Introduction
The operational amplifier is a commonly used component in signal processing and conversion. Operational amplifier is short for Op Amp. The op amp is a circuit unit named from a functional point of view. Since it employed in analog computers in the early days to realize mathematical operations. In electronic circuits, a certain functional module is usually combined with a feedback network. It consists of discrete devices or in a semiconductor chip. With the development of semiconductors, most of op amps exist in monolithic form now.
Non-Inverting and Inverting Amplifiers
There are many types of op amps today. They are widely used in almost all industries. But there are two types of non-inverting input and inverting input in common. The typical circuit is as follows:
Figure 1. Simple Amplifying Circuit
Non-Inverting Amplifiers vs Inverting Amplifiers
Non-Inverting Amplifiers
1) The output is in phase with the input.
2) High input impedance.
3) The Non-inverting output works for bootstrap.
4) The inverting input is idle for other purposes.
5) As a voltage follower.
6) Input voltage in series with negative feedback.
7) The input has no virtual ground, large common mode voltage, and poor anti-interference ability.
Inverting Amplifiers
1) The output and the input are reverse.
2) The amplifying circuit can use feedback compensation.
3) To increase conversion rate.
4) Low signal source impedance, and high signal-to-noise ratio.
5) As a current input.
6) To reduce input impedance or maintain a certain value in the circuits.
7) The non-inverting input end is idle for other purposes.
Feedback Circuit of Non-inverting Amplifier
For the non-inverting amplifier, since the feedback loop reaches the inverting end, its amplification factor has nothing to do with the input signal. Even if the internal resistance R of the input signal changes greatly, it will not affect the amplification factor of the op amp. But the the internal resistance of the signal will affect inverting amplifier. However, the non-inverting amplifier also has certain inconveniences. If have zero adjustment on the inverting terminal of the non-inverting op amp or an addition circuit, the impedance of the signal source will change to affect the gain. Generally, when using a non-inverting amplifier, the inverting end does not connect other circuits except for the feedback circuit.
Voltage Follower Circuit
A common application of non-inverting amplifiers is voltage followers, following is the voltage follower circuit:
Figure 2. Voltage Follower Circuit
In this circuit, R7 is a protection resistor. It prevents a large current from flowing into the clamp diode of the operational amplifier and burning the component. Generally, require a large phase compensation capacitor when using a non-inverting amplifier to make the system stable. So use the voltage follower with phase compensation capacitor with the condition that the input signal rises slowly and the conversion rate is small. When processing signals with high rise speed and large amplitude, the emitter follower or FET source follower designed by transistors or a dedicated voltage follower operational amplifier are popular.
RC Circuit
When using a voltage follower, if self-oscillation occurs, the first thing that comes to mind is phase compensation. Reduce the electric shock by moving the pole position. As for the first method, the RC circuit connectes with the non-inverting and inverting ends of the operational amplifier in series, as follows:
Figure 3. RC circuit
Another method is to connect a resistor in series between the load and the voltage follower (the load behaves as a capacitor). At this time, it is necessary to confirm that if the load is non-capacitive through calculation, oscillation will not occur. In addition, the effect of this method is not very obvious because of amplifier oscillation.
Figure 4. RC circuit
Load in Amplifier Circuits
In the electronic circuit design, usually, the circuit becomes oscillating due to carelessness to the characteristics of the load. At this time, we should pay attention to the load.
Normally, when the load is capacitive and less than 2000pF, eliminate the oscillation by connecting a small resistor in series with the output of the load and the op amp. The resistor R2 is between 10-300Ω.
Figure 5. Amplifier Circuit
Compensation Capacitor
When the load is large, we use the following method to eliminate:
Figure 6. Amplifier Circuit
The compensation capacitor C2 and the feedback resistor R3 form an advanced compensation network, forming a new zero point, which offsets the new pole formed by the capacitive load Cl and the op amp output resistance R1, thus achieving the purpose of eliminating oscillation. At this time, the size of the compensation capacitor C2 is C2=Cl(R1+R2)/R3, and R2 takes an value of 10~300Ω.
Voltage Attention
When using a non-inverting amplifier, it is necessary to care about the voltage range. If the voltage exceed the rated voltage of the op amp, which will damage the device. So requires a limiting circuit.
Figure 7. Non-inverting Amplifier Circuit
When the voltage signal is input through the resistor R15, the signal input to the non-inverting terminal of the operational amplifier may rise slowly due to the influence of the amplifier’s own input capacitance and other stray capacitance. If this happens, the bootstrap circuit may also be used. At this time, connect a small capacitor C4 to compensate, the value of C4 is generally C4=(R14/R16)*C3
The C3 is the total capacitance at the input end. If the value of C4 is greater than C3, the circuit will oscillate. Therefore, C4 mostly uses ceramic fine-tuning capacitors with good temperature characteristics. And it is convenient for adjusting when observing the waveform.
Figure 8. RC Circuit
Conclusion
Although the non-inverting op amp has various limitations and inconveniences during use, its unique characteristics are still useful in some typical circuits.
You should obey the following rules when choosing them.
Unique Characteristics in Amplifier
Inverting Amplifier:
1) The voltages of the two input terminals are equal to zero. So there is no common mode input signal. In addition, there is no requirement for the common mode rejection of the op amp.
2) Vp=Vn=0, the inverting terminal is virtual ground.
3) Have a deep negative feedback. The input resistance is R1 and the output resistance is approximately zero.
Non-inverting Amplifier:
1) Have high input resistance and low output resistance.
2) Since Vp=Vn≠0, a common-mode input signal is introduced. It requires the op amp to have a higher common-mode rejection ratio.
Common Rules
1) The input impedance of non-inverting amplifier is close to infinity. So it is often used as a voltage follower for isolation.
2) The positive and negative potential difference at the input end of inverting amplifier is close to 0. In addition, only differential mode signals exist, and the anti-interference ability is strong.
3) There is no “virtual ground” in the input of non-inverting amplifier. But there is a large common-mode voltage, and poor anti-interference ability. When used, the operational amplifier is required to have a higher common-mode rejection ratio.
4) The input impedance of inverting amplifier is very small. It is equal to the resistance of the series resistance at the signal input terminal.
2) In non-inverting amplifier circuit, the introduced voltage connects negative feedback in series.
3) In inverting operational amplifier circuit, the voltage introduced is in parallel with negative feedback.
4) Both amplifiers output resistances are basically zero. Because introduce a deep voltage negative feedback.
5) Consider the “virtual break” and “virtual ground” principles together.
Note
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