1. The main types of circuits are divided into four types according to the input and output mode: dual-end input dual-end output, dual-end input single-end output, single-end input dual-end output, and single-end input single-end output. According to the form of common-mode negative feedback, there are two types: typical circuit and emitter with constant current source. 2. Main features of the circuit (1) The circuit structure is symmetrical. The parameters of the two transistors are the same, and the two sides of the circuit are completely symmetrical. (2) Suppressing zero drift Since the two sides of the circuit are completely symmetrical, the zero drift of the collectors of the two transistors is also equal. When the circuit is output at the dual end, the zero drift of the circuit is zero; when the output is single-ended, the common-mode negative feedback of the emitter resistor RE has the ability to suppress zero drift. That is, the differential amplifier circuit uses the symmetry of the circuit and the common-mode negative feedback to suppress zero drift. (3) Suppressing common-mode signals When the two input signals are "common-mode signals" of equal magnitude and the same direction, due to the symmetry of the circuit and the effect of common-mode negative feedback, the output common-mode signal is very small. When the output is double-ended, the output common-mode signal is approximately zero. (4) Amplifying differential-mode signals When the two input signals are "differential-mode signals" of equal magnitude and opposite directions, due to the symmetry of the circuit, the two output terminals have "differential-mode signals" of equal magnitude and opposite directions. When the output is double-ended, the output differential-mode signal is equal to the sum of the output voltages on both sides, that is, the circuit has a greater amplification capability for differential-mode signals. (5) Common-mode rejection ratio The differential amplifier circuit has a stronger amplification capability for differential-mode signals and a stronger suppression capability for common-mode signals. That is, the differential mode gain is large, while the common mode gain is small. In order to comprehensively evaluate the performance of the differential amplifier circuit, the common mode rejection ratio is defined as 42121, and the larger the better. 2. The static analysis method uses the symmetry of the circuit to decompose the circuit into two halves. The RE (current is 2IE1) in the original circuit should be 2RE (current is IE1) in the equivalent circuit. The static operating point is solved according to the circuit equation. 3. Dynamic analysis method (1) Small signal differential mode characteristics Draw the differential mode equivalent circuit according to the characteristics of the differential mode signal, analyze and calculate the differential mode voltage gain, differential mode input resistance and output resistance. For the typical circuit shown in Figure 4.2, when the circuit inputs a differential mode signal, the signal current flowing through the emitter resistor RE is equal to zero. When analyzing the differential mode voltage gain, the emitter is treated as "AC ground", and when analyzing the differential mode input resistance, the emitter resistor RE is treated as an open circuit. According to the symmetry of the circuit, the load resistance is halved for double-ended output, and it is not necessary to halve for single-ended output. (2) Small signal common mode characteristics Draw the common mode equivalent circuit according to the characteristics of the common mode signal, analyze and calculate the common mode voltage gain, common mode input resistance and common mode rejection ratio. For the typical circuit shown in Figure 4.2, when the circuit inputs a common mode signal, the signal current flowing through the emitter resistor RE is equal to twice the emitter current of a single tube, and the emitter resistor in the common mode equivalent circuit is treated as 2RE. (3) Decomposition of arbitrary input signals If the two input signals of the circuit are neither differential mode signals nor common mode signals, the two arbitrary input signals uI1 and uI2 can be decomposed into input signals of differential mode and common mode. uI1=uI+uId/2, uI1=uI-uId/2 Where
The single-ended input mode of the differential amplifier circuit is equivalent to the case where uI1 or uI2 is equal to zero in the dual-ended input mode. The technical indicators of the typical circuit shown in Figure 4.2 are listed in Table 4.1. (4) Large signal characteristics When the input signal is within the range of ±26mV, there is a good linear relationship between current and voltage. When the input signal exceeds ±100 mV, the current of the two amplifier tubes almost no longer changes with the input voltage. One tube enters the cutoff region, while the current of the other tube is close to IE (=IE1+IE2). This is a very useful limiting characteristic.
Table 4.1 Technical indicators of typical differential circuits
Take three linear variable resistors of the same specification (VR1 and 2 are used as a bridge, an AC voltmeter is placed across the two moving pieces, and VR3 is connected in series with a power supply). When the bridge is balanced, turn VR3 or change the power supply voltage. What will the voltmeter indicate? If the bridge is unbalanced, what effect will the adjustment of VR3 or the fluctuation of the power supply have on the output?
VH3 replaces the constant current element, which can greatly reduce the impact of power supply fluctuations on the load voltage, and weaken the common mode amplitude modulation effect. However, the differential structure cannot be made carelessly, because when it is used for amplitude modulation (multiplication), the influence of the asymmetry of the parameters of each component will reappear. To improve the common mode rejection ratio, we still have to start from the core (the symmetry of the differential bridge)!
Originally posted by hk6108 on 2010-3-27 20:54 Take three linear variable resistors of the same specification (VR1 and 2 are used as a bridge, an AC voltmeter is placed across the two moving pieces, and VR3 is connected in series with the power supply). When the bridge is balanced, what will the voltmeter indicate when VR3 is moved or the power supply voltage is changed? If the bridge is unbalanced, what effect will the adjustment of VR3 or the fluctuation of the power supply have on the output?
Two purely resistive voltage dividers can form a purely resistive bridge, right? If the voltages of the two arms are equal, then no matter how the power supply changes, the voltages of the two arms are always equal. If there is a difference between the two voltages, then this difference should be multiple of the power supply voltage. This is the amplitude modulation effect. If it does not follow this multiple relationship, it is common mode distortion.
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