Why logarithmic amplifier

However the RC time constant of the low pass filter determines the maximum rise time of the output. Setting the corner frequency too low will result in the log amp having a sluggish response to a fast-changing input envelope. The ability of a log amp to respond to fast changing signals is critical in applications where short RF bursts are being detected.

The upper figure shows the response of the AD to a short MHz burst. The table below compares the rise times and other important specifications of different Analog Devices log amps. Now take a look at the lower figure. This shows you what will happen if the frequency of the input signal is lower than the corner frequency of the output filter. As might be expected, the full wave rectified signal appears unfiltered at the output. However this situation can easily be improved by adding additional low-pass filtering at the output.

I notice that there is an unusual tail on the output signal in the lower figure. What is causing that? That is an interesting effect that results from the nature of the log transformation that is taking place. Looking again at transfer function plot i. That is what is expected from a logarithmic amplifier. However, looking at the input signal i. What's happening in the figure is that the burst does not turn off instantly but drops to some level and then decays exponentially to zero.

And the log of a decaying exponential signal is a straight line similar to the tail in the plot. Is there a way to speed up the rise time of the log amp's output? This is not possible if the internal low-pass filter is buffered, which is the case in most devices. The 5-pF capacitance in parallel with the In the figure, an external 1. Now, the overall load resistance is reduced to around 1. This will decrease the rise time ten-fold.

However the overall logarithmic slope has also decreased ten-fold. You may also want to take a look at the Application Note AN This shows how to improve the response time of the AD Returning to the architecture of a typical log amp, is the heavily clipped signal at the end of the gain chain in any way useful? The signal at the end of the linear gain chain has the property that its amplitude is constant for all signal levels within the dynamic range of the log amp.

This type of signal is very useful in phase- or frequency demodulation applications. Remember that in a phase-modulation scheme e. QPSK or broadcast FM , there is no useful information contained in the signal's amplitude; all the information is contained in the phase. Indeed, amplitude variations in the signal can make the demodulation process quite a bit more difficult. So the signal at the output of the linear gain chain is often made available to give a limiter output.

This signal can then be applied to a phase or frequency demodulator. The degree to which the phase of the output signal changes as the input level changes is called phase skew.

Remember, the phase between input and output is generally not important. It is more important to know that the phase from input to output stays constant as the input signal is swept over its dynamic range. The figure shows the phase skew of the AD's limiter output, measured at MHz. I noticed that something strange happens when I drive the log amp with a square wave. Log amps are generally specified for a sine wave input.

The effect of differing signal waveforms is to shift the effective value of the log amp's intercept upwards or downwards. Graphically, this looks like a vertical shift in the log amp's transfer function see figure , without affecting the logarithmic slope. The figure shows the transfer function of the AD when alternately fed by an unmodulated sinewave and by a CDMA channel 9 channels on of the same rms power.

The output voltage will differ by the equivalent of 3. The table shows the correction factors that should be applied to measure the rms signal strength of a various signal types with a logarithmic amplifier which has been characterized using a sinewave input. So, to measure the rms power of a squarewave, for example, the mV equivalent of the dB value given in the table In your datasheets you sometimes give input levels in dBm and sometimes in dBV. Can you explain why? Signal levels in communications applications are usually specified in dBm.

The dBm unit is defined as the power in dB relative to 1 mW i. Since power in watts is equal to the rms voltage squared, divided by the load impedance, we can also write this as. Because impedance is a component of this equation, it is always necessary to specify load impedance when talking about dBm levels.

Log amps, however fundamentally respond to voltage, not to power. The input to a log amp is usually terminated with an external W resistor to give an overall input impedance of approximately 50 W, as shown in the figure the log amp has a relatively high input impedance, typically in the W to W range. If the log amp is driven with a W signal and the input is terminated in W, the output voltage of the log amp will be higher compared to the same amount of power from a W input signal.

As a result, it is more useful to work with the voltage at the log amp's input. An appropriate unit, therefore, would be dBV, defined as the voltage level in dB relative to 1 V, i. However, there is disagreement in the industry as to whether the 1-V reference is 1 V peak i. Most lab instruments e. Based upon this, dBV readings are converted to dBm by adding 13 dB. So dBV is equal to 0 dBm. As a practical matter, the industry will continue to talk about input levels to log amps in terms of dBm power levels, with the implicit assumption that it is based on a 50 W impedance, even if it is not completely correct to do so.

As a result it is prudent to provide specifications in both dBm and dBV in datasheets. He has worked at Analog Devices for 30 years in various field and factory roles covering mixed-signal, precision, and RF products. He is currently focused on RF amplifiers and beamformer products for satcom and radar. He holds a Bachelor of Engineering B.

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Edit Save. Create your myAnalog password, and log in to access this article. Confirm Password. MAR Part Number. Input Bandwidth. Dynamic Range. Log Conformance. Limiter Output? Signal Type. Square Wave or DC. Triangular Wave. GSM channel all time slots on. This section discusses about the op-amp based logarithmic amplifier in detail.

An op-amp based logarithmic amplifier produces a voltage at the output, which is proportional to the logarithm of the voltage applied to the resistor connected to its inverting terminal. In the above circuit, the non-inverting input terminal of the op-amp is connected to ground. That means zero volts is applied at the non-inverting input terminal of the op-amp.

According to the virtual short concept , the voltage at the inverting input terminal of an op-amp will be equal to the voltage at its non-inverting input terminal. So, the voltage at the inverting input terminal will be zero volts. Observe that the left hand side terms of both equation 1 and equation 3 are same. An anti-logarithmic amplifier , or an anti-log amplifier , is an electronic circuit that produces an output that is proportional to the anti-logarithm of the applied input.

This section discusses about the op-amp based anti-logarithmic amplifier in detail. An op-amp based anti-logarithmic amplifier produces a voltage at the output, which is proportional to the anti-logarithm of the voltage that is applied to the diode connected to its inverting terminal.

In the circuit shown above, the non-inverting input terminal of the op-amp is connected to ground.