logarithmic amplifier and antilog amplifier

[zhi_yi]

antilog amplifier

hi there, please help me, what is wave form from the output signal of logarithmic amplifier and antilog amplifier? where do we use those kinds of amplifier? what makes them called logarithmic and antilog amplifiers?

thank you

 

[fredflinstone]

log and antilog amplifier

A logarithmic amplifier is one which gives an output equal to the logarithm of the input and Anti log Amp does the reverse operation. One operation can be that, these Log and Antilog amps can be used for making multipliers.

 

[Kral]

log amplifier

zhi_yi,

Other uses are signal compression/decompression.

Generating exponential functions. For example, if you want a signal that is equal to X^1.56, you can run X through a log amplifier, multiply the result by 1.56, and then run this result thruough an anti-log amplifier. Analog functions such as this have largely been supplanted by DSPs, but they do have their uses.
Regards,
Kral

 

[zhi_yi]

antilog amplifiers

thank you all

but i still a little confused

what is the meaning of signal compression/decompression? is it we compress the signal like we use winzip/winrar, so the signal become smaller ?

if i want to generate signal at x^1.56, what is x here? is it the input signal?

thank you once again

 

[fredflinstone]

logarithmic amplifier

signal compression is done for signals which have a large dynamic range, like speech signals. Basic idea is to have a reversible signal level dependent amplification. So that the signal can be digitized linearily.
for getting x^1.56 pass x through a log amp to get y.
y=logx. Give 1.56v and y as inputs to a log amp cofigured as a summing amplifier to get z. Take the anti log of z by passing it through an antilog amp.
Hope this helps
regards
fred

 

[zhi_yi]

log amp

for getting x^1.56 pass x through a log amp to get y.
y=logx. Give 1.56v and y as inputs to a log amp cofigured as a summing amplifier to get z. Take the anti log of z by passing it through an antilog amp. <-- sorry, i am still a little confused may i see the schematic? is it the meaning is to digitize the signal linearly?

thank you

 

[Kral]

log and antilog amplifiers

zhi-yi,
Yes, x is the input.
~
Regarding compression-decompression: In this context, we're referring to a situation in which we are processing an input signal that has a large dynamic range, and a large Signal/Noise ratio is desired for low level inputs. You can not simply amplify the input, because the output would saturate. The idea of compression is to provide a non-linear output vs input characteristic, so that low level signals have a high gain, and high level signals have a low gain. A logarithmic response has this characteristic. Once the processing of the signal has been performed, the signal can be "decompressed" by using the inverse of the log function, which is an anti-log or exponential function.
Regards,
Kral

Added after 15 minutes:

zhi-yi,
Yes, x is the input.
~
Regarding compression-decompression: In this context, we're referring to a situation in which we are processing an input signal that has a large dynamic range, and a large Signal/Noise ratio is desired for low level inputs. You can not simply amplify the input, because the output would saturate in the presence of high level inputs. The idea of compression is to provide a non-linear oputput vs input characteristic, so that low level signals have a high gain, and high level signals have a low gain. A logarithmic response has this characteristic. Once the processing of the signal has been performed, the signal can be "decompressed" by using the inverse of the log function, which is an anti-log or exponential function.
Regards,
Kral

 

[zhi_yi]

antilog amp

oo.. so the purpose is to compress the signal, and i read from google that we use signal compression for a.) improving signal-to-noise ratios prior to digitizing an analog signal for transmission over a digital carrier system, b.) preventing overload of succeeding elements of a system, or c.) matching the dynamic ranges of two devices. Signal compression always introduces distortion.

i don't understand with the preventing overload of succeeding elements of a system, what does it mean? what is the meaning of overload here? and then for the c.), why do we need to matching the dynamic ranges of two devices? and then signal compression is always introduces distortion, why??

and also, what is the meaning with the output will be saturate? why we want the low level signal to have high gain and high level signal have low gain? why not both of them have high gain??


thank you i am sorry if there are a lot of error with my grammar, please notice me again, thank you

Added after 3 minutes:

erm.. i want to try to answer my question, please correct the answer, about the preventing overload for succeeding elements of a system, is it the overload mean that the maximum signal power that the system can tolerate without distortion of the signal to the noise level of the system? and so that we need the high gain for low level signal and low gain for high level signal so it won't have any distortion. is it? please correct me thank you

 

[sunjiao3]

logarithmic amp

Well, low level signal get high gain while high level signal get low gain, it seems that the output of both kinds of signal will fall into a same region. It reminds me of LA(limiting amplifier) and AGC(Automatic gain control). Could anyone of you please shed light on this topic? What are the differences and relationships between them? Thank you all.
As for the "saturation", I remember in LA, when the input signal level are high enough, the amplifier will go to nonlinear operation area, which means the amplitude of the output is "limited". But, :?: I am not so sure, what does the saturatin here means.

 

[electronics_kumar]

antilog amps

KRAL wrote

Regarding compression-decompression: In this context, we're referring to a situation in which we are processing an input signal that has a large dynamic range, and a large Signal/Noise ratio is desired for low level inputs. You can not simply amplify the input, because the output would saturate in the presence of high level inputs. The idea of compression is to provide a non-linear oputput vs input characteristic, so that low level signals have a high gain, and high level signals have a low gain. A logarithmic response has this characteristic. Once the processing of the signal has been performed, the signal can be "decompressed" by using the inverse of the log function, which is an anti-log or exponential function.
Regards,
Kral
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i want endorse the view expressed by KRAL.....in the case of PCM we are log and antilog to achieve high gain for lowlevel input and low gain for high input .tHIS process is called as COMPANDING

 

[zhi_yi]

log antilog amplifier

thank you all

from all of the replied messages, is it the summary is : the logarithmic and the antilogarithmic amplifier could be used for signal compression and decompression, the purpose of signal compression is to avoid the overmodulation when we modulate the signal? because with the logarithmic amplifier, it would give the high gain for low level signal and low gain for high level signal, so, there would be no distortion occured, and the signal could be modulated properly, please correct me, thank you very much