Log Amplifier Symbol
Definition: The Log Amplifier symbol represents an analogue circuit block whose output voltage is proportional to the logarithm of its input voltage or current, used in circuit diagrams to indicate signal compression, wide dynamic-range processing, or true-RMS and decibel conversion.
Also known as: logarithmic amplifier, log amp, translinear log amp, log-ratio amplifier, decibel amplifier.
What the Log Amplifier symbol means
The Log Amplifier symbol denotes an analogue function block that implements the mathematical relationship V_out = K · log(V_in / V_ref), where K is a scaling constant and V_ref is an internal or external reference. Because a bipolar transistor's base-emitter voltage follows a logarithmic relationship with collector current (Ebers-Moll equation), discrete and integrated log amps are built using matched transistors or dedicated ICs such as the AD8304, LOG112, or LT1088.
In schematic diagrams the symbol marks a stage that compresses signals spanning many decades of amplitude into a manageable linear voltage range. This is essential in communications receivers, power meters, and scientific instruments where the signal of interest can vary by 60 dB or more.
How to identify the Log Amplifier symbol
The symbol is drawn as a rectangle or trapezoid functional block labelled 'LOG' or 'Log Amp', with a single analogue input pin (In) on the left and a single output pin (Out) on the right. Some renderings include a curved transfer-function graph inside the block to distinguish it from a linear amplifier. Power supply pins (V+, V−, GND) may be shown but are often omitted for clarity in block diagrams.
Function in a circuit
A log amplifier produces an output voltage proportional to the logarithm of the input signal level. For a decade change in input voltage, the output changes by a fixed number of millivolts (the log slope, typically 200–500 mV/decade for IC versions). This compresses large dynamic ranges into a compact linear output swing, enables multiplication and division by addition and subtraction of log outputs, and allows direct conversion of signal amplitude to decibels (dB) without microcontroller computation.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617 does not define a specific symbol for a log amplifier; it is represented as a general amplifier block (IEC 60617-13, amplifier symbol) with a 'LOG' functional qualifier label inside. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2 / IEEE 315-1975 similarly treats the log amplifier as a general amplifier symbol (a triangle or rectangle) with a 'LOG' or 'f(log x)' qualifier; no dedicated glyph is standardised. |
| Key difference | Both IEC and ANSI rely on a text qualifier to distinguish the log amplifier from a linear amplifier; the underlying amplifier glyph shape is identical in both standards. |
Terminals / pins
| Pin | Name |
|---|---|
| in | In |
| out | Out |
Typical values
Output slope: typically 200–500 mV/decade of input. Dynamic range: 40–120 dB depending on IC. Input range: 1 µA to 1 mA (transimpedance types) or 1 mV to 10 V (voltage-input types). Supply voltage: ±5 V to ±15 V. Bandwidth: DC to several MHz.
Where the Log Amplifier symbol is used
- RF power meters and signal-strength indicators measuring power across a 70–100 dB dynamic range
- Optical power meters for laser and fibre-optic systems requiring linear-in-dB output
- Ultrasonic time-gain-compensation (TGC) amplifiers in medical imaging
- True-RMS AC measurement circuits requiring logarithmic averaging
- Chemical concentration sensors (pH, dissolved oxygen) exploiting the Nernst logarithmic equation
- Audio VCA and compressor circuits using log/anti-log amplifier pairs for gain control
Example
In a 70 dB RF power monitor, an AD8307 log amplifier IC accepts the detected RF input signal at its In pin, produces a log-linear output of 25 mV/dB at the Out pin, and feeds an ADC on a microcontroller. A 1 mW input (0 dBm) produces approximately 1.5 V output; a 1 µW input (−30 dBm) produces approximately 0.75 V, allowing the MCU to calculate power level in dBm directly from the ADC reading without complex computation.
Key facts
- The Log Amplifier symbol represents an analogue block whose output voltage is proportional to the logarithm of the input, implementing V_out = K · log(V_in / V_ref).
- Log amplifiers are built using the Ebers-Moll characteristic of bipolar transistors: the base-emitter voltage V_BE = (kT/q) · ln(I_C / I_S), making transistors the natural element for a log function.
- Typical IC log amplifiers (AD8304, AD8307, LOG112) offer dynamic ranges of 40–120 dB and output slopes of 25–500 mV/decade.
- A log amp output in mV can be directly calibrated in dB: 20 dB per decade of voltage corresponds to one decade of amplitude change.
- Two log amplifiers plus a difference amplifier implement a log-ratio circuit, which measures the ratio of two signals in dB — the core of many power-meter and loss-measurement instruments.
- The inverse function (anti-log or exponential amplifier) is implemented by swapping the transistor and resistor positions; log/antilog pairs are used in true analogue multipliers.
- IEC 60617-13 and IEEE 315-1975 represent the log amplifier as a standard amplifier block with a LOG qualifier label, not a unique glyph.
Frequently asked questions
What does the log amplifier symbol mean in a circuit diagram?
The log amplifier symbol represents an analogue circuit block that produces an output voltage proportional to the logarithm of the input signal. It is used to compress signals with a wide dynamic range (many decades of amplitude) into a compact linear output voltage, enabling decibel-scale measurements and signal processing.
What does a log amplifier look like on a schematic?
The log amplifier appears as a rectangle or trapezoid block labelled 'LOG' or 'Log Amp', with an input pin (In) on the left and an output pin (Out) on the right. It is visually similar to a standard amplifier block but is distinguished by the LOG text qualifier inside the symbol. Some renderings include a curved logarithmic transfer-function curve drawn inside the block.
What is the difference between a log amplifier and a linear amplifier?
A linear amplifier produces an output proportional to the input (V_out = A · V_in), while a log amplifier produces an output proportional to the logarithm of the input (V_out = K · log(V_in / V_ref)). The log amp compresses large variations in input amplitude into a small linear output swing, whereas the linear amp preserves amplitude relationships.
What standard defines the log amplifier symbol?
Neither IEC 60617 nor ANSI Y32.2 / IEEE 315 defines a dedicated glyph for a log amplifier. Both standards represent it as a general amplifier symbol (IEC 60617-13 for amplifiers) with a 'LOG' functional qualifier label inside the block, indicating the logarithmic transfer function.
What is the output slope of a log amplifier?
The output slope defines how many millivolts the output changes per decade (or per dB) of input change. Common IC log amps provide slopes of 25 mV/dB (AD8307) or 200–500 mV/decade. The slope is also called the log conformance constant and determines the sensitivity of the measurement.
What are typical applications for a log amplifier?
Log amplifiers are used in RF power meters and RSSI circuits (measuring power across 70–100 dB), optical power meters for fibre-optic systems, medical ultrasound time-gain-compensation amplifiers, audio compressors and VCAs, and chemical concentration sensors. They are chosen wherever the signal of interest spans many decades of amplitude.
What are common log amplifier ICs?
Widely used log amplifier ICs include the AD8307 (Analog Devices, 500 MHz, 92 dB range, 25 mV/dB), AD8304 (for optical power measurement), LOG112 (Texas Instruments, precision log-ratio), and LT1088 (Analog Devices, wideband). These ICs integrate matched transistors, reference circuits, and temperature compensation on a single die.
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