What Is a Logarithmic Amplifier?
A logarithmic amplifier is an amplifier that outputs a logarithmic conversion of an input signal. As it compresses the input signal and then outputs it, logarithmic amplifiers find application in situations where handling signals with a broad input range is necessary. Since the frequency and temperature characteristics vary depending on the configuration of the logarithmic amplifier, it is necessary to select the appropriate configuration for the application. An amplifier that restores the signal compressed by a logarithmic amplifier is called an anti-log amplifier (inverse logarithmic amplifier).
Uses of Logarithmic Amplifiers
Logarithmic amplifiers can be used, for example, to compress an analog signal with a wide dynamic range that varies by digits to fit the input range of the ADC when the analog signal is converted to a digital signal by an A/D converter (ADC). For example, logarithmic amplifiers are used to adapt a signal whose amplitude variation exceeds 100dB to a standard operational amplifier or ADC with a dynamic range of 60dB to 100dB. Logarithmic amplifiers are used in many systems, including communication systems, measurement systems, and acoustic systems.
Principle of Logarithmic Amplifiers
Logarithmic amplifiers are configured to provide high gain for low-level signals and progressively lower gain for higher-level signals. There are two main types of logarithmic amplifier configurations. These are the multi-stage logarithmic amplifiers and the DC logarithmic amplifiers.
In a multistage log amplifier, several stages of linear amplifiers are connected in series, and the outputs of each stage are added together. Low-level signals are amplified sequentially by several stages of linear amplifiers to achieve high gain. As the signal becomes higher level, a logarithmic output characteristic can be obtained by controlling how many stages of linear amplifiers are added up to the output of the linear amplifiers.
DC logarithmic amplifiers are achieved by using a diode in the feedback of the inverting amplifier circuit of an operational amplifier. Instead of a diode, a diode-connected transistor can be used. By using the logarithmic portion of the voltage and current characteristics of the diode or transistor, a logarithmic output can be obtained.