Input frequency for amplifier

Thread Starter

bessou55

Joined Oct 14, 2022
13
Hello,

I am using an amplifier with a gain of 100 (40dB) to amplify 20-400 Hz signals.
The pic from the data sheet shows that the minimum frequency for 40dB needs to be 1kHz.
What happens if the frequency is lower than 1kHz? will it affect the amplification?

thank you
 

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MrChips

Joined Oct 2, 2009
27,692
The chart shows the high frequency response of the amplifier, i.e. the gain-bandwidth product.
1kHz is off the chart on the left-hand side.
Ignore the chart for your situation.
 

Thread Starter

bessou55

Joined Oct 14, 2022
13
The chart shows the high frequency response of the amplifier, i.e. the gain-bandwidth product.
1kHz is off the chart on the left-hand side.
Ignore the chart for your situation.
would that mean that the gain is the same for lower frequencies?
 

WBahn

Joined Mar 31, 2012
27,900
Hello,

I am using an amplifier with a gain of 100 (40dB) to amplify 20-400 Hz signals.
The pic from the data sheet shows that the minimum frequency for 40dB needs to be 1kHz.
What happens if the frequency is lower than 1kHz? will it affect the amplification?

thank you
That's not what the chart shows. What it is showing is that IF you design the circuit to give a 40 dB gain, that the amplifier can do it up to about 100 kHz, after which the gain will start dropping. Below 100 kHz, there are no problems.

What is being graphically depicted is the "gain-bandwidth product" of the amplifier, which says that the product of the gain and the bandwidth defines a limiting boundary on the behavior of the amplifier. As long as the gain-bandwidth product you need is less than what the amplifier can do, this limitation doesn't apply to you (well, it always applies, but you can ignore it because you are going to run afoul of it).
 

Audioguru again

Joined Oct 21, 2019
5,418
You show the frequency responses of an opamp circuit but you do not show the resistor and capacitor values then your response might be much different.
The response is shown to have a gain of 100 times from at least 1kHz to 80kHz but the capacitance of any coupling capacitors will reduce the gain at lower frequencies.
Please post your schematic of the amplifier circuit.
 
Last edited:

DickCappels

Joined Aug 21, 2008
9,513
The chart is showing the open loop gain of the amplifier as a function of frequency.

One point is that in all the cases I've seen, before the rolloff the gain is constant going back to zero Hz.

Another thing is that the open loop gain of your amplifier must exceed the closed loop gain, otherwise amplifier performance will be poor at best, but how much of a problem that is depends on your application.

A little supplemental reading: https://toshiba.semicon-storage.com...-loop-and-closed-loop-gains-of-an-op-amp.html
 

WBahn

Joined Mar 31, 2012
27,900
The chart is showing the open loop gain of the amplifier as a function of frequency.

One point is that in all the cases I've seen, before the rolloff the gain is constant going back to zero Hz.

Another thing is that the open loop gain of your amplifier must exceed the closed loop gain, otherwise amplifier performance will be poor at best, but how much of a problem that is depends on your application.

A little supplemental reading: https://toshiba.semicon-storage.com...-loop-and-closed-loop-gains-of-an-op-amp.html
The envelope (limiting right-edge of the curves) is for the open loop gain, but the chart is showing the response for specific closed loop gains (every 20 dB from 0 dB up to 60 dB).
 

Audioguru again

Joined Oct 21, 2019
5,418
If the input to the amplifier has no RC highpass filter then the gain of the 40dB amplifier is 40dB from and including DC up to almost 100kHz. that is why the graph does not show lower frequencies because the response below 70kHz or 80kHz is a straight horizontal line at 40dB.
 
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