differential amplifier

Thread Starter

bug13

Joined Feb 13, 2012
2,002
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:
 

Thread Starter

bug13

Joined Feb 13, 2012
2,002
The V_in_2 input resistance is 10k. What about V_in_1?
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?
 

Ron H

Joined Apr 14, 2005
7,063
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:

Thread Starter

bug13

Joined Feb 13, 2012
2,002
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.
I am lost, can you tell me a bit more?
 

t_n_k

Joined Mar 6, 2009
5,455
So - do you require the resistance as "seen" by an input signal source connected between the differential input terminals to be 10kΩ?
 

Thread Starter

bug13

Joined Feb 13, 2012
2,002
So - do you require the resistance as "seen" by an input signal source connected between the differential input terminals to be 10kΩ?
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.
 

t_n_k

Joined Mar 6, 2009
5,455
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?
 

Thread Starter

bug13

Joined Feb 13, 2012
2,002
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?
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.
 

t_n_k

Joined Mar 6, 2009
5,455
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?
 

Ron H

Joined Apr 14, 2005
7,063
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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Thread Starter

bug13

Joined Feb 13, 2012
2,002
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?
I got R2+R1||R4, is that what you expected?

but isn't it become an inverting amplifier?
 

Thread Starter

bug13

Joined Feb 13, 2012
2,002
This is the circuit (attached) I would draw in response to that question.
With two voltage sources, there are two input resistances.
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?
 

Thread Starter

bug13

Joined Feb 13, 2012
2,002
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?
Arrrh, are you trying to guild me to R_in_1 = R2+R1||R4, and R_in_2 = 0?
 

t_n_k

Joined Mar 6, 2009
5,455
Since the context is differential amplifier I would tend to make a distinction (if any) between differential mode and common mode input resistance.
 

Ron H

Joined Apr 14, 2005
7,063
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?
This is homework. What do you think, and why?
 

Thread Starter

bug13

Joined Feb 13, 2012
2,002
Since the context is differential amplifier I would tend to make a distinction (if any) between differential mode and common mode input resistance.
more to google: differential mode and common mode input resistance
 
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