# how to find cmos amplifier's non-inverting input?

Discussion in 'Electronics Homework Help' started by only6512, Mar 22, 2014.

1. ### only6512

18
0
Feb 15, 2014
hello everyone,I have a question:if the diagram doesn't show non-inverting and inverting input,then how to identify them?

what I know:
If I set the non-inverting input voltage up,
then I(M2) decreases,(....)finally,the output voltage increases!(otherwise,it's an inverting input)

what i don't know:
the (.....) steps which lead to the result'the output voltage increases!'

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2. ### Harald KappModeratorModerator

11,653
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Nov 17, 2011
The inverting and non-inverting input are clearly marked on the diagram. What's the issue then?

Can you expand these braces? I have no idea what you mean by them.

3. ### only6512

18
0
Feb 15, 2014
my problem is:if the diagram doesn't show '+' and '-' input,then how to identify them?

"(....)" are steps which lead to the result "the output voltage increases"

Last edited: Mar 22, 2014
4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,497
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Jan 21, 2010
For useful background:

First look at M3 and M4. What is the relationship between the Id for each?

Now look at M6, M7, and M8. What is the relationship between Id for each of these?

Now, assuming all other things remain constant:

What happens to the output if the gate of M5 goes more positive?

What happens if the gate of M2 goes more positive.

So in general, if the gate of M2 goes more positive, what happens to the output?

From the answers to the first two background questions, what will happen when the gate of M1 goes more positive?

Now can you look at this circuit and figure out how to tell which is the inverting and which is the non-inverting input?

5. ### only6512

18
0
Feb 15, 2014
thank you~can you help me check the answer?
I(1)+I(2)=I(7)=I(8)=50uA;
I(1)=I(3)=I(4);
I(4)=I(2)+I(Cc)

I(2) down->I(1) up->I(4) up->I(Cc)up->V(M5,gate)up
is it right?

6. ### only6512

18
0
Feb 15, 2014
thank you~can you help me check the answer?
I(1)+I(2)=I(7)=I(8)=50uA;
I(1)=I(3)=I(4);
I(4)=I(2)+I(Cc)

I(2) down->I(1) up->I(4) up->I(Cc)up->V(M5,gate)up
is it right?
( i don't really know why M5's gate voltage will increase?)

Last edited: Mar 22, 2014
7. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
You're on the wrong track.

That is wrong. It might look like it's right, but it's not. In fact it can't even look right because Id(m1) + Id(m2) = Id(m7), so your equation only holds when the current is zero.

It would help you and me if you could answer the questions in the order I gave them to you.

BASIC UNDERSTANDING

1) what is the relationship between Id(m3) and Id(m4)? (just the relationship -- one word will suffice)

2) What is the relationship between Id(m6), Id(m7), and Id(m8)

HINT: there's something different about M8, M3, and M4 compared to the other mosfets

2a) To make question (2) easier, perhaps you can just describe the current through M8 using words (a single word will do -- and it's not the same single word as the answer for (1)).

2b) Given that word, can it also describe Id(m7) and Id(m6)?

2c) What is Id(m8) (in uA)?

2d) What is Id(m7) (in uA)?

HINT: question (2c) and (2d) do not have the same answer! (check the previous hint)

2e) What is Id(m6) (in uA)?

OUTPUT STAGE

3) What happens when the gate of M5 goes more positive? (describe in relative terms the effect on the output voltage)

3a) Is this an inverting or non-inverting relationship?

3b) Does the current Id(m5) change?

HINT: See the answer for (2a) & (2b)

3c) What could you say about the equivalent resistance through M6 and M5?

HINT: The sum of the voltages across M6 and M5 is constant, and what was your answer to (3b)?

3d) describe how the output voltage can change in this circumstance?

DIFFERENTIAL PAIR

I'm not concerned about Id(m1) and Id(m2). I'm not concerned, and neither should you be, because you have answered (1) above.

4) Why is the above statement true?

5) What is the relationship between Id(m3), Id(m4) and Id(m7)? (This is a very simple question)

HINT: Why is it a very simple question? Read what I have written from the beginning.

SIMPLIFYING THINGS

Now, let's assume that M1 and M3 simply are not there. The circuit now looks a lot like the output stage (albeit inverted).

6) What happens to the voltage at the gate of M5 when we increase the voltage at the gate of M2?

6a) When you increase the gate voltage on M2, does this tend to turn M2 on, or off?

6b) Given this action, what happens to the voltage at the gate of M5?

6c) Is this an inverting or non-inverting action?

7) What happens at the output when the gate voltage of M2 rises (all other things being equal)?

HINT: look at the answer to questions (3) and (6b).

8) Is this an inverting, or non-inverting action?

9) can you resolve the answers to (3a), (6c), and (8)?

