How can measure the OP open loop gain?

Dear all:

I just have a question about measure the OP open loop gain. It is not simulation on the PC or workstation, I means measure the OP open loop gain in the Lab. But it is too difficult. The difference voltage of N- and N+ is too small to be measure.

So, someone can give me suggestion. Thank you!

you can refer to
cmos analog circuit design
allen
chap 6.6

sunking said:

you can refer to
cmos analog circuit design
allen
chap 6.6

Click to expand...

Thank you very much. I try to find this book

sunking said:

you can refer to
cmos analog circuit design
allen
chap 6.6

Click to expand...

https://www.amazon.com/exec/obidos/...104-5222959-9508710?v=glance&s=books&n=507846

You can download the pptsof Allen's book in EDA E-books Upload/Download. They are detailed enough, just like the book.

1. Set the DC gain to 1 by connecting op. ampl. output to the (-) input via resistor (the resistor value is not critical , say 10 - 30 kohm).
2. Set the low frequency AC gain to the open loop gain by shorting the negative input to the ground via capacitor. Its value is not critical, but the lower resistor the bigger capacitor is needed, for the start the time constant should be greater than say 10 sec.
3. Apply low frequency 1V, 1Hz signal at the (+) input through the 1:1 000 000 divider (or 1:100 000 000 if necessary ).
4. Measure the AC component at the output (output swing should not reach the saturation region).
5. Divide apropriate values .
6. Check if the AC frequency and the capacitor were selected correctly:

a) Repead the measurement using the half of the original frequency (1/2 Hz)
b) Repead the measurement using the original frequency (1 Hz) and double the capacitor value.

The calculated gain should be the same for all measurements.
If they are not, then lower the frequency and increase the capacitor as long as you will get the same gain for different frequences/capacitors.

What you need to do is find a paper by R.D.Middlebrook called "Measurement of loop gain in feedback systems"
It is old, written in 1975, but I think if you do a search in google, you'll find it.

Thank AndrzejM, It is very kind of you get me suggestion.

Thank sutapanaki,I have find this page by google

Thank everyone

Use network analyzer if you can find one. It's much more convinent.

hi,

best way to cal openloop gain ; firt u connect a large resistane from o/p to inverting terminaland connect a large capacitance from inverting terminal to ground. apply ac signal at non inverting terminal and do ac analysis

since u r connecting a large feed back resistor it in open loop configuration and rc feed back is for stabilisation


use resistor in the order of M ohms and capacitor in nano or micro

To nani
Will u explain the theory of ur measuring open loop response? Thanks in advance!

introducing higher RC feedback will provide dc feedback and for ac its openloop

refer allen section 6.6 page 311 figure 6.6-2

substitue higher RC value u can observe only the openloop freq reponse of opamp

A subtalty of this method is to chose a suitable point where to put the RC, because ultimatly it changes the loading in the loop at that point. One should be careful with this when measurements/simulations are done for high frequencies.