PHY C21: Inverting & Non-Inverting Amplifiers

Now, let’s take a closer look at negative feedback.

• Effects of Negative Feedback
• Inverting amplifiers
• Virtual earth approximation
• Non-inverting amplifiers
• Expressions for voltage gain of inverting & non-inverting amplifiers

Throughout this post, remember the key thing about negative feedback:

Negative feedback prevents an amplifier from saturating.

This will be handy in understanding its usage below.

What are the benefits of negative feedback?

• increased bandwidth
• less output distortion
• greater stability

What are the applications of negative feedback?
You can create INVERTING & NON-INVERTING AMPLIFIERS.

These differ slightly from the negative feedback shown above since not all of the output voltage is fed back to the inverting input.

General Characteristics of Amplifiers using Negative Feedback

Both function in similar ways: I can provide an input signal (maybe from a microphone recording my voice) & the amplifier will amplify it.
The differences between them are where Vin is connected to.

Now, let’s explore each type of amplifier.

Inverting amplifier

How can you identify an inverting amplifier?
Vin is connected to the inverting input.

Before looking at its characteristics, we need to get a concept down first: VIRTUAL EARTH.

What is meant by Virtual Earth?
A point where potential can be approximated to be 0V (Earth).

Why?:

• an ideal op-amp has infinite gain
• the non-inverting input is connected directly to the ground, so it is at 0 V
• remember: negative feedback prevents the amplifier from saturating
• if the amplifier is not to saturate, the inverting input must be almost at 0 V as well
• thus, we can say it is at the same potential as the inverting input

What does this mean?
This causes an inverting amplifier to have a gain given by:

Vout/Vin = -Rf/Rin

Here’s why:

How does this effect the characteristics of an inverting amplifier?

Non-inverting amplifier

How can you identify a non-inverting amplifier?
Vin is connected to the non-inverting input.

There is no virtual earth point here, unlike an inverting amplifier.

A non-inverting amplifier’s gain is given by:

Vout/Vin = 1 + Rf/R1

Here’s why:

How does this effect the characteristics of a non-inverting amplifier?