Double-ended input double-ended output (differential)

Replace R in Figure 1-24 with the constant current source circuit in Figure 1-25, and it becomes a differential amplifier circuit with a constant current source, as
shown in Figure 1-27. In this way, as long as c3 remains constant, cl and cl cannot increase or decrease at the same time, thus suppressing
the voltage drift.
    Differential circuits are divided into four types according to their signal input and output modes: double-ended input, double-ended output; double-ended
input , single-ended output; single-ended input, double-ended output; single-ended input, single-ended output . As shown in Figure 1-28.
    The amplification factors of the four connection methods can be summarized as follows: No matter which connection method is used, as long as it is a single-ended output, its amplification factor is
half of the basic amplification circuit; if it is a double-ended output, its amplification factor is the same as that of the basic amplification circuit. o
    Differential The amplifier circuit is generally always used as the front-stage input part of the amplifier circuit. Therefore, its output end must always be
connected to the basic amplifier circuit. This is not a problem for a single-ended output differential circuit, but it is
inconvenient for a double-ended output differential circuit. Because although it has two output terminals, there is no common terminal between the input signal and the single-ended output differential circuit. terminal output,
the advantages of the differential circuit cannot be exerted. Therefore, a double-ended to single-ended output circuit is needed to match it. This is
the mirror constant current source circuit, as shown in Figure 1-29.
    In Figure 1-29, VT3 and VT4 form a mirror constant current source, so there is: Ic,3=J d, and, c3 - Icl,
so: Ic4=Icl
    output current D o= Id - lc2=Icl - Ic2 0
    In the case of common mode input (when the two input terminals input signals with equal amplitude, it is common mode input), I{:1=Ic2,
so f. =, & 1, c2 =0
is the same as double-ended output. Differential mode input ( when the two input terminals input signals
    with equal amplitude and opposite polarity , it is called differential mode input):     Icl≈Icl-△Icl, Ic2≈, c2+△Ic2 because Icl≈, so,. ≈2AlC. That is, when using differential mode input, the output current is twice the output current of a single tube, which is the same as double-ended output.     The above circuit principles will be encountered in subsequent examples.