Amplification is one of the most fundamental functions in analog signal processing. It is achieved through an amplifying circuit, which serves as a core building block in most analog electronic systems. Amplifying circuits are not only used for signal boosting but also form the basis for other essential analog circuits like filtering, oscillation, and voltage regulation.
Common types of analog amplifying circuits include single-transistor configurations such as common-emitter, common-collector, and common-base amplifiers. Multi-stage amplifiers can be classified into RC-coupled, transformer-coupled, and directly coupled designs. Additionally, there are field-effect transistor (FET) amplifiers, vacuum tube amplifiers, feedback amplifiers, and power amplifiers. These circuits are further categorized into low-frequency and high-frequency amplifiers based on their operational range.
Regardless of the type, all amplifying circuits share certain key characteristics:
First, an amplifying circuit acts as an energy converter rather than an energy source. A small change in the base current of a transistor controls a much larger change in the collector current, while a small variation in the gate voltage of a FET or vacuum tube controls the screen current. This makes FETs and tubes voltage-controlled devices, while transistors are current-controlled. Unlike a magnifying glass that directly enlarges images, an amplifier superimposes an AC signal onto a DC bias, causing variations in the DC signal that are then converted back into an amplified AC signal via a load resistor. The transistor essentially acts as a switch or modulator in this process.
Second, amplifying circuits contain both DC and AC components. For analysis purposes, the DC path refers to the route through which the DC current flows, while the AC path is where the signal travels. Capacitors block DC but allow AC to pass, so during DC analysis, capacitors are treated as open circuits, and during AC analysis, they are considered short circuits. The DC power supply is treated as a short in AC analysis since its voltage remains constant. Understanding the signal flow through these paths is crucial for proper circuit operation.
Third, amplifying circuits operate in two main states: static and dynamic. Static operation occurs when no input signal is present, and the transistor is biased to operate in its active region, with the emitter junction forward-biased and the collector junction reverse-biased. Dynamic operation refers to the circuit's ability to amplify an input signal once it is applied.
When analyzing an amplifying circuit, it’s important to first check the static operating point to ensure that the DC voltages across the transistor, tube, or FET are correct. Only after confirming the static condition should the AC path be analyzed. Both DC and AC paths coexist in the same circuit and influence each other. The AC signal is superimposed on the DC bias, and the performance of the AC circuit depends heavily on the stability of the DC operating point.
Fourth, in a negative feedback amplifier, the output amplitude is controlled by the feedback mechanism. If the negative feedback component fails—such as due to an open circuit, poor soldering, or a change in resistance—the feedback effect is lost, leading to increased gain and potential distortion. When distortion occurs, the amplifier may be operating in a nonlinear region, either saturated or cut off. To diagnose the issue, it’s essential to measure the operating point voltage and check the integrity of the components and the parameters of the active device.
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