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How to identify input terminals in OpAmps ?

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suraj

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Question about OP AMP

Hello Friends,
I have question about OP Amp:

In the Symbol we have two input terminals of the OP AMP (called positive and negative respectively). In the schematic we have two input transistors (say NMOS), then how we can say which transistor is held at positive and negative terminal respectively.
Please help me to clarify this doubt.

Thanks

Suraj
 

Re: Question about OP AMP

Normally we say that the potential at the inputs remain the same. How are the transistors applied to the inputs?
 

Re: Question about OP AMP

for me
first, i put one input to 0.5*vdd and the other input to vdd
just trace the signal and check the output
if the output is high then the input with vdd is positive, else it is negative
 

Re: Question about OP AMP

If you just want to know what is the positive and negative input, you can do what dumbfrog suggested.

If you want to understand, you can do this by imagining that the voltage at one of the inputs rises and the voltage at the other input falls, and see what happens to the output voltage (following the path of the currents from the input transistors until you reach the output node). Something like "If the voltage at this input rises, than the current on this transistor will rise, which also increases the current on this other transistor (...) and then the output voltage will fall".
 

Re: Question about OP AMP

What I find interesting is that opamps are found everywhere and people will throw anything around them. I have been looking at the opamp from a different perspective. I feel that there should always be a voltage differential between the two inputs. Sometimes there isn't and sometimes the output bias current is not high enough. You can simply calculate the required ouput current if you look at the schematic. Normally there is a resistor in parallel with a diode and you can approximate the current. Has anybody come across these two problems?
 

Re: Question about OP AMP

for the schematics I have seen, generally the left transistor is the positive input (non-inverting) and the right transistor is the negative input (inverting input)...
 

Re: Question about OP AMP

well its easy ..you can start on one of the gates of you input and assume that its positive and go & test the sign of its output (..negative)..& so on till you reach the output
if there the sign was negative the gate that you choosed first was negative one.
 

Re: Question about OP AMP

if the opamp is diferential input and single output, you can find negative input watching the gain polarization. if you increase input voltage in a terminal and the output decreases, you had found your negative input, and viceversa.
 

Question about OP AMP

Two input terminals of the OP AMP really are the inputs of the Differntial Amplifire of the Operational Amplifier that those are the base pin of the transistors.
In this case the voltage difference between bases is important.
 

Re: Question about OP AMP

Kevin Weddle said:
What I find interesting is that opamps are found everywhere and people will throw anything around them. I have been looking at the opamp from a different perspective. I feel that there should always be a voltage differential between the two inputs. Sometimes there isn't and sometimes the output bias current is not high enough. You can simply calculate the required ouput current if you look at the schematic. Normally there is a resistor in parallel with a diode and you can approximate the current. Has anybody come across these two problems?
Click to expand...

Can you kindly elaborate?
 

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