# Logarithmic amplifier thesis

Basically there are two log amp configurations: Opamp-diode log amplifier and Opamp-transistor log. In this configuration a transistor is placed in the feedback path of an opamp wired in inverting mode.

The necessary condition for a log amp to work is that the input voltage must be always positive. 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.

I've just been reading datasheets of some recently released Analog Devices log amps and I'm still a little confused about what exactly a log amp does.

The table below compares the rise times and other important specifications of different Analog Devices log amps.

To understand how this signal transformation yields the log of the input signal's envelope, consider what happens if the input signal is reduced by 20 dB. Take a look a the scope photo below.

The upper figure shows the response of the AD to a short MHz burst.

## Logarithmic detector

What is causing this? Collector of the transistor is connected to the inverting input of the opamp, emitter to output and base is grounded. Well, I suppose that I would expect to see an output proportional to the logarithm of the input voltage or current, as you describe in the Nonlinear Circuits Handbook and the Linear Design Seminar Notes. The table below compares the rise times and other important specifications of different Analog Devices log amps. Anti-Logarithmic Amplifier 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 is costing me dynamic range at the low end. The focus of this discussion, however, is on envelope-detecting log amps, also referred to as demodulating log amps, which have interesting applications in RF and IF circuitry for communications. Such devices, which calculate the instantaneous log of the input signal are called baseband logamps the term "true logamp" is also used. The outputs of these rectifiers are summed together as shown and applied to a low pass filter, which removes the ripple of the full-wave rectified signal. Circuit diagram of an Opamp-transistor log amplifier is shown below.

Part Number. Setting the corner frequency too low will result in the log amp having a sluggish response to a fast-changing input envelope. If the input signal is reduced by a further 20 dB, the summed output will drop to about 2V.

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