THE OTHER SIDE OF THE DIFFERENTIAL PAIR

10) What single word describes the sum of voltages across M1 and M3 as compared to the sum of the voltages across M2 and M4?

11) Given the answer to (10), what can you say about the equivalent resistance through M1 and M3 as compared to that between M2 and M4?

12) Does (11) hold as either (or both) M1 and M2 are turned on a little more or less by their gate voltages (presume that they are not completely turned off)?

13) If the gate voltage of M1 rises slightly, what will have to change to maintain (11)?

14) How does this affect the voltage at the gate of M5?

15) How does this affect the output?

16) How can you compare the effect on the output of small changes to the gate voltage of M1 as compared to M2?

Number your answers, and stop as soon as you're unsure because later answers depend on understanding (and getting correct) the answers to former questions.

If you don't number your answers I'll assume I'm not helping and I'll stop trying.

8. ### only6512

18
0
Feb 15, 2014
(1)equal
(2) 2I(8)=I(7)=I(6);
2(a)different;
2(b) No;
2(c)50uA ;
2(d)100uA
2(e)100uA
however,i don't know the answer to 3 & 3(a)

9. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Try again on 2(a) it might be different, but what else is it? (The current source symbol below it is a big hint)

The answer to 2(b) is wrong. You'll work out why when you get the right word for 2(a).

2(c), 2(d), and 2(e) are correct.

OK, the word I'm looking for for 2(a) is "constant". The fact that it's a constant current (i.e. a current source -- or sink in other cases) is important.

You might review what happens to the voltage gain in a common source amplifier when the drain load is changed from a resistor to a current source. Current sources are used all over the place in analogue integrated circuit design and it's important to have a good awareness of this.

10. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
For the answer to question 3, refer to the operation of a mosfet. Is this an N or a P channel mosfet? Identify which terminals are source, drain, and gate. What happens in this circuit if you make the gate more positive (assume the mosfet is already partially on)

11. ### only6512

18
0
Feb 15, 2014
(3) it's Nmos,then i(d5) up,so the output voltage will be...what?(sorry ,i don't know it )

12. ### only6512

18
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Feb 15, 2014
question 3,M5 is nmos ,so v(out)/vi=-gm*ro,so vi up,v(out) down right?

13. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Remember you're talking about CMOS, so you have both N and P channel devices.

M5 is an N channel device, so as the gate voltage rises, the drain current will increase for a constant drain voltage.

But the drain voltage (Vds) is not constant! The drain current is constant!

What happens to the drain voltage in this case as the gate voltage (Vgs) increases?

Yes, the output voltage drops as the gate voltage increases.

14. ### only6512

18
0
Feb 15, 2014
(3a)since v(M5.gate) up,v(out) down->inverting!
3(b)I(M5)=I(M6)=const,so I(M5) doesn't change
3(c) ro5//ro6
right?

15. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
3a and 3b -- yep, that's right.

3c -- yeah, but what about their values. If they have the same voltage across them and the same current through them, then the resistance is...?

16. ### only6512

18
0
Feb 15, 2014
3(c) ro/2!!
3(d) inverting effect
4.I(1)=I(3),I(2)=I(4),I(3)=I(4),so we know I(1)=I(2)
5,I(3)+I(4)=I(7)
6 decrease,cause v5=-gm*ro*v2
6(a)off 6(b) decrease 6(c) inverting
7 increase
8 non-inverting
9 .for two times of inverting gain->We get an non-inverting output!
10 equal
11 same
12 since ro=VA/Id, Vgs change->Id change->ro change,so the answer is no!

Last edited: Mar 23, 2014
17. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
That looks right. I'm using a phone right now so its harder to do a side by side comparison with the answers, but the last ones are right, and the rest look like what i remember

18. ### only6512

18
0
Feb 15, 2014
thanks a lot!!
but I still don't know (13)
if M1gate voltage goes up,I(M1)will go down,ro1 will go up~right?

19. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
(13) is the trickiest part (and I hope I get this right...)

Let's assume initially that both Vi- and Vi+ are equal. And lets also assume all components are perfectly identical to each other. Id(M1) = Id(m2)m Vds(M1) = Vds(M2), and therefore Vgs(M1) = Vgs(M2)

Now Vi- becomes slightly higher than Vi+.

This switches M1 off a little. Consequently M3 can't maintain the constant current (because it now has a reduced Vgs). Because of this the voltage at the point marked (2) rises.

In order to maintain the constant current through M4, Vds(M4) increases. (At the same time Vgs(M2) has increased, requiring a further increase in Vds(M4).

Thus, increasing the gate voltage on M1 has a similar effect to *reducing* the gate voltage on M2.

20. ### only6512

18
0
Feb 15, 2014
summarize:
vgs(M1) down->I(M1) =I(M3) down->V(point 2) up->vgs2 up->v+ down
but,why V(point 2) will go up?