General form of differential input

In an input circuit using a transistor as a differential input stage, the resistance Ri connected from the base of the left tube to ground and the resistance
R connected from the base of the right tube to output point n generally have the same value, as shown in Figure 1-34 As shown, since the output point D is very close to the ground potential,
the bias currents of the two tubes are basically the same. After removing q, Rn is directly connected to ground , and the bias resistance value of the right tube is approximately equal to Rfl. Considering
that there must be sufficient amplification and R is not too large, R fl is generally taken as 1 m, and the left tube bias resistor Ri is usually
30kn, so the bias current of the left and right tubes is obviously different, and the open-loop characteristics of the amplifier It must be bad. Especially when the differential output
is a double-ended output, the output voltage will differ too much, which
will have a negative impact on the subsequent stage. If the left tube is also changed to 1kfl,
of course the left and right bias currents can be consistent, but such a low input
resistance is difficult to match with the front stage, which increases the difficulty of production
. To solve this problem, the following methods can be adopted.