# Input frequency for amplifier

#### 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
30,964
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.

#### 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?

#### MrChips

Joined Oct 2, 2009
30,964
would that mean that the gain is the same for lower frequencies?
Yes.

#### WBahn

Joined Mar 31, 2012
30,246
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
6,758
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.

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#### DickCappels

Joined Aug 21, 2008
10,231
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.

#### Audioguru again

Joined Oct 21, 2019
6,758
I found an opamp with a similar response as yours at 1MHz and below:

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#### WBahn

Joined Mar 31, 2012
30,246
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.