# Voltage Gain for a inverting and non inverting amp.

Discussion in 'Homework Help' started by Retrogalaxy, Aug 14, 2013.

1. ### Retrogalaxy Thread Starter New Member

Aug 14, 2013
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Hi guys,
was wondering if the voltage gain be less than 1 for a non-inverting amp or a inverting amp.

or can none of them be less than 1?

2. ### JoeJester AAC Fanatic!

Apr 26, 2005
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Write the formulae for the two circuits and discuss the possibility of a gain of less than 1.

3. ### LDC3 Active Member

Apr 27, 2013
920
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For a non-inverting op-amp, the gain is
Vout = Vin (1 + Rf / Rg)
so the gain can never be less than 1.

4. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
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@LCD3,

This is homework - Joe was asking the OP to come to this same conclusion.

I'm surprised you haven't caught on about the homework thing yet. Mind you it took me while to get the message.

5. ### LDC3 Active Member

Apr 27, 2013
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160
Sorry, my mistake. I didn't look up.

6. ### Retrogalaxy Thread Starter New Member

Aug 14, 2013
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Sorry guys i worded it wrong,

i meant to ask if the voltage at the non-inverting input will always be at Zero or the voltage at the inverting input will always be at zero volts for an op-amp circuit. or are these both not true.

7. ### LDC3 Active Member

Apr 27, 2013
920
160

One of the common ways to use an op-amp is temperature measurement. A RTD is placed in a bridge and the voltage level from the 2 legs are connected to both inputs. If you only have a single power source, then the voltage levels of the 2 legs are different, but greater than 0V. The op-amp uses the feedback to adjust the 2 inputs according to the defined relation between the 2 inputs.

8. ### WBahn Moderator

Mar 31, 2012
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That's quite a change in wording!

You are basically asking two T/F questions:

1) Is the voltage at the non-inverting input ALWAYS zero?

2) Is the voltage at the inverting input ALWAYS zero?

Note the emphasis I added. If you can find a single example where it is NOT zero, then you have your answer. Pick the simplest opamp circuit you can think of, namely one that doesn't use any external components at all, and ponder it in this light.

9. ### bountyhunter Well-Known Member

Sep 7, 2009
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Unless you put a resistive divider before the input.....

10. ### screen1988 Member

Mar 7, 2013
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Is the voltage at the non-inverting input ALWAYS equal to inverting input?

11. ### WBahn Moderator

Mar 31, 2012
18,080
4,917
No. If the amplifier is saturated, then all you can determine from knowing the output is which voltage is higher than the other, but it could be by any amount.

If the amplifier is in the active region of operation, then the output voltage is equal to the open-loop gain of the amplifer times the difference between the input voltages (ignoring voltage offsets and common-mode gain). The results in the difference between the inputs -- again, if the amplifier is in the active region of operation -- being extremely small (fractions of a millivolt).

But, with any opamp that does not have infinite gain, there has to be SOME difference in order to drive the output to a non-zero output.

screen1988 likes this.
12. ### LvW Active Member

Jun 13, 2013
674
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Yes - and to complete the scene: It is the negative feedback principle that keeps the amplifier within its linear/active region (with a negligible differential voltage between both opamp input nodes) for a relatively large range of external signal voltages.

screen1988 likes this.