Discussion in 'Homework Help' started by !!Miss.EE!!, Nov 7, 2007.

1. ### !!Miss.EE!! Thread Starter Active Member

Oct 17, 2007
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Hi

my teacher asked us to design an amplifier with normal gain of (-10) using [ 741 Op-amp] I searched through the internet but I couldn't understand any thing .. I looked at the data sheet but it was to complicated. If you could give me any hint or the steps and rules I should go for Please........

2. ### recca02 Senior Member

Apr 2, 2007
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gain of -10;
that shud mean an opamp in inverting mode (- sign) and magnitude of gain = rf/r1 so the
rf/r1 ratio should be 10.

3. ### JoeJester AAC Fanatic!

Apr 26, 2005
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If someone told me to design an amplifier with a gain of -10, that means the output would be 0.1 of the input. I don't think the inverted output of an op amp qualifies as -10, unless your using it strickly as a DC amplifier. You should seek clarification from your teacher.

I'm thinking in the context of gain ... +10 [signal increase] -10 [attenuation].

4. ### recca02 Senior Member

Apr 2, 2007
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so -10 gain will correspond to -10 dB right?
if so i apologize for the incorrect response.i'll try to correct it if possible with further inputs from the op.
thanks Mr. Joe
Miss.ee you should seek clarification and do post for further help from the members.

May 16, 2005
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6. ### Distort10n Active Member

Dec 25, 2006
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When I see Av= -10 I assume an inverting gain.

7. ### JoeJester AAC Fanatic!

Apr 26, 2005
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Hence the advice to seek clarity.

Using this lab as a guide, I can see that Recca02 provided the proper advice using a inverting amplifier.

Of the first one hundred responses at google using the search terms ... +"gain of -10" +"op amp" ... about 6 percent refered to a gain of -10 as the inverting input to the op amp. Even the classical inverting amplifier page at williamson lab's doesn't use a gain of -10 as such a reference.

8. ### Ron H AAC Fanatic!

Apr 14, 2005
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OK, Joe, then what would "gain=0.1" mean?
The minus sign may be confusing to the "uninitiated", but to any hard-core analog engineer (like me ), it means an inverting amplifier, whether AC or DC.

9. ### recca02 Senior Member

Apr 2, 2007
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what about Vo/Vi = 1.02329?
gains are many times measured in dB arent they?
but again its confusing for me since for voltage the gain is 20log(Vo/Vi) so that wud mean vo/vi =1.0115????
or m i adding a new confusion to the discussion?

10. ### The Electrician AAC Fanatic!

Oct 9, 2007
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It can be important that an inverting gain be specified as a negative value. If the inverting amplifier is embedded in a network, such as an active filter, the analysis of the network will not give the correct result if the inverting gain is not negative. The units for gain in this case are volts (out) per volt (in), or V/V. If the units are V/V, they are often omitted.

See: http://en.wikipedia.org/wiki/Gain

If the units are decibels, that should be specified, and of course units of decibels would not be used in (for example) a nodal analysis of an active filter.

Units of decibels might be used for the overall gain of an active filter taken as a two port, but if you want to know the voltages at all the internal nodes, you don't want to use decibels in the analysis.

11. ### recca02 Senior Member

Apr 2, 2007
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i was thinking the same
yes, that way it would be hard to represent the inversion of signal wont it?
thanks.

12. ### cumesoftware Senior Member

Apr 27, 2007
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Google for inverting amplifier. That circuit should be your answer. Plus, you will only need two resistors for the feedback network.

Probably, the gain is expressed in the means of Vout / Vin, being Vout = -10 x Vin.

13. ### JoeJester AAC Fanatic!

Apr 26, 2005
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I'm not saying I don't see the reasoning ... I do see it. I'm just suprised that only 6 percent of the first hundred examples explained it as such. This does not preclude the student from asking the instructor to clarify.

The voltage gain formula has always been Av = Eout / Ein.

In the case of the common emitter vacuum tube amplifiers, Av = -(u * (RL / (RL + rp))). The minus sign was included to show there was a phase inversion between input and output.

Such is the case with OP amps as Av = - ((Rf / Rin) * Vin). Again, the negative sign shows a phase inversion between input and output.

So the -10 gain would be proper and I stand corrected.

14. ### cheddy Active Member

Oct 19, 2007
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Maybe you should ask the teacher why she is asking you to use a component before explaining the data sheet to the class.

15. ### The Electrician AAC Fanatic!

Oct 9, 2007
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I think you intended the search clause +"gain of -10" +"op amp" to force Google to find only pages that included the exact phrase "gain of -10", but if you carefully examine the results, you'll see that it doesn't do that. Apparently the minus sign is ignored.

The explanation for what you find on the Williamson lab page is probably just plain carelessness, and that may also account for some of the other pages where the gain of an inverting op amp stage is not given as negative.

Sometimes an author may intend to discuss the magnitude of the gain, but fail to fully qualify it by using the term "absolute" or "magnitude". Again, I would just call it carelessness. If that's what an author intends, then they should say so.

16. ### JoeJester AAC Fanatic!

Apr 26, 2005
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Yes I did. +" " must have the phrase, as well as -" " should reject any pages with that phrase. It works for single words also. I must have opened a dozen or so lab assignments, but the answer came from a 1965 book by Millman. I did look at HHs AOE and the accompaning Student's Manual.

Yes, I saw that google dropped the minus sign. I think it still would have had more than 6 percent showing in one hundred pages as the reasoning should have stood out. I can accept the carelessness factor as a reason.

17. ### Distort10n Active Member

Dec 25, 2006
429
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A negative sign indicates phase inversion? What about a DC signal? Phase is undefined at DC.

18. ### JoeJester AAC Fanatic!

Apr 26, 2005
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Polarity inversion between input and output.