+ Post New Thread
Results 1 to 20 of 27

10th June 2018, 11:24 #1
 Join Date
 Dec 2013
 Posts
 21
 Helped
 0 / 0
 Points
 1,004
 Level
 7

10th June 2018, 13:47 #2
 Join Date
 May 2008
 Location
 Germany
 Posts
 5,738
 Helped
 1722 / 1722
 Points
 40,060
 Level
 48
Re: Loopgain calculation
Are you familiar with the loop gain definition?
Do you know how you would measure/simulate the loop gain function?

10th June 2018, 20:38 #3
 Join Date
 Dec 2013
 Posts
 21
 Helped
 0 / 0
 Points
 1,004
 Level
 7
Re: Loopgain calculation
thanks
i read a file about loopgain and also i know from control lesson that A(CLOSE LOOP)= A(openloop)/[1+B.A(openloop)].
and the multiplication of B.A(OPENLOOP) is called loop_gain

Advertisment

11th June 2018, 07:27 #4
 Join Date
 May 2008
 Location
 Germany
 Posts
 5,738
 Helped
 1722 / 1722
 Points
 40,060
 Level
 48
Re: Loopgain calculation
So  the question remains: What is "B" in your circuit?
What is the name of the quantity "B"?

11th June 2018, 08:08 #5
 Join Date
 Jan 2008
 Location
 Bochum, Germany
 Posts
 43,155
 Helped
 13131 / 13131
 Points
 248,140
 Level
 100
Re: Loopgain calculation
So  the question remains: What is "B" in your circuit?
G = C*A/(1 + A*B)
signs are a matter of convention, assuming positive A and B you get
B = R1/(R1 + R2)
C = R2/(R1+ R2)
   Updated   
To be done: derivation of open loop amplifier gain A

12th June 2018, 19:43 #6
 Join Date
 Dec 2013
 Posts
 21
 Helped
 0 / 0
 Points
 1,004
 Level
 7
Re: Loopgain calculation
hi
i thinked befor that for obtaining loop gain , we had to multiply B(feedback factor) and A_O(open loop gain). BUT I KNOW IT IS NOT RIGHT.
Thanks for answers

12th June 2018, 22:14 #7
 Join Date
 Jan 2008
 Location
 Bochum, Germany
 Posts
 43,155
 Helped
 13131 / 13131
 Points
 248,140
 Level
 100
Re: Loopgain calculation
In fact it is right.

Advertisment

13th June 2018, 10:00 #8

Advertisment

13th June 2018, 10:51 #9
 Join Date
 Aug 2015
 Posts
 1,125
 Helped
 432 / 432
 Points
 7,214
 Level
 20
Re: Loopgain calculation
Respectfully  there is no loop in the ckt.

13th June 2018, 11:14 #10

13th June 2018, 13:50 #11
 Join Date
 Jan 2008
 Location
 Bochum, Germany
 Posts
 43,155
 Helped
 13131 / 13131
 Points
 248,140
 Level
 100
Re: Loopgain calculation
The circuit has internal feedback, one of the possible readings is that its loop gain should be calculated, see analysis in post #5.

13th June 2018, 13:56 #12

13th June 2018, 23:09 #13
 Join Date
 Aug 2015
 Posts
 1,125
 Helped
 432 / 432
 Points
 7,214
 Level
 20
Re: Loopgain calculation
is there is some marginal internal feedback?  nope. Loop gain refers to gain around the complete loop, there is no ckt shown to complete the loop.
what you are seeking is the forward transfer function of input to output with specified load.

14th June 2018, 08:37 #14
 Join Date
 Jan 2008
 Location
 Bochum, Germany
 Posts
 43,155
 Helped
 13131 / 13131
 Points
 248,140
 Level
 100
Re: Loopgain calculation
Consider a setup without additional external feedback, e.g. Vin connected to a voltage source. Then the internal feedback through R2 (feedback factor R1/(R1 +R2) ) is the only feedback in this circuit. It's not "marginal" but  among other parameters  determining the circuit gain. A loop gain can be calculated.

14th June 2018, 09:54 #15
 Join Date
 May 2008
 Location
 Germany
 Posts
 5,738
 Helped
 1722 / 1722
 Points
 40,060
 Level
 48
Re: Loopgain calculation
The two transistors can be used as a linear amplifier.
The same applies to an opamp.
However, the problem in both cases is that  without negative feedback  there will be no usable DC bias point in the middle of the quasilinear region.
(I know  the reason for the missing bias point may be not the same...but this is not of any importance with respect to the subject under discussion).
Therefore, we use two resistors (forming a simple voltage divider) for feeding back a part of the developped output voltage to the inverting input.
However, it is to be mentioned that  in the circuit under discussion  there must be 100% feedback for DC (CMOS inverter with Dcin=DCout).
Therefore, an additional capacitor at the input will be necessary. This results in a feedback factor k=1 for DC (no DC current through R2) and k=R1/(R1+R2) for AC.
For calculating the gain of the complete circuit, we are using the classical formula for the closedloop gain of an amplifier with feedback (given in post#5): G=C*A/(1+A*B).
Easy peasy  anythig wrong ?

14th June 2018, 10:02 #16
 Join Date
 Aug 2015
 Posts
 1,125
 Helped
 432 / 432
 Points
 7,214
 Level
 20
Re: Loopgain calculation
R2 provides degeneraton  a loop requires connection to the actual input ...

14th June 2018, 15:47 #17
 Join Date
 Jan 2008
 Location
 Bochum, Germany
 Posts
 43,155
 Helped
 13131 / 13131
 Points
 248,140
 Level
 100
Re: Loopgain calculation
a loop requires connection to the actual input

14th June 2018, 17:05 #18
 Join Date
 May 2008
 Location
 Germany
 Posts
 5,738
 Helped
 1722 / 1722
 Points
 40,060
 Level
 48
Re: Loopgain calculation
What is an "actual input"? What is your definition? The CMOS inverter has only one single input node  the common node for both gates.
Is there any doubt that R2 is connected between output and input?
As an example, consider the noninverting opamp configuration.
The feedback path is connected to the inverting input  and the "actual" input (do you mean: signal input?) is the noninv. opamp terminal.
The circuit under discussion shows the classical negative feedback scheme.
(by the way  "degeneration" is nothing else than negative feedback; see the "degeneration resistor" RE in the emitter path of a common emitter gain stage).

14th June 2018, 18:03 #19
 Join Date
 Dec 2015
 Location
 Madrid, Spain
 Posts
 1,217
 Helped
 299 / 299
 Points
 6,852
 Level
 19

Advertisment

14th June 2018, 18:31 #20
 Join Date
 Jan 2011
 Posts
 3,316
 Helped
 1179 / 1179
 Points
 20,065
 Level
 34
Re: Loopgain calculation
I had a professor in undergrad who absolutely insisted that simple circuits like this be analyzed in terms of shunt/series feedback systems. What fun we had, spending half an hour analyzing calculating the gain of an inverting opamp amplifier, which could be solved in seconds using KVL and KCL.
For more info on this horribly inefficient and outdated method of analysis, see: http://cc.ee.nchu.edu.tw/~aiclab/tea...cs3/lect08.pdf
+ Post New Thread
Please login