The basic circuit structure of a differential amplifier

The basic circuit structure of a differential amplifier

Since the zero point drift and the input signal are both "DC" quantities with slow signal changes, if the drift is close to or exceeds the input signal
, the input signal will be overwhelmed by the zero point drift. Therefore, reducing zero point drift has become an important goal in making high-quality DC amplifiers
, and various circuit forms of DC amplifiers. The differential amplifier circuit is a DC amplifier circuit that can effectively suppress zero point drift. It is also called
a differential amplifier circuit. It is commonly used in the prestage of multi-stage amplifier circuits and is also the basic circuit in operational amplifiers.
    Circle 1 shows the basic circuit structure of a differential amplifier .
    As can be seen from the picture, it is actually composed of two symmetrical single-tube common-emitter amplifier circuits. Among them, VTlj and VT2 are two
transistors with the same characteristics, ‰Il and R1 are bias resistors, Rbl2 and Rb22 are base current-limiting resistors, and &I and R2 are collector load
resistors. Usually in order to meet the symmetry requirements, RbII=Rb21, Rbl Z=Rb22, fart. i=R. 2. Ul is the input signal voltage, which passes through
Rb. and R. The divided voltage is Gong l and r 1, which are added to the bases of the two tubes respectively (called double-ended input); Un is the output voltage, which is the difference between the output voltages of the two tubes,
that is, Uo=Uor Uuz (called double-ended output ).
    (1) Suppression of zero point drift
    Since VT1 and VT2 are completely symmetrical, when no input signal is added (that is, Ut=o), UoI=U02, Uo-U01-U2-o.
If for some reason, the currents of the two collector plates change, then their respective changes will be equal in magnitude and direction. That is
△UCl-AUC2. Then the output variation AUo=AU01-AU220, which means that the zero point drift is canceled due to mutual compensation, and the output voltage is still
0. Obviously, the better the symmetry on both sides of this differential amplifier circuit, the better the effect of suppressing zero drift.
    (2) The principle of differential amplifier amplifying useful signals.
    The actual input signal (i.e. useful signal) voltage is usually added between the two input terminals (called double-terminal input). Since the circuit is symmetrical and
has Uol- UoZ, so Uo - Uo wide U02=2 Uo, so the output voltage will not be 0, but will be twice as large as the output of a single tube. This is
the principle that the differential amplifier can effectively amplify the useful input signal.
    (3) Connection method of differential amplifier circuit
    The differential amplifier circuit has two input terminals and two output terminals, which can be combined into four different connection methods, namely; double-terminal input and double-
terminal output, double-terminal input single single-ended output, single-ended input, double-ended output, and single-ended input single-ended output.
    Table 1-2 lists the schematic diagrams of the differential amplifier circuits of these four different connection methods, and
compares the differential mode voltage amplification factor, input and output resistance and other characteristics.
    It can be seen from Tables 1 and 2 that regardless of whether the signal is a single-ended input or a double-ended input, the differential mode voltage amplification factor depends on whether the output is single
-ended or double-ended. If it is a double-ended output, the voltage amplification factor is the same as the unilateral basic amplifying circuit that constitutes it; if it is a single-ended output, it is
half of the basic amplifying circuit.