier is also called stepwise detection logarithmic amplifier. It has piecewise linear approximation properties. After forming a logarithmic cascade, a good logarithmic transfer function can be obtained. The logarithmic accuracy is high throughout the dynamic range. The baseband logarithmic amplifier is similar. It also uses multiple cascaded linear amplifiers with a large dynamic range.

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noise

All signal processing systems are limited by random noise, which sets a threshold that can be detected or identified for the smallest signal. Random noise is closely related to the bandwidth of the signal input. Random noise is often defined by the "noise spectral density (SND)". The total noise power is proportional to the system's noise bandwidth BN (expressed in Hz). In a linear system, the output noise power N is related to the bandwidth of the system. The bandwidth here usually refers to the 3dB bandwidth. For an ideal low-pass system, the 3dB bandwidth is the equivalent noise bandwidth of the system. In a non-linear system, such as a logarithmic amplifier, the situation is different. Even small noise at the input end will cause the overload of the amplifier's final stage. Therefore, the main disadvantage of the logarithmic amplifier is that it will reduce the signal-to-noise ratio of large signals.

Dynamic Range

The lower end of the system's dynamic range is limited by noise within the range that can guarantee measurement accuracy, while the upper end of the signal range is affected by the non-linearity of the amplifier.

Log consistency error

After eliminating the reference current error and the offset component, the difference between the actual voltage value presented at the output of the logarithmic amplifier and the ideal value calculated by the transmission characteristic equation is called the log consistency error. It is closely related to the dynamic range, frequency characteristics and temperature of the device ^[2] .