**Ⅰ.** **Introduction**

In electronics, an operational amplifier is a circuit unit with a very high amplification factor. In the actual circuit, usually combined with the feedback network to form a certain functional module. It is an electronic device with a special coupling circuit and feedback. The output signal can be the result of mathematical operations such as addition, subtraction or differentiation, integration, etc, thus it was used in analog computers to implement mathematical operations.

An op amp is a functional unit that can be implemented in discrete devices or in semiconductor chips. With the development of semiconductor technology, most of the op amps exist in the form of a single chip, but there are many types of op amps, which are widely used in the electronics industry.

The op amp can be simply viewed as a high-gain direct-coupled voltage amplifying unit with one signal output port (Out) and two high-impedance inputs, non-inverting input and inverting input, so op amps can be used to make the non-inverting, inverting, and differential amplifiers.

This video talks about inverting and non-inverting op amps, and explains how to calculate the closed loop voltage gain of such amplifiers as well as the output signal voltage.

**Ⅱ.** **Non-inverting Amplifiers and Inverting Amplifiers**

An operational amplifier in an electronic circuit has a non-inverting input and an inverting input. The same polarity of the input and the output is a non-inverting amplifier, on the contrary, it is an inverting amplifier. And the inverting amplifier circuit has a function of amplifying the input signal and inverting the output.

Non-inverting: When a positive phase is received, a positive phase is output, whereas the negative phase is output. The phases of non-inverting end and the output end are the same.

**Figure 1. Non-inverting Amplifier**

Vin and V-Virtual are short circuit in the figure, where **Vin=V-……a**

Because of the virtual open circuit, there is no current to the inverting input, the current through R1 and R2 is equal, and the current is set to I, which is obtained by Ohm's law:

**I=Vout/(R1+R2)……b**

Vin equal to the partial voltage on R2, where **Vin=I*R2……c**

By abc, where **Vout=Vin*(R1+R2)/R2**

Inverting: When the positive phase is received, the negative phase is output, whereas the positive phase is output. And the non-inverting end and the output end are keeping inverting relation.

Figure 2: The grounding of the op amp is 0V, the inverting end and the non-inverting end are short circuit, so it is also 0V. The input resistance of the inverting input is very high, while it is virtual open. So that there is almost no current injection and outflow, then R1 and R2 are equal to a series connection, the current flowing through each of the components in a series circuit is the same, that is, the current flowing through R1 and the current flowing through R2 are the same.

**Figure 2. Inverting Amplifier**

Current flowing through R1: **I _{1}=(Vin-V-)/R1………a**

Current flowing through R2: **I _{2}=(V--Vout)/R2……b**

**V-=V+=0………………c**

**I _{1}=I_{2}……………………d**

By solving the above algebra equation, we can get the result:

**Vout=(-R2/R1)*Vi**

The inverting amplifier circuit has the function of amplifying the input signal and inverting output, which is a negative feedback technique. Negative feedback returns a portion of the output signal to the input. The reason why the inverting amplifier can only connect the signal to the inverting input is because the negative feedback can be formed only in this way, otherwise it will not work in the linear amplification region.

When inputting from both ends simultaneously, the size and phase are the same, that is the common mode signal, and the theoretical output is zero.

**Figure 3. Integrated Circuit Using Op-amp**

The integrated operational amplifier can be connected to the non-inverting or to the inverting amplifier. Is it better to non-inverting amplification or inverting amplification? Let's look at the difference between them.

1）non-inverting amplifier

a. Advantages

The input impedance is equal to the input impedance of the op amp, which close to infinity.

b. Disadvantages

The amplifying circuit has no virtual ground, so it has a large common mode voltage, and the anti-interference ability is relatively poor. So that the op amp requires a higher common mode rejection ratio, and another disadvantage is that the amplification factor can only be greater than one.

2）inverting amplifier

a. Advantages

The potential of the two input terminals is always approximately zero (the non-inverting terminal is grounded, and the inverting terminal is virtual-grounded), in addition, only the differential mode signal exists, and the device has strong anti-interference ability.

b. Disadvantages

The input impedance is small, which is equal to the resistance of the series resistance of the signal to the input.

3) The gain calculation of the two are different, and their phases are opposite.

**Ⅲ.** **Note: Input Impedance**

The input impedance of the non-inverting input is high, and the input impedance of the inverting input is low. The input impedance of the non-inverting input is basically determined by the bias resistor connected in parallel with the non-inverting terminal, and the resistance can be very large. When the inverting input is connected, the feedback resistor is connected between the inverting terminal and the output terminal, and the resistance is small, so the input impedance of the inverting input is relatively low.

1. The magnitude of the input resistance of the non-inverting amplifier does not affect the input impedance, and the inverting amplifier input resistance is approximately equal to the input impedance.

2. When the input impedance is required to be high, the non-inverting amplifier should be selected.

3. If the input impedance is not required to be large, the non-inverting or inverting can be selected at this time. When the phase is not considered strictly, the inverting amplification is preferred because it only has the differential mode signal.

4. The CMRR of the inverting amplifier is better when the CMRR is decisive.

Inverting amplifier, the input common mode of the op amp is almost constant, the common mode amplification is not reflected to the output, and the input common mode of the op amp in the non-inverting amplifier changes with the input signal, the common mode amplification of the op amp will be reflected Output. Therefore, the CMRR of the inverting amplifier is better when the CMRR of the op amp is decisive.

**Ⅳ.** **Summary**

It can be seen that comparing them is from the following aspects: input and output impedance, common mode anti-interference.

1. The input impedance of the non-inverting amplifier is equal to the input impedance of the op amp, and they are close to infinity. The input resistance of the non-inverting amplifier does not affect the input impedance; and the input impedance of the inverting amplifier is equal to the resistance of the series resistor of the signal to the input. Therefore, when the input impedance is required to be high, the non-inverting amplifier should be selected.

2. The input signal range of the non-inverting amplifier is limited by the op amp's common-mode input voltage range, while it is not the case with the inverting amplifier. Therefore, if the input impedance is required to be low and the phase is free, the inverting amplification is preferred because it only has a differential mode signal. And the anti-interference ability is strong, thus a larger input signal range can be obtained.

3. In the design where the same magnification is required, try to select a resistor with a small value, which can reduce the influence of the input bias current and the influence of the distributed capacitance. If you are more concerned about power consumption, you have to compromise on the resistance.

4. Determine if the input signal is a non-inverting input or an inverting input. If the input resistance of the amplifier circuit is required to be large, the non-inverting input amplifier circuit should be used because the increase of the input resistance of the amplifier circuit will affect the voltage gain. When the inverting input resistance is increased, the voltage gain of the circuit is reduced, and the voltage gain is also affected by the internal resistance of the signal source. Therefore, when designing the inverting input amplifying circuit, sometimes the input resistance and the voltage gain is difficult to balance. If the bias resistor or the voltage divider is appropriately increased, the input resistance of the amplifier circuit can be increased, and the voltage gain has little or no effect on the voltage gain, which requires a better understanding of the circuit.

When the input voltage is from the inverting end, the output voltage and the input voltage are inverted.

When the input voltage is from the non-inverting end, the output voltage and the input voltage are non-inverted.

## 1 comment

When distinguish the non-inverting op amp and the inverting op amp, look at which end of the op amp the signal enters. If it is the inverting end, it is the inverting op amp, entered from the non-inverting end is opposite. If the signals enter from the two, it is most likely differential or addition and subtraction op amp.