Multistage amplifier coupling analysis

The design of the circuit is inseparable from the multi-stage amplifier, so do you know what precautions should be taken with it? Regarding multi-stage amplifier coupling, it may seem unfamiliar, but once you understand what is going on, it is easy. Engineers who don’t know how to do this should quickly learn! Multi-stage amplifier circuits are composed of several single-stage amplifier circuits connected in series. The amplification factor of a single-stage amplification circuit is not large, generally no more than 200. In practical electronic equipment, the amplification factor is often as high as tens of thousands. In this way, the single-stage amplification circuit is not competent, and several single-stage amplification circuits need to be combined. They are connected in series to form a multi-stage amplifier circuit. The form of the multi-stage amplifier circuit is shown in Figure 2-17.

In this form of amplification circuit, the signal is transmitted through the capacitor C between the VT1 and VT2 two-stage amplification circuits, the signal is transmitted through the resistor R between the VT2 and VT3 two-stage amplifier circuits, and the signal amplified by VT3 is transmitted to the lower stage by the transformer T.

Characteristics of multi-stage amplifier coupling methods

1. Resistor-capacitor coupling

1) The working points at the front and rear levels do not affect each other, making maintenance easy.

2) Since capacitors attenuate low-frequency signals greatly, they are not suitable for transmitting slowly changing signals.

3) Due to the large size of the capacitor, it cannot be integrated.

2. Direct coupling

1) Few components, easy to integrate.

2) The working points of the front and rear stages influence each other. If there is a problem with any stage, the working point of the entire circuit will change; "zero drift" is easy to occur. "Zero drift" means that when the input stage is short-circuited (no signal input), the DC voltage at the output terminal changes slowly. "Zero drift" is very harmful to amplifier circuits.

3. Transformer coupling

1) It can perform impedance transformation, and the working points of the front and rear stages do not affect each other. This is its biggest advantage.

2) Because the transformer is slightly larger and cannot be integrated, the frequency characteristics are poor.

Advantages and disadvantages of coupling methods of multi-stage amplifiers

1. Direct coupling: The stages are connected directly or with resistors.

Advantages: transmits various signals such as DC or AC, has good frequency characteristics and is easy to integrate.

Disadvantages: Static working points at all levels affect each other. Zero point drift occurs.

2. Resistor-capacitor coupling: Multi-stage amplifier circuit stages are connected through resistors and capacitors to transmit signals.

Advantages: Static working points at all levels are independent of each other and do not affect each other.

Disadvantages: only transmits communication, is not easy to integrate and match, and has poor low-frequency response.

3. Transformer coupling: Transformers are used to transmit AC signals between stages.

Advantages: good matching, low energy consumption, independent Q point, impedance conversion;

Disadvantages: narrow frequency band, large size, bulky, large non-linear distortion, only transmits communication, and cannot be integrated. The above are some instructions on multi-stage amplifier coupling. I hope it will be helpful to everyone.