EEngiGrind
Sign inGet started
← All problems
ElectricalElectronicseasyFE ExamCourseworkInterview

Output of an Inverting Op-Amp

An ideal op-amp is configured as an inverting amplifier with input resistor Rin=10 kΩR_{in} = 10\ \text{k}\Omega and feedback resistor Rf=100 kΩR_f = 100\ \text{k}\Omega.

The input voltage is Vin=0.50 VV_{in} = 0.50\ \text{V}.

Find the output voltage.

Given
Rin10 kohmInput resistor
Rf100 kohmFeedback resistor
Vin0.5 VInput voltage
Hint 1

The inverting configuration has a gain set purely by the resistor ratio.

Hint 2

A_v = −R_f/R_in.

Hint 3

Do not lose the minus sign — that is what 'inverting' means.

Worked solution — try the problem first

For an ideal inverting amplifier:

Av=−RfRin=−100k10k=−10A_v = -\frac{R_f}{R_{in}} = -\frac{100\text{k}}{10\text{k}} = -10

Vout=AvVin=−10×0.50=−5.0 VV_{out} = A_v V_{in} = -10 \times 0.50 = -5.0\ \text{V}

Vout=−5.0 V\boxed{V_{out} = -5.0\ \text{V}}

Why the gain is a plain ratio. The ideal op-amp forces a virtual ground at the inverting input, so Iin=Vin/RinI_{in} = V_{in}/R_{in}. No current enters the ideal input, so that same current flows through RfR_f, giving Vout=−IinRfV_{out} = -I_{in}R_f. The resistor units cancel in the ratio, so kΩ needs no conversion.

The sign is not optional. It is the defining feature of this topology — the output is inverted relative to the input. A non-inverting amplifier would give 1+Rf/Rin=+111 + R_f/R_{in} = +11.

Concepts:Operational amplifiersVirtual groundInverting amplifier

Sign in to submit

Grading needs an account so your progress, attempts, and daily quota can be tracked. The problem statement and worked solution stay open to everyone.

Create an account