Can anyone think of any OP-AMP questions to expect on my interview?

Question

i have an interview coming up this week . It's an entry level electrical engineering interivew,
what kind of operational amp questions do you think I should expect ? I am kind of nervous , if they are gona ask me any question that i am not sure of I will start to panic. but if i have any idea of the kind of question ( relating to op amp ) then i think I will be fine.
What do you think ?

Answer 1

I have actually given technical interviews for entry level engineers. Here is my list of questions:

1. Draw (on the chalkboard or whiteboard) an inverting amplifier using an op-amp.

1a. What is the gain?
1b. What is the input impedance?
1c. What is the output impedance?
1d. Why did you choose those values of input and feedback resistor?
1e. Discuss some of the limitations of this circuit (frequency response, gain limitations, gain stability (i.e. some op-amps are not stable at low gains), bias and offset currents and voltages, etc.

For more advanced engineers I would include more detailed aspects of the circuit.
One time some of us interviewers had a thing among ourselves to name as many 2nd and 3rd order error sources as we could, concerning op-amps. One guy came up with 22. The best I could do off the top-of-my head was 17 of them.

If you can name 15 2nd-order and 3rd-order error sources to an ideal op-amp and discuss how you could mitigate their effects, or why they wouldn't be a problem in certain circuits, you would impress the interviewer.

2. Draw a non-inverting amplifier using an op-amp.
2a, 2b, 2c, 2d, same as above.

Those are the basic 2 op-amp question groups. They may ask about active filters, but that is generally not for entry level engineers.

.

Answer 2

I just started as an entry level electrical engineer back in May... when I was interviewing I really didn't get asked many technical questions... the most technical question I was asked is "What is a FET, and how are they used."

IF you were to get asked about op-amps, I wouldn't expect it to be more than "what is an op amp," "can you draw the symbol of an op-amp," "how are they used," Some more difficult questions might be related to spec's on op-amps like "slew rate" or some simple design problems like drawing a schematic for an integrator/ differentiator or a high/lowpass filter (without any specific values).

If you aren't interviewing with a really high tec company, like motorola, or texas instruments or something I wouldn't worry about it. If you are, then brush up on some of the basic concepts you learned in your analog electronics class.

Answer 3

I would ask questions like

"Which are the noise sources in an op-amp amplifier?"

"How do you calculate total noise?"

"Can you draw a few typical noise curves?"

"What does total noise as a function of source impedance look like? Why?"

"How do you pick an op-amp for a circuit with 10Ohm source impedance?"

"What type of op-amp do you use for a circuit with 10GOhm source impedance?"

"Do you know part numbers of op-amps suitable for these applications?"

"Which manufacturers make the best op-amps? Why? Do you have a favorite? Why?"

Then I would give you a slightly non-trivial schematic and ask you to calculate a few voltages and currents for me.

If you can answer these questions without cold sweat on your forehead, I might hire you.

Good luck!

PS: I might also ask about stability and how to achieve it in practice (measuring loop gain in closed loops and compensating the circuit until it is unconditionally stable).

And if they don't ask questions like these, you will either be given the opportunity to learn on the job (good for you)... or you will be working for idiots who don't understand analog electronics themselves (not so good for you because you won't learn from anyone but your own and other's mistakes).

To the previous poster: these are questions I would ask to hire for a company with 40 design engineers. It's true, a really large high tech company does not need people with such "advanced" basic expertise. So I wouldn't personally worry about knowledge as much as I would while interviewing with the small ones who need real engineers because they can't support a >>10:1 ratio between worker bees and experts and still live to tell the tale of how great they are.

Answer 4

I doubt if they will

but you might look up the parameters
Gain 200,000 at dc with a single pole giving 20 db per decade from about 6Hz

bias current 6 pA (for fet opamps)

bias current 10nA (for bipolar opamps)
offset current 3nA

Offset voltage 6mV
or 1mV (trimmed)

Voltage offset temp coeff 1uV/deg C

Output impedance (open loop / under feedback)

Slew rate Volts/uSec

Unity gain bandwidth (1MHz to 4MHz)


Go to Ti.com OPA277 OP07 TL081
or National.com LM7301 LM324 LM351 LF351

and look at a few opamps datasheets (PDF)

and a comparator LM211

rail to rail inputs and / or outputs
(cmos opamps)

Low Voltage opamps


read up on that lot .... and you can bet they'll not ask a single question on opamps lol

Answer 5

Can you write the gain for a non-inverting amplifier in terms of Rf and Ri ? You need to be able to do that first. Now set the Rf to 0 and then what is the gain of the output? This will answer your question. You should be able to find a formula for the gain of a non-inverting amplifier or use some of the principals that you have been learning to create the formula for yourself.