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questions :Configuration of differential amplifier
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[Visitor (223.166.*.*)]answers [Chinese ]Time :2021-03-23
Differential amplifiers can use transistors (transistors) or tubes as their active devices. The output voltage of u01-u02 is the difference between the output voltage of the transistor T1 and T2 sets of electrode output voltage of u01 and u02. When the input voltage amplitudes of T1 and T2 are equal but the polarity is opposite, i.e. us1-us2, the gain Kd (called differential mode gain) of the differential amplifier and the gain of the single-tube amplifier are equal, i.e. Kd≈Rc/re, in the mode Rc=Rc1=Rc2, re is the radio resistance of the transistor. Usually re is very small, so Kd is larger. When us1 is us2, i.e. the amplitude of the two input voltages is equal to the polarity, the amplifier's output u0 should be equal to zero and the gain equal to zero. The actual amplification circuit cannot be completely symmetrical, so there is still a certain gain. This gain is called common mode gain and is recorded as Kc.In practice, factors such as temperature change and supply voltage instability have an effect on amplification, equivalent to producing a drift voltage at the input of each transistor. The symmetry of the circuit can be used to cancel each other out or weaken it, so that the drift voltage at the output is greatly reduced. Obviously, the smaller the common mode gain, the smaller the drift voltage when the better the circuit symmetry...
The performance of differential amplifiers is usually represented by the ratio Kd/Kc of differential gain Kd and common mode gain Kc. This ratio is called the common mode suppression ratio (CMRR). The common mode suppression ratio of the general differential amplifier is about a few very dB, and the performance is higher than 100 dB.

The analysis shows that the common mode suppression ≈ beta Re/hie, in which hie represents the input resistance of the transistor. Therefore, the use of current amplification coefficient β large transistors or composite tubes, or the use of constant current source circuit instead of the transmitter common resistance Re can improve the common mode suppression ratio of differential amplifiers. Figure 2 is a differential amplifier that replaces Re with a constant flow source. This circuit has been widely used in a variety of integrated circuits.

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