# differential amplifier

Discussion in 'Homework Help' started by bug13, May 7, 2013.

1. ### bug13 Thread Starter Senior Member

Feb 13, 2012
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I am not sure about the following question, as the values look a bit odd.

Q:
Draw the circuit of a differential amplifier with a gain of 2 that has an input resistance of 10K, show pin details

here is my drawing:

2. ### Ron H AAC Fanatic!

Apr 14, 2005
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The V_in_2 input resistance is 10k. What about V_in_1?

3. ### bug13 Thread Starter Senior Member

Feb 13, 2012
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I am assuming they are looking for something R1 = R2, R3 = R4, as the question only asks for input resistance, not asks for R_Vin_1 and R_Vin_2.

Did I answer what you are expected?

4. ### Ron H AAC Fanatic!

Apr 14, 2005
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If V_in_1 and V_in_2 are two independent sources, then the input resistances in your schematic are NOT equal.

If you have a single voltage source connected across the inputs, the input resistance is STILL not 10k.

Last edited: May 7, 2013
5. ### bug13 Thread Starter Senior Member

Feb 13, 2012
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I am lost, can you tell me a bit more?

6. ### t_n_k AAC Fanatic!

Mar 6, 2009
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So - do you require the resistance as "seen" by an input signal source connected between the differential input terminals to be 10kΩ?

7. ### bug13 Thread Starter Senior Member

Feb 13, 2012
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The question in the OP is word by word from our exercise, I am not sure, but from what I understand it, is the resistance to the ground.

8. ### t_n_k AAC Fanatic!

Mar 6, 2009
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What do you mean by "resistance to ground". You suggest this is your understanding of the wording. How do you know this is what was meant by the wording?

9. ### bug13 Thread Starter Senior Member

Feb 13, 2012
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I mean the resistance see by the inputs, not the resistance between the inputs, but I don't think I truly understand the question, I will need some help.

10. ### t_n_k AAC Fanatic!

Mar 6, 2009
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OK - so let's say we connect V_in_2 directly to ground and connect a non-zero ground referenced input to V_in_1. What is the input resistance on each input?

11. ### Ron H AAC Fanatic!

Apr 14, 2005
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This is the circuit (attached) I would draw in response to that question.
With two voltage sources, there are two input resistances.

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12. ### bug13 Thread Starter Senior Member

Feb 13, 2012
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I got R2+R1||R4, is that what you expected?

but isn't it become an inverting amplifier?

13. ### bug13 Thread Starter Senior Member

Feb 13, 2012
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We have talk about input impedance (of differential amplifier) in class, I am guessing the tutor want us to look the info up.

but good to know there are two input resistances with two voltage source.

so do you consider a high side current sensing circuit as one voltage source?

14. ### bug13 Thread Starter Senior Member

Feb 13, 2012
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Arrrh, are you trying to guild me to R_in_1 = R2+R1||R4, and R_in_2 = 0?

15. ### t_n_k AAC Fanatic!

Mar 6, 2009
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Since the context is differential amplifier I would tend to make a distinction (if any) between differential mode and common mode input resistance.

16. ### Ron H AAC Fanatic!

Apr 14, 2005
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This is homework. What do you think, and why?

17. ### bug13 Thread Starter Senior Member

Feb 13, 2012
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more to google: differential mode and common mode input resistance

Feb 13, 2012
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