What Is an In-Line Amplifier?

The voltage amplification factor of a single-stage amplifier circuit can generally reach tens of times. However, in many occasions, such an amplification factor is not enough. It is often necessary to connect several single-tube amplifier circuits in series to form a multi-stage amplifier to signal After multiple zooms, the desired magnification is obtained.

Each single-tube amplifier circuit in a multi-stage amplifier is called a "stage," and the connection between stages is called coupling. There are three common coupling methods: resistance-capacitance coupling, transformer coupling, and direct coupling.

No matter what kind of coupling method the multi-stage amplifier uses, it must meet the following basic requirements in order to work properly.

(1) Ensure that the signal can be smoothly transmitted from the previous stage to the subsequent stage.

(2) The amplifiers at all levels can still have normal static operating points after connection.

(3) The distortion of the signal during transmission should be small, and the transmission efficiency between stages should be high.

In practice, in order to obtain a sufficiently large amplification factor or take into account special requirements such as input resistance and output resistance, an amplifier is often composed of a multi-stage circuit. Figure 1 shows the block diagram of the multi-stage amplifier. The input stage is mainly used to complete the connection with the signal source and amplify the signal. The intermediate stage is mainly used for voltage amplification to amplify the weak input voltage to a sufficient amplitude. The output stage It is mainly to complete the power amplification of the signal to achieve the power required to meet the output load, and to match the load. ^[1]

In multi-stage amplifiers, the connection of a single-stage amplifier is called coupling to achieve smooth signal transmission. There are three commonly used inter-stage coupling methods, namely resistance-capacitance coupling, transformer coupling and direct coupling.

Multistage amplifier RC coupling

Figure 2 shows a two-stage RC-coupled amplifier. The two-stage amplifier is connected by a capacitor. The input resistance of the post-stage amplifier acts as the load of the pre-stage amplifier, so it is called resistance-capacitance coupling. Because the capacitor has the function of blocking DC and AC, the output signal of the pre-amplifier is transmitted to the input of the post-amplifier through the coupling capacitor under the condition that the capacitor value is appropriate. The impact is beneficial to the design, debugging and maintenance of the amplifier. The resistance-capacitance coupling circuit is small in size and light in weight, and is widely used in multi-stage amplifiers. Its disadvantage is that the signal will be greatly attenuated when it is added to the next stage through the coupling capacitor. The resistance-capacitance coupling method is not suitable for transmitting DC signals, so the resistance-capacitance coupled amplifier cannot amplify the DC signal. In addition, it is difficult to manufacture large capacitors in integrated circuits, so resistance-capacitance coupling is only suitable for discrete component circuits.

figure 2

Multistage amplifier transformer coupling

Amplifier circuit using transformer to achieve inter-stage coupling is shown in Figure 3. Transformer T1 passes the output signal of the first-stage amplifier to the second-stage amplifier, and transformer T2 couples the output signal of the second-stage amplifier to the load. Because there is no direct connection between the primary and secondary sides of the transformer, the static operating points of the amplifiers using transformer coupling are independent at all levels. This facilitates design, commissioning and maintenance. The biggest advantage of this coupling method is that it can realize the transformation of voltage, current and impedance, which is especially suitable for matching between amplifiers, amplifiers and loads. This is an important consideration in the transmission of high-frequency signals and the design of power amplifiers. The problem. The disadvantage of transformer coupling is that it is bulky and cannot amplify DC signals and cannot be integrated. Due to the poor frequency characteristics, it is generally only used in low frequency power amplification and intermediate frequency tuning circuits.

image 3

Multistage amplifier direct coupling

The first two coupling methods have the disadvantage of poor frequency characteristics of the amplifier. In order to solve this problem, people have designed direct coupling amplifiers, which directly connect the front and rear amplifiers. Directly coupled amplifiers can not only amplify AC signals but also DC signals, and their frequency characteristics are the best. However, the direct-coupled DC paths of the amplifiers are interconnected, and the static operating points of amplifiers at all levels affect each other, which is not convenient for debugging and maintenance. One of the biggest problems with direct coupled amplifiers is zero drift. Zero drift makes it impossible to distinguish whether the output of the amplifier is a useful signal or an unwanted signal. This problem must be solved, otherwise the direct coupling amplifier cannot be used. Because the direct coupling amplifier is easy to integrate, it is a coupling method commonly used in integrated circuits.

Figure 4 ^[1]