Differential circuit using mirror current source as load (b)

In the two-stage differential amplifier with resistive load shown in (a), only the collector of VT4 outputs the signal.
Although VT3 has gain, it is not utilized. This shortcoming can be overcome by using a mirror constant current source as the load of VT3 and VT4,
as shown in Figure 8-23(b).
    It can be seen from the working principle of the mirror current source that in Figure 8-23(b), the current IC3 - works. =Jr West, when there is no input signal,
the static current Ia=, Zi, no current flows through the load resistor RL. But when there is a signal input, due to J. with J. Changes in
the opposite direction, the current flowing through the load resistor RL, L=, C4-, island o. For the output end, it is equivalent to
the push-pull operation of VT4 and VT6. That is, the output signal of VT3 is converted into the output of VT6, thereby doubling the voltage gain
(6icB).
    In Figure 8-23(b), the collector load of VT3 is approximately equal to (Ra+R聒)=5-82kfl, while the collector load
resistance of VT4 is approximately equal to RL. RL is the input terminal of the subsequent power output stage. Assume that the output stage is a Darlington connected OCL
circuit, and its ^re= 5000. When the load impedance is an 8n speaker, then RL=8n×5000=40kCID. It can
be seen that the loads of VT3 and VT4 are very different, and the symmetry condition of the circuit cannot be met, and the result will inevitably be an increase in distortion.
    In order to overcome the above shortcomings, the circuit in Figure (b) is changed to the connection in Figure (c), that is, a one-stage differential amplifier circuit using a mirror current source as the load
. Due to the negative feedback of the emitter resistance ‰ and R function, the distortion can be improved. However,
    in modern high-fidelity amplifiers, the main pursuit is that the amplifier should have smaller open-loop distortion and smaller negative
feedback to reduce transient intermodulation distortion. The gain of the amplifier is often not the main conflict. Therefore, it
is better to use the circuit shown in Figure 8-23(c).
    By the way, in a power amplifier circuit, generally the number of voltage amplification stages should not exceed two. Otherwise,
a large phase shift will occur . In this case, it is not only necessary to set up a phase correction circuit , but also it will be difficult to obtain pure sound quality.