# Op Amp

Discussion in 'General Electronics Chat' started by alphacat, Feb 7, 2010.

1. ### alphacat Thread Starter Active Member

Jun 6, 2009
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I understand pretty well how an op-amp operates at small signal.
You need to bias the op-amp in order to set the transistors at their constant current region, to achieve max output voltage swing, and to eliminate DC voltage/current at output.

Once these are set, you can calculate the small-signal gain of the amplifier.

However, how do you know what is the large-signal gain of the amplifier?

Moreover, how can you tell the DC input and output resistances of the amplifier?

2. ### Jony130 AAC Fanatic!

Feb 17, 2009
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The only thing you need to do is ensure that there is a path for dc current that can be sourced or sinked by the opamp input.
Rin depends on the specific circuit.
And Rout is very low thanks to negative feedback
Rout=Rout_open_loop/(1+Aol*K)
Where
Aol is a open-loop gain of a opamp (see datasheet)
K - feedback factor ( feedback gain ), sometime we use letter "β" for feedback gain.
http://forum.allaboutcircuits.com/showthread.php?t=24971&highlight=loop

And for example this circuit

Rin≈R5; Au=Vout/Vin=1+(R2/R1); F_cut-off=GBW/Au
GBW - Gain bandwidth product (in the datasheet )
And low corner frequency depends on C1 and R5, C3 and R1, C4 and Rload.
And ideal opamp has
Rin=∞; Rout=0; Aol=∞;

3. ### Jony130 AAC Fanatic!

Feb 17, 2009
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What is the difference between small-signal gain and large-signal gain.
In opamp large-signal gain is Aol
http://forum.allaboutcircuits.com/showpost.php?p=173559&postcount=9

Last edited: Feb 7, 2010
4. ### alphacat Thread Starter Active Member

Jun 6, 2009
186
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Thank you very much Jony.

Well, the small-signal input resistance of a BJT-based Op-amp is 2*rπ.
How do you calculate the large-signal input resistance?

How do you reach the large-signal Rout_open_loop?
I know how to calculate the small-signal Rout_open_loop but not the large-signal one?

You are right, they are the same, my mistake.
Do you calculate the large-signal gain by calculating first the small signal gain and placing ω=0?

5. ### Jony130 AAC Fanatic!

Feb 17, 2009
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When we design circuit with opamp we usually treat them as ideal opamp. And if we want to know Rin or Aol , Rout wee look to the datasheet, we don't need to do any calculations .
For example for old op amp LM741

http://pdf.elenota.pl/pdf/Fairchild/lm741.pdf
Rin=250KΩ (Figure 2.Input Resistance and Input
Capacitance vs Frequency)
Rout_open_loop=100Ω (Figure 1. Output Resistance vs Frequency)
And Aol=15000[V/V] ω=0
http://www.national.com/an/AN/AN-A.pdf
http://users.ece.gatech.edu/~mleach/ece4435/chap02.pdf

And can you tell me the difference between small-signal vs large-signal resistance ?
For me there is no such thing as large-signal resistance.

Last edited: Feb 7, 2010
6. ### alphacat Thread Starter Active Member

Jun 6, 2009
186
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Thank you very much Jony!
I got it now.
My problem was that i didnt know there was no difference between Rin, Rout, Aol in small signal (for low frequencies) and in large-signal.

Theoretically, in large signal analysis, I can draw a 2*rπ resistor between the op-amp's input pins, in order to calculate DC voltage across the circuit?

7. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Well, yes but we never do this. We treat op-amp as black box and assume that:
Rin=∞, so input current is 0A. (No current is flow to the op-amp inputs)
Rout=0Ω
And the open loop gain is infinite large.
And with this assumptions we get that in linear region non-inventive input voltage "V+" is equal inverting input voltages "V-" and we call it "virtual short". And that is why is this diagrams V+=V-

http://focus.ti.com/lit/an/slod006b/slod006b.pdf

8. ### alphacat Thread Starter Active Member

Jun 6, 2009
186
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Got it!

Thank you very much!

You taught me very important things.