Textbook: "Op-Amps output voltage is opposite in sign to inverting terminal" What if both terminals negative?

Question

The output voltage is supposed to be out of phase with the inverting terminal (-), and in phase with the non-inverting (+) terminal. So what happens if both input terminals are negative, say V(-)= -5 and V(+)=-7? The output is the same sign as V(-) as:

Vo = A * ( V(+) - V(-)).

THX!!

Answer 1

Once the input signal exceeds op-amp saturation voltages the amplifier will no longer act as an amplifier. If the signal is within the saturation voltage range, then the output will be proportional to the difference in current multiplied be the open-loop gain.

Answer 2

An op amp is alway employed in a circuit with feedback.  When this is done, the actual voltage at both the amp inputs is almost exactly the same.  A typical op amp may have a gain of 10^4.  This means that one millivolt difference between the inputs will produce a 10V output which may saturate the amp.  In operation,  it is only the difference in voltage between the inputs that matters, not the absolute voltage (assuming a differential-input balanced op amp with + and - power supply).  Feedback forces this input difference to be near zero for normal output levels.

The rules for solving op-amp circuits are:

The voltage difference at the inputs is zero.

No current flow into or out of either input.

Answer 3

Op amps have huge gain. They take the difference between the + and - terminals and multiply by a huge number. A small difference between them is multiplied, and ends up driving the output all the way to as close to the power voltage as it can (saturation). If the + terminal is more positive than the - terminal, the output is positive. If it is negative to the - terminal, the output is negative.

The huge gain, coupled with feedback in most applications means that both inputs are normally very close together in voltage.

Answer 4

to simplify,

V(-)= -5 and V(+)=-7 is the same as

V(-)= 0 and V(+)=-2

The opamp amplifies the difference between the two inputs by a very large number. So in this case the output will be -2 volts x a million and the output will be at the negative rail.

The best way is to look at an opamp is that you always use it with feedback, and the opamp will, if possible, adjust the output so that the difference between the inputs is zero volts (actual number is usually less than 1 mV). If the opamp can't get the input to zero, it will saturate, ie, the output will be at one of the rails.

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Answer 5

it will be of the same sign.

when one terminal is grounded and other has the input signal, the output determines what kind of input terminal it is. for a change in phase (AC) and inversion of magnitude i mean + to - or - to +. its called an inverting terminal. when we define this we make statement that other terminal is grounded.l.

remember this when both terminals have some signal we have to do the way u followed..

All the best