I. Composition of Amplifier Circuits and Functions of Each Component
Rb and Rc: provide suitable bias – forward bias for the emitter junction and reverse bias for the collector junction. C1 and C2 are DC blocking (coupling) capacitors, blocking DC and allowing AC.
Common-emitter amplifier circuit
Vs, Rs: Source voltage and internal resistance; RL: Load resistance, converting the change in collector current Δic into the change in voltage between the collector and emitter ΔVCE.
II. Basic Working Principle of Amplifier Circuits
Static analysis (Vi=0, assuming operation in amplification mode), also known as DC analysis, calculates the current and inter-electrode voltage of the transistor. A DC path (open capacitor) should be used.
Base current: IB = IBQ = (VCC - VBEQ) / Rb
Collector current: IC = ICQ = βIBQ
Collector-emitter voltage: VCE = VCEQ = VCC - ICQRc Dynamic (vi≠0) analysis:
The amplification of signals by an amplifier circuit is achieved by using the current control of a transistor; in essence, it is an energy converter.
III. Basic Principles of Constructing Amplifier Circuits
An amplifier circuit must have a suitable quiescