The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> Analog Design >> What is the difference between "large signal voltage gain" and "open loop gain"?
https://designers-guide.org/forum/YaBB.pl?num=1240574161

Message started by ryang on Apr 24^th, 2009, 4:56am

Title: What is the difference between "large signal voltage gain" and "open loop gain"?
Post by ryang on Apr 24^th, 2009, 4:56am

Hi,all

Could anybody here tell me what is the difference between large signal voltage gain and open loop gain.

I wanted to build a buffer amplifier using an existing amplifier. To determine the settling accuracy, I have to know the information of the open loop gain of this Opamp. However, when I searched the datasheet, I saw these two different definations, each of them has different value of gain. Suppose I'm going to connect the output of this buffer to the input of an instrumentation amplifier afterwards, which of these two gains should I use to calculate the settling accuracy?

Thank you for your help in advance.

Title: Re: What is the difference between "large signal voltage gain" and "open loop gain"?
Post by vamshikrishna on Apr 25^th, 2009, 2:33am

I am not sure what they mean by the large signal gain, but the open loopgain has to be used in settling accuracy,as this is responsible for the gain error.

-vamshi

Title: Re: What is the difference between "large signal voltage gain" and "open loop gain"?
Post by thechopper on Apr 25^th, 2009, 11:41am

I´m not sure either what the mean with large signal voltage gain, but for sure open loop gain refers to a small signal gain. Therefore they have to be different definitions.
May be they refer to as "large voltage gain" as the closed loop gain of such buffer?

Tosei

Title: Re: What is the difference between "large signal voltage gain" and "open loop gain"?
Post by raja.cedt on Apr 25^th, 2009, 12:44pm

hi ryang,
the basic difference i can say between those two gains are when you apply very low swing signal(small enough to to be in some specified distortion level) and if it is the above level then distortion will be very high and usually no body will think about large signal gain.But i am sure for open loop and for all small signal calculations small signal gain will be relevant.Can you direct me that data sheet because i have seen any data sheet with two gains..if so u can pick larger value because always small signal gain will be larger.

Thanks,
Rajasekhar.

Title: Re: What is the difference between "large signal voltage gain" and "open loop gain"?
Post by Asmodeus on Apr 26^th, 2009, 3:00am

Large signal gain refers to input with larger swings, which changes the Q-point of operation and hence the current etc. The ouput is distorted in this case as the output swing is more than what we thought of.

Usually large signal gain is always less that small signal gain. The concept of ckt linearization don't work for this case. You can't use small signal models to calculate this gain as the parameters are not correct. Use large signal ( DC ) equations for it.

Also, ac analysis won't give correct gain, use transient analysis to get this gain. :)

Title: Re: What is the difference between "large signal voltage gain" and "open loop gain"?
Post by raja.cedt on Apr 26^th, 2009, 4:08am

hi Asmodeus,
correct,like that only we have to find but you forgot to mention that we have to do Fourier analysis and generally large signal gain defines ratio of fundamental harmonic at the output to the input.

thanks,
Rajasekhr.

Title: Re: What is the difference between "large signal voltage gain" and "open loop gain"?
Post by ryang on Apr 27^th, 2009, 4:12am

Hi, gus,

Thank you for your help. The datasheet I am referring is actually AD8628 of Analog device. As we can see from the datasheet, the large signal voltage gain is typically 110dB. However, as is shown in fig.16, the open loop gain is even less than 70dB.

By the way, I noticed that the large signal voltage gain is tested with a load resistor, while the open loop gain is tested without any load. Is it affects the result? Which one exactly should I take?

Thank you.

Title: Re: What is the difference between "large signal voltage gain" and
Post by pancho_hideboo on Apr 27^th, 2009, 5:40am

ryang wrote on Apr 27^th, 2009, 4:12am:

The datasheet I am referring is actually AD8628 of Analog device.
As we can see from the datasheet, the large signal voltage gain is typically 110dB.
However, as is shown in fig.16, the open loop gain is even less than 70dB.

Former 110dB is open loop gain for very low frequecny or almost DC signal.

ryang wrote on Apr 27^th, 2009, 4:12am:

By the way, I noticed that the large signal voltage gain is tested with a load resistor, while the open loop gain is tested without any load.
Is it affects the result? Which one exactly should I take?

Both are open loop gain.
But former is DC open loop gain. Here you can ignore frequency characteristics due to load.

On the other hand, data sheet argue frequency characteristics of open loop gain in Fig.15 and Fig.16.
Here you can not ignore frequency characteristics due to load.