In electronics, analog circuits is a very important and difficult course for many people. Here is a description of the understanding of the analog circuit class, I hope it can help you.
1 Summary
Analog electricity itself is a very complex subject, and the analog course is only the most basic thing in it. Analog Circuit (Analog Circuit) means an electronic circuit that processes analog signals. Most signals in nature are analog signals, which have continuous amplitude values, such as the voice signal when speaking. Analog circuits can process such signals directly (of course, they need to be converted into electrical signals first), for example, amplifiers can amplify sound signals, and broadcasters can send analog sound signals and image signals. It can even be argued that the basis of all circuits is analog (even digital circuits, whose underlying principles are based on analog circuits). Its importance cannot be overstated.
Due to the rapid development of digital circuits and programmable devices, many superior characteristics are reflected. Many electronic devices are slowly being digitized, but they are always still inseparable from analog circuits.
Currently, the most important devices in analog circuits are semiconductor devices. The most basic and commonly used semiconductor devices are diodes, triodes, field-effect tubes and operational amplifiers.
Diodes have many roles, such as ordinary diodes can be used for rectification, light-emitting diodes can be used for indicators and lighting, voltage regulators for voltage regulation, varactor diodes can be used for signal modulation, etc. In the analog course, the part involving diodes is relatively simple, while many characteristics of field-effect tubes are similar to triodes, so they are often taught as the main body of the triode or op-amp.
2 transistors and amplifiers
The basic function of the triode is amplification, and through this characteristic, the triode forms a variety of circuits that reflect many engineering ideas.
The basic circuit of a triode is an amplifier, for example, an amplifier is an amplifier where the input is small and the output is large. The ratio of the amplifier’s output to the input voltage (or current) is called the amplification, also called the gain.
For a voltage, if a graph is made with time as the horizontal axis and voltage as the vertical axis, the graph will be the waveform of the voltage.
If an amplifier with an amplification of 5 has a constant input voltage of 1V (waveform as shown in the left graph of Figure 1), the output should always be 5V (waveform as shown in the middle graph of Figure 1), which will neither change with time nor with temperature, and the output and input voltages will have exactly the same shape. However, if the amplification is unstable and keeps changing, the original input signal will be distorted (as shown in the right panel of Figure 1), and the signal may change from a horizontal straight line to a curve. This waveform change is called distortion.
Figure 1
An ideal amplifier, hopefully, has a constant amplification. If the amplification of the amplifier is not stable, the sound will be large and small, and the change of waveform will also lead to changes in the sound, i.e. distortion.
Reality always contradicts the ideal. Unfortunately, the characteristics of the triode is not ideal, it works in the amplifier circuit, the amplification is not only affected by the input voltage, power supply voltage, and their own heat caused by temperature changes, will also affect its amplification. This is really a headache for many engineers, if you can not find an effective way to reduce the impact of this characteristic, the triode is difficult to apply to the actual.
3 Negative feedback
3.1 Basic concepts
So some very powerful people have found a good way: negative feedback. What is negative feedback?
Feedback refers to the system’s output and return to the input and affect the input, thereby producing an effect on the overall output of the system. Feedback can be divided into positive feedback and negative feedback. Negative feedback is to make the output to play the opposite role of the input, so that the system output tends to stabilize.
The above explanation is not easy to understand, I give two examples.
- When playing with an inverted pendulum, we use our hands to support an inverted stick, when the stick tilts in a certain direction, we offset this change by moving our hands to the direction of the stick tilt, so that the stick can be balanced in the hands.
- In high school, we often took monthly exams, and I found that some students had this habit: when they did poorly in one exam, they would start studying hard, and then their grades would go up next time; and when they did better, they would slack off again in the next month, so their grades would go down again, and so on and so forth.
Both of these examples fully illustrate that negative feedback can make the system more stable.
3.2 Negative Feedback Amplifier
Let’s ignore the specific circuit and just draw a simple block diagram to illustrate how a triode amplifier circuit makes use of negative feedback.
The triangle in Figure 2 represents an amplifier composed of a triode with amplification A and input I , then the output O=A*I , and the output waveform will be distorted because the amplification A is unstable.
Figure 2
A few devices were added to the circuit, as in Figure 3.
The purple circle is the summing device, combined with the purple “+” and “-” symbols, indicating that its output Y = (+I)+(-X) = I-X , which can be achieved in the actual circuit with a resistor;
The box F is a feedback device, indicating that the signal is taken out from the output O and multiplied with F to get X , so X=0*F , where F<1 (this part can be implemented with a resistor in the actual circuit);
The triangular representation of the amplifier A, which is mainly composed of triodes, satisfies O=A*Y , and the amplification of A is unstable and easily disturbed.
Figure 3
A system of equations can be listed as
The amplification of the whole circuit is solved as follows
If the circuit is designed so that the amplification A is very large and at the same time F is not too small, then
According to the idea of approximation, the entire circuit above has the following amplification.
Since the feedback device can be realized by a resistor, the resistance value of an ordinary resistor is not easily disturbed by the outside world, so the value of F is stable, so the amplification of the whole circuit is stable. We have successfully solved the problem of amplification stability of the triode by negative feedback.
As you can see here, the feedback part and the amplification part form a loop, so the amplification of the whole circuit is called loop gain, or closed-loop gain; and the amplification of the circuit before adding feedback, A, is called open-loop gain. Because of the negative feedback, although the circuit gain stability is improved, there is a cost.
Since AF>>1 so A>>1/F
That is, the open-loop gain is much greater than the closed-loop gain, which means that the amplifier gain is greatly reduced. But in general, this is worth it for stability.
4 Operational Amplifier
In the above circuit, in order to actually create an amplifier with a large open-loop gain A, it is often designed with a multi-stage triode amplifier circuit connected in series. Since the need for such high gain amplifiers was common, someone historically made them into a finished circuit board module, and it was very convenient to use them directly as a component when they were to be used. This was the original operational amplifier, or op-amp for short.
The development of integrated circuits made it possible to integrate a large number of transistor components on a small chip, and so there was the integrated operational amplifier that is very commonly used today.
The name “operational amplifier” was derived from its original use in analog computers to perform mathematical operations. Although digital computers, which are now widely used, no longer use op amps for computational operations, the name has been retained. Today, op amps play a very important role in analog circuits and are one of the main focuses of analog courses.
4.1 The false short and false break characteristics of op amps
Usually op amps have two inputs U+ and U, and one output Uo, which satisfy between them.
Uo = A(U + U)
The op-amp loop gain A is often as high as hundreds of thousands to millions, but the output voltage of the op-amp is limited by the supply voltage and cannot exceed the supply voltage. So the input-output relationship of the op-amp is similar to the shape of Figure 4.
The horizontal axis in Figure 4 is (U+U) and the vertical axis is U.
Figure 4
In the middle section of the linear region, the operator is placed in the normal amplification state, called the linear region, satisfying.
Uo=A(U+U)
When the absolute value of the input is a little larger, the output is limited by the power supply and no longer satisfies the above relationship. The value of Uo is usually slightly smaller than the supply voltage range (note that the op-amp can use dual power supplies, i.e., the supply voltage range can be between a negative and a positive value), which is called the nonlinear region.
The output of rail-to-rail op-amp can reach the supply voltage, you can search and learn by yourself on the internet if you are interested.
When the op-amp operates in the linear region, the value of Uo is very limited, but A is large, so.
(U + U) = Uo / A ≈ 0 that U + ≈ U
At this time, the positive and negative input voltages of the op-amp are almost equal, just like a short circuit, called a false short. Therefore, only when the op-amp is working in the amplification area, it will have the characteristic of “virtual short”, not the inherent property of the op-amp itself.
On the other hand, the input impedance of the op-amp is very large due to its internal structural characteristics. Input impedance can be simply understood as follows
Input impedance = Voltage at input / Current at input
A high input impedance means that only a small amount of current is required at the input of the op-amp to operate properly. Because of this, op amps can be used to detect weak currents, such as human brain waves and EMG waves, where the maximum voltage value is only a few mV and the current value is very small.
This characteristic of op-amp is called virtual break, which means that the input is the same as a broken circuit and almost no current flows in.
Unlike the virtual short, the virtual break is an inherent property of the op-amp itself and does not change with different circuits.
4.2 Non-ideal Characteristics of Op-Amps
The op-amp is composed of triode, and obviously, like triode, it will have a lot of non-ideal characteristics. The above mentioned are the characteristics of ideal op amps. The actual op-amp, it will not fully meet the virtual short and false break characteristics, the normal operation of the input requires current inflow, the current is called the input bias current. This current is called the input bias current. The same op-amp also has non-ideal parameters such as input bias voltage, input detuning voltage and input detuning current.
These non-ideal characteristics, such as the input bias current, although small, can sometimes have a significant impact on the circuit, causing it to fail to work. Therefore, it is necessary to reduce the impact of these factors by some means. In practical applications, the non-ideal characteristics of op amps are a very important issue. There are many ways to eliminate the non-ideal characteristics of op amps, so we will not introduce them here.
5 Comparison of Op-Amp and Triode
In the actual design of the circuit, op amps are relatively more used than transistors. Because many characteristics of op-amps are better than triodes, the circuit design is simple, and often the cost of op-amps is not high. In many cases, the same effect is achieved with a triode and an op-amp, but the cost of using an op-amp is lower. Because op amps integrate a large number of transistors into one piece, the average manufacturing cost per transistor is very low.
For example, a conventional audio preamplifier, a general-purpose op-amp can be done, the cost may be 0.2 yuan, while the use of transistors to achieve the same effect, may require 10 or even more transistors, the cost of perhaps 0.5 yuan, and the design of the human cost is much higher than the op-amp program.
Of course, the triode also has its advantages. In some very simple circuits, and not strictly required amplification stability, one or two transistors to complete the task, often using transistors to save costs. In addition, in some more extreme conditions, such as working in a high-frequency, high-power environment (such as RF signal transmission circuit), the performance of a well-designed triode circuit will be much better than the op-amp effect, or much lower cost, and even in some cases only the direct use of transistors to complete the circuit, when it is necessary to use transistors to build the circuit.
