# Log/exponential calculations re. amp gain?

• posted

I have an amplifier circuit with AGC. I am told that the AGC circuit attenuation of this amplifier can be determined from the voltage across a diode in the AGC circuit. As I understand it, the voltage across a diode is logarithmically related to the current, with the current, in this case directly related to the attenuation.

Problem is, I don't understand what I think I know about logarithmic & exponential functions.

What I ultimately want to do is:

• Measure the signal level out of the amplifier.
• Measure the voltage across the diode, & calculate the attenuation.
• From this, calculate the input signal level (I am actually only concerned with relative signal levels, not the absolute level of the input).

Here's what I measure:

Input Voltage Diode Voltage < 0.02 .4241 Quiescent voltage of diode with

1M bias resistor to 5V+ 0.02 .4242 The AGC is just starting to kick in at this point. 0.05 .4279 0.10 .4334 0.20 .4424 0.5 .4620 1.0 .4827 2.0 .5085 5.0 .5483 10.0 .5827 20.0 .6235

Can anybody help me to understand where I need to go from here?

Thanks Scott Kelley

• posted

There are a number of unknowns, the primary one being we can't see the circuit. Without that, it's hard to help.

In general, though, AGC feedback is proportional (indirectly, as you deduce) to input signal level, but that specific relationship depends on it's implementation. Another thing; are your input voltages ac? One would assume so, but it's nice to clarify.

Cheers

PeteS

• posted

Yes, the input listed is AC, and is in the middle of the passband.

I have been assured by the circuit designer that the attenuation can be determined by the voltage across this diode, and that it is a logarithmic function. Unfortunately, he is no longer around to ask for further details. Although I know that I have it, I have not been able to find the schematic.

I would think that the numbers shown would at least give a clue as to the relationship between the diode voltage and attenuation.

For the sake of current discussion, the output, within the AGC range of voltages listed is constant.

Scott

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.