If the input voltage of an inverting amp is positive, which description best matches the output?

If the input voltage of an inverting amp is positive, which description best matches the output?

• A. The output voltage equals the input voltage
• B. The circuit output voltage is greater than the input voltage and has opposite polarity
• C. The output voltage is less than the input voltage
• D. The output voltage is unaffected by the input

Answer: (B)

In an inverting amplifier, the input signal is inverted and scaled by the resistor ratio. With the noninverting input at ground, the output follows Vout = - (Rf/Rin) · Vin. This means a positive Vin produces a negative Vout, i.e., opposite polarity. If the resistor ratio is greater than 1, the magnitude of the output can be larger than the input, so the output can be described as greater in magnitude while being inverted. That’s why the description that the output has opposite polarity and, for a gain > 1, a larger magnitude fits best. The other statements don’t fit because: the output is not simply equal to the input (that would be a non-inverting follower behavior); the magnitude isn’t guaranteed to be smaller (it depends on the gain); and the output is not unaffected by the input (the whole point of an inverting amplifier is to amplify and invert the signal).

In an inverting amplifier, the input signal is inverted and scaled by the resistor ratio. With the noninverting input at ground, the output follows Vout = - (Rf/Rin) · Vin. This means a positive Vin produces a negative Vout, i.e., opposite polarity. If the resistor ratio is greater than 1, the magnitude of the output can be larger than the input, so the output can be described as greater in magnitude while being inverted. That’s why the description that the output has opposite polarity and, for a gain > 1, a larger magnitude fits best.

The other statements don’t fit because: the output is not simply equal to the input (that would be a non-inverting follower behavior); the magnitude isn’t guaranteed to be smaller (it depends on the gain); and the output is not unaffected by the input (the whole point of an inverting amplifier is to amplify and invert the signal).