Operational Amplifiers - IB Physics

IB Subjects - IB Physics

I have some problems about Operational Amplifiers from the communication option. I have had three mental breakdowns in class now.. Why do we need an inverting and a non-inverting amplifier. Its name suggest the concept of amplifying signals, then WHY would we want an inverting one? I mean, it doesn't help by inverting the signal sent. It's like playing a song backwards. It just doesn't make sense!
I know all the theory of drawing it and calculating gain and stuff, but why do we have them?
What use do they have?
- Camilla

Answer

The question you pose is in fact a very good one. Unfortunately the reason you were never given a straight answer is because you could essentially take an entire university course on op-amps and their characteristics before you can say you fully understand the reasoning. As a result, I'm going to try and give you only a few reasons - the most obvious ones and hope that your breakdowns will stop.

1. First, consider the formulas for the gains of the two configurations:

inv. amp -> G = -(R_f/R_in)

non inv. amp -> G = 1+R₂/R₁

As you notice, the gain of the non inverting amplifier will ALWAYS BE greater than or equal to 1. This means that when you have R₂ = R₁ (which is not TRULY achievable in practice) your output voltage (or signal) will be multiplied by 1 (i.e. it will not be amplified at all).

Sometimes however, you might be interested in say halving the input signal. In such a situation, you MUST use an inverting amplifier as it does allow for a fractional gain (consider R_f = 1 ohm and R_in = 2 ohms). Here, we are of course talking about small signals. As you must have realised, for large voltage signals a transformer will do the trick.

2. With the inverting amplifier, R_in determines the input resistance of the ENTIRE circuit. It is sometimes very important that you have a PARTICULAR input resistance to a amplifier as, depending on what you connect to it, not having a particular input resistance can lead to huge losses and distortions in the signal. (The input resistance of a non inverting amplifier is that of the actual op amp. Ideally infinite. Which is not always good.)

3. On many occasions, you are actually interested in turning the signal upside down. Particularly in digital applications.

Consider having a signal representing a binary value of 1. To that value you want to add 1, giving you the output 0 (keep in mind this is binary). How would you then be able to change the value of the signal from 1 to 0 in the most effective way? You simply invert it! As a result, inverters are used in digital electronics as counters, adders, multipliers and in many other circuits.

4. Inverting amplifiers tend to be more stable as currents leaking into the + or - terminals do not have such a large effect on the output as for the non inverting amplifiers.

I really hope this helps, and as you read this keep in mind that NONE of this information is actually relevant to your IB diploma - this is purely for your interest. If there's anything you're still unclear about, let me know!

- Bogdan, Lanterna Tutor