How to Suppress Zero Drift in Direct-Coupled Amplifier Circuits

0 Introduction

Direct coupling is the simplest way to connect stages. It is to directly connect the input of the latter stage with the output of the previous stage. The coupling method in which the output of one amplifier circuit is directly connected to the input of another amplifier circuit is called direct coupling. In addition, the direct coupling amplifier circuit can amplify both AC signals and slowly changing signals. And because there is no large-capacity capacitor in the circuit, it is easy to integrate all the circuits on a silicon chip to form an integrated amplifier circuit. Due to the rapid development of the electronics industry, the performance of integrated amplifier circuits is getting better and better, the types are increasing, and the prices are getting cheaper, so the use of direct coupling amplifier circuits is becoming more and more widespread. In addition, many physical quantities such as pressure, liquid level, flow, temperature, length, etc. are converted into weak, slowly changing non-periodic electrical signals after being processed by sensors. Such signals are not enough to drive the load and must be amplified. Because such signals cannot be transmitted step by step through coupling capacitors, it is obviously not possible to amplify such signals by using resistor-capacitor coupling amplifier circuits. Direct coupling amplifier circuits must be used. However, after the direct coupling connection method was adopted between each stage, the static working points between the front and rear stages influenced each other and the zero-point drift occurred. This paper mainly analyzes the cause of the zero-point drift and finds a solution.

1 Characteristics of direct coupled amplifier circuit

When a multi-stage amplifier circuit needs to amplify extremely low frequency signals, or even DC signals, neither resistor-capacitor coupling nor transformer coupling between stages is applicable, and direct coupling as shown in Figure 1 must be used.

The RC coupling method in Figure 1 uses only one capacitor to connect the two-stage amplifier circuits, which is simple. The coupling capacitor has the function of blocking direct current and passing alternating current. The capacitance of the capacitor is selected according to the frequency of the signal, so that the capacitive reactance is very small, and the AC signal can be transmitted smoothly; the DC blocking function of the capacitor makes the static working points of each stage of the amplifier circuit independent of each other, and does not affect each other. As long as the static working points of each stage are relatively stable, the operation of the entire amplifier circuit is relatively stable. Therefore, the RC coupling amplifier circuit is widely used. However, in various automatic control systems and some measuring instruments, most of the transmitted signals are extremely slowly changing, non-periodic signals, or even DC signals. For example, the speed of the hydro-generator set, the terminal voltage of the generator, the oil temperature of the transformer, the water level of the forebay of the hydropower station, etc. change slowly. To realize the measurement and automatic control of these slowly changing physical quantities, these physical quantities must be converted into electrical signals (i.e., analog signals). Since these electrical signals are not only slowly changing, but also weak, they must be amplified. The frequency of the slowly varying signal is extremely low. If it is coupled with a capacitor, the capacitance must be very large. Such a capacitor is difficult to make. It is not only costly and bulky, but also has poor performance, which is unrealistic. People will naturally think of directly connecting the two-stage amplifier circuit with a wire, so that even lower frequency signals and even DC signals can pass smoothly. This is the direct coupling method. The direct coupling amplifier circuit can amplify both AC signals and slowly varying signals and DC signals (so it is called a DC amplifier circuit in some books). The lower limit frequency of its frequency characteristics is zero, and it is widely used in automatic control systems and electronic instruments.

2 Special Problems of Direct-Coupled Amplifier Circuits——Zero Drift

Zero drift is a special problem in direct-coupled amplifier circuits. The so-called zero drift refers to the fact that when the input of the amplifier circuit is short-circuited (that is, when there is no input signal), a sensitive DC meter is used to measure the output end, and a slowly changing output voltage will also be generated, which is called zero drift phenomenon, as shown in Figure 2. The signal of zero drift will be transmitted between the circuits of each level of amplification, and after multiple levels of amplification, it will become a larger signal at the output end. If the useful signal is weak, in the direct-coupled amplifier circuit with zero drift phenomenon, the drift voltage and the effective signal voltage are mixed together and amplified step by step. When the drift voltage can be compared with the effective signal voltage, it is difficult to distinguish the voltage of the effective signal at the output end; in the case of severe drift phenomenon, the effective signal will often be "submerged", making the amplifier circuit unable to work normally. Therefore, it is necessary to find out the cause of zero drift and the method to suppress zero drift.

3 Causes of zero drift

There are many reasons for zero drift, mainly in three aspects: first, the fluctuation of power supply voltage will cause output voltage drift; second, the aging of circuit components will also cause output voltage drift; third, semiconductor devices will change with temperature, which will also cause output voltage drift. The first two factors cause a small zero drift. Practice has proved that temperature change is the main cause of zero drift and the most difficult factor to overcome. This is because the conductivity of semiconductor devices is very sensitive to temperature, and the temperature is difficult to maintain constant. When the ambient temperature changes, it will cause changes in transistor parameters VBE, β, ICBO, thereby changing the static operating point of the amplifier circuit. Moreover, since the inter-stage coupling adopts direct coupling, this change will be amplified and transmitted step by step, and finally cause the voltage at the output end to drift. The more stages of the direct coupling amplifier circuit and the greater the amplification factor, the more serious the zero drift. In the zero drift generated at each stage, the zero drift generated at the first stage has the greatest impact. Therefore, the key to reducing zero drift is to improve the performance of the first stage of the amplifier circuit.

4 Measures to suppress zero drift

The specific measures to suppress zero drift are as follows:

(1) Select high-quality silicon tubes. The ICBO of silicon tubes is several orders of magnitude smaller than that of germanium tubes. Therefore, almost all high-quality DC amplifier circuits currently use silicon tubes. In addition, the manufacturing process of transistors is also very important. Even if they are the same type of transistors, if the process is not strict enough and the semiconductor surface is not clean, the drift degree will increase. Therefore, qualified semiconductor devices must be strictly selected.

(2) Introduce DC negative feedback into the circuit to stabilize the static operating point.

(3) Use the temperature compensation method to use thermistors to offset the changes in the amplifier tube. Compensation means using the drift of another component to offset the drift of the amplifier circuit. If the parameters are properly matched, the drift can be suppressed to a lower limit. In circuits composed of discrete components, diode compensation is often used to stabilize the static operating point. This method is simple and practical, but the effect is not ideal. It is suitable for circuits that do not require high temperature drift.

(4) Modulation means converting DC changes into other forms of changes (such as changes in the amplitude of a sine wave), amplifying them through a resistor-capacitor coupling circuit with very small drift, and then trying to restore the amplified signal to changes in the DC component. This method has a complex circuit structure, high cost, and poor frequency characteristics. The cost of implementing this method is relatively high.

(5) Inspired by the temperature compensation method, people use two transistors of the same model and characteristics for compensation, which has achieved a better effect of suppressing zero drift. This is the differential amplifier circuit. The most widely used unit circuit inside the integrated circuit is the differential amplifier circuit based on the parameter compensation principle. In the direct-coupled amplifier circuit, the most effective way to suppress zero drift is to use a differential amplifier circuit.

4.1 Principle of Suppressing Zero-Point Drift in Differential Amplifier Circuit

The differential amplifier circuit is also called the differential circuit. It can not only effectively amplify DC signals, but also effectively reduce the zero drift caused by power supply fluctuations and temperature changes of transistors. Therefore, it is widely used, especially in integrated operational amplifier circuits. It is often used as the pre-stage of a multi-stage amplifier.

The basic differential amplifier is shown in Figure 3. In the figure, VT1 and VT2 are transistors with the same characteristics, the circuit is symmetrical, and the parameters are also symmetrical. For example: VBE1=VBE2, RCl=RC2=RC, Bl=RB2=RB, β1=β2=β. The circuit has 2 input terminals and 2 output terminals. Because the left and right two amplifier circuits are completely symmetrical, when there is no signal, that is, when the input signal UI=0, Uo1=Uo2, so the output voltage Uo=0, which means that the differential amplifier has the characteristic of zero output when there is zero input. When the temperature changes, the output voltages Uo1 and Uo2 of the left and right tubes will change, but because the circuit is symmetrical, the output change of the two tubes (that is, the zero drift of each tube) is the same, that is, △Uo1=△Uo2, then Uo=0. It can be seen that the zero drift of the two tubes is offset at the output end, thereby effectively suppressing the zero drift. The differential amplifier circuit shown in Figure 3 can suppress zero drift because of the symmetry of the circuit. However, this circuit has defects: the ideal situation of complete symmetry does not exist; so there is a limit to suppressing zero drift by simply improving the symmetry of the circuit. The drift of the collector potential of each tube in the above differential circuit is not suppressed. If a single-ended output is used (the output voltage is taken from the collector of one tube and the "ground"), the drift cannot be suppressed at all. For this reason, the typical differential amplifier circuit shown in Figure 4 is often used.

4.2 Typical differential amplifier circuit structure and the principle of suppressing zero drift

The typical differential amplifier circuit is shown in Figure 4. Compared with the simplest differential amplifier circuit, this circuit adds a zero adjustment potentiometer RP, an emitter common resistor RE, and a negative power supply UEE. The following analyzes the principle of the circuit suppressing zero drift, the role of the emitter common resistor RE (the zero adjustment potentiometer RP can be considered as a part of RE), and the negative power supply EE.

The main function of RE in the circuit is to stabilize the static operating point of the circuit, thereby limiting the drift range of each tube and further reducing the zero drift. For example, when the temperature rises and both IC1 and IC2 increase, there is a drift suppression process as shown in Figure 5.

It can be seen that due to the negative feedback of RE, the collector potential remains basically unchanged, reducing the drift of the output end. The feedback resistor RE can suppress common-mode signals and has no effect on differential-mode signals. Zero-point drift belongs to common-mode signals, so the drift of each tube is suppressed to a certain extent. Obviously, the larger the resistance value of RE, the stronger the negative feedback effect of current, the better the current stabilization effect, and thus the more significant the drift suppression effect of each tube.

The role of the emitter negative power supply UEE: When the collector current and collector potential of the two tubes drift in phase due to various reasons (such as: both input signals contain common-mode signal components or 50 Hz AC common-mode interference signals, etc.), then RE has a current negative feedback effect on them, so that the drift of each tube is weakened, which further enhances the ability of the differential circuit to suppress drift and suppress signals with the same phase. Although, the larger the RE, the more significant the effect of suppressing zero drift; however, when UCC is constant, an excessively large RE will make the collector current too small, which will affect the static operating point and voltage amplification factor. For this reason, the negative power supply UEE is connected to compensate for the DC voltage drop across RE, so that the emitter point is approximately zero, and a suitable static operating point is obtained. The role of the resistor RP: The potentiometer RP is used for balancing, also known as the zero potentiometer. Because the circuit is not completely symmetrical, when the input voltage is zero (both input terminals are connected to the "ground"), the output voltage is not necessarily equal to zero. At this time, the initial working state of the two tubes can be changed by adjusting RP, so that the output voltage is zero. However, RP will play a negative feedback role for signals with opposite phases, so the resistance value should not be too large. Generally, the RP value is between tens of ohms and hundreds of ohms.

5 Conclusion

From the above analysis, we can know that the typical differential amplifier circuit can not only use the symmetry of the circuit and adopt the double-ended output method to suppress the zero drift, but also use the emitter common resistor RE to suppress the zero drift of each transistor and stabilize the static working point. Therefore, even if this typical differential amplifier circuit adopts single-ended output, its zero drift can be effectively suppressed. So this circuit has been widely used.