Dry information | Selection of non-inverting amplification and inverting amplification of op amps

The op amp is a commonly used device for analog power feedback and various signal conditioning circuits. Depending on whether the input signal is connected to the non-inverting end or the inverting end of the op amp, we can divide it into a non-inverting input amplification circuit and an inverting input amplification circuit. What is the difference between these two amplifier circuits, or what are their advantages and disadvantages? This article discusses this basic topic.

one. Non-inverting amplifier operational amplifier circuit analysis

In a non-inverting amplified op amp, the input signal is connected to the non-inverting terminal of the op amp, and the signal source faces an infinite impedance, as shown in Figure 1.

Figure 1 Schematic diagram of a common-mode amplifier

According to the characteristics of an ideal operational amplifier, the input impedance of both input terminals is high impedance, so the current on the input pin of the operational amplifier is 0. At the same time, since the voltage at both ends of the operational amplifier is the same, the voltage at the negative input terminal is VIN. At this time, we can find the relationship between VIN and VOUT based on the resistor voltage division relationship .

Figure 2 Input-output relationship of the op amp with in-phase amplification

As can be seen from Figure 2 above , the gain of the output over the input is always greater than 1 , so the output signal is always greater than the input signal. This is subject to certain limitations in circuit use, so it is relatively rare in power supply feedback circuits.

Similarly, from the basic relationship in Figure 2 , we can know that when RF is much smaller than RG , or when RG is much larger than RF , the input signal VIN is equal to the output signal VOUT. At this time, the op amp is a single-gain op amp ( In-phase amplification).

Figure 3 Input and output relationship of the same-phase single-gain op amp

At this time, the corresponding circuit form is RG infinite impedance (open circuit) and RF infinitesimal impedance (short circuit), which is equivalent to directly short-circuiting the input inverting terminal and the output.

It is worth noting that the RF resistor can never be removed in a current feedback amplifier because it involves the stability of the amplifier. We will discuss the topic of current feedback amplifiers later. In some voltage feedback amplifier circuits, the RF resistor is also retained to limit the current flowing through the input ESD circuit.

2. Analysis of inverting amplifier op amp circuit

Figure 4 Schematic diagram of reverse amplifier

The input signal of the inverting amplifier is connected to the inverting input terminal through the RG1 resistor, and the output signal is also connected to the inverting input terminal through the feedback resistor RF1 . We place a reference voltage VREF at the non-inverting terminal .

After derivation, we get the input and output relationship of the DC operating point as shown in Figure 5 .

Figure 5 DC input-output relationship of the inverting amplifier circuit

When we analyze only the small signal disturbance at the DC operating point, we can regard VREF as 0 , and we can see that the basic relationship becomes as shown in Figure 6 .

Figure 6 Input-output relationship of inverting operational amplifier in small signal state

As can be seen from Figure 6 , when the inverting input is used, the gain of the op amp can be greater than 1 or less than 1 , which depends on the relative relationship between the values ​​of RF and RG , and has nothing to do with the parameters of the op amp itself. Since it is inverting amplification, there is a negative sign ( 180 degree phase shift) between the output and input.

For this circuit, if we only look at the AC small signal, the input non-inverting terminal is grounded, then the signal input impedance is determined by RG .

In summary, by analyzing the difference in gain relationship between in-phase amplifier and inverting amplifier, the concept of single-gain amplifier is introduced, and the main functions of feedback resistor and gain resistor are analyzed, which is helpful for understanding the basic feedback amplifier circuit of op amp. Do you understand?

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