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Dynamic range of Log circuit

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Hello Everyone,

I’m looking for some help with calculating the dynamic range of the logarithmic op-Amp circuits ( both Basic Log Amplifier using Transistor and Diode). Can someone please direct me?
In the following MT-077 TUTORIAL
**broken link removed**
in Page number 4 mentioned
"the dynamic range of this configuration (The Diode / Op-Amp Log Amp) is limited to 40-60 dB because of non-ideal diode characteristic, but if the diode is replaced with a diode-connected transistor as shown in Figure 7, the dynamic range can be extended to 120 dB or more"

Can anyone explain this please? and I would like to calculate the dynamic range for both circuits!!

You can assume any values. Opamp dynamic range define as follows:

DR = 20 Log (Vout(max)/Vout(min))
 
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Could not see your pdf. As the current increases through a diode, the ohmic resistance of the die predominates. When you use a transistor and the current increases the drain or collector current can increase without the ohmic penalty.
Frank
 

You should repair the MT-077.pdf link

But chuckey has explained the reason for better performance of transistor log circuit, also without the link.

In addition, I don't understand the below expression related to the discussion. Dynamic range of a log amplifier refers to input rather than output signal. A dynamic of 120 dB (6 decades) can be usually only achieved for current input, not for voltage input due to offset voltage of the involved OP.
DR = 20 Log (Vout(max)/Vout(min))
Click to expand...
 

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