Why dont Operational Amplifier, Amplify the signal in Peak detectors and Clippers etc

[waqasai]

Hello,
I am kinda new to Op-Amps, but i was reading its basic applications and i cam across peak detectors and clippers. It made me thinking that Op-Amps are basically amplifiers so why dont it amplify the signal in these situations?

 

[MikeMl]

The output pin of an opamp cannot output a voltage more positive than the opamp's Vdd pin; nor more negative than its Vee (Gnd) pin (true for Rail-to-Rail Opamps, not true for some older designs, like a 741). Also, the ability of an opamp to source/sink current at its output pin is quite small, typically +-25mA or less.

 

[waqasai]

but it does amply the signal if u adjust the gain via feed back resistor etc.. am i right?

 

[MikeMl]

If you put negative feedback around an opamp, then the "gain" is 99.9999% determined by the gain setting resistors. In a simple inverting amplifier, the Gain is -Rf/Ri, so changing either resistor will effect the gain. In a simple non-inverting amplifier, the gain is 1+Rf/Ri.

 

[waqasai]

yeah you are right about that, but the thing that i am confused about is that which this much gain it should amplify the signal. i also read about unity gain Op-Amp, that things make sense that if the closed loop gain gain is 1 then input signal will be equal to output. but still i am confused that how can it be true for peak detector and clippers!!

 

[RMMM]

It will amplify the signal up to the SUPPLIED POWER for the opamp, not the signal.

If you are powering your opamp with 10v and you have a 1v signal, you cannont amplify it past 10v.

If you supplied it with 20v you can amplify the signal to 20v (For a rail-to-rail opamp)

 

[crutschow]

yeah you are right about that, but the thing that i am confused about is that which this much gain it should amplify the signal. i also read about unity gain Op-Amp, that things make sense that if the closed loop gain gain is 1 then input signal will be equal to output. but still i am confused that how can it be true for peak detector and clippers!!

I don't quite understand your confusion. Perhaps you do not understand the operation of peak detectors and clippers.

Normally peak detectors and clippers do have fixed gains as determined by the feedback circuits. If they are configured open loop then they would not operate as peak detectors or clippers.

Let us assume that gain is a typical value of one:

A peak detector then simply amplifies the signal with a gain of one but holds the peak value.

A clipper will pass the signal with a gain of one until the signal limit is reached. Then the output is clamped (limited) to the limit voltage.

 

[MrAl]

Hello,
I am kinda new to Op-Amps, but i was reading its basic applications and i cam across peak detectors and clippers. It made me thinking that Op-Amps are basically amplifiers so why dont it amplify the signal in these situations?

Hello there,

Op Amps are used in many applications that dont actually appear to amplify anything, but rest assured, internally the op amp is amplifying all the time by as much as 100,000 or more (one hundred thousand). If you would like more details about this i can provide more information, but basically put the device amplifies the tiny difference between it's inverting and non-inverting inputs by a large amount, and this action is utilized in the external circuit to perform some sort of real world function like peak detecting. This works out because of a special property of feedback amplifiers: they can correct an error in the output signal using large amplification gains, and that is the basic reason the op amp exists today.

 

[waqasai]

ummmm i thnk i got the point now it still amplifies the signal with a gain of one it stays the same almost... thanks alot guys ..