Two types of multistage negative feedback amplifiers a

Two types of multistage negative feedback amplifiers a

 According to the AC and DC properties of the feedback signal, it can be divided into DC feedback and AC feedback. If the feedback signal only contains DC components, it is
called DC feedback; if the feedback signal only contains AC components, it is called AC feedback. In many cases, both AC and DC feedback are present.
    The figure shows two types of multi-stage negative feedback amplifiers . In the circuit shown in figure (a), assuming that the bypass capacitor c:? of the emitter of transistor VT2 is large enough, it can be considered that the AC signal at both ends of the capacitor is basically zero. Then the feedback signal led from the emitter of transistor VT2 through the resistor R back to the base of transistor VT1 will only have a DC component, so DC feedback is introduced in the circuit. In the circuit shown in Figure 1-38 (b) , the feedback is led from the output end back to the emitter of the transistor VT1 through the capacitor C and the resistor R. Due to the DC blocking effect of the capacitor , the feedback signal will only have an AC component. So this feedback is communication negative feedback.      The function of DC negative feedback is to stabilize the static operating point, but has no effect on the dynamic performance of the amplifier, such as amplification, passband, input and output resistance, etc. Various types of AC negative feedback will have different impacts on the dynamic performance of the amplifier circuit, and are the main means to improve the AC performance of the circuit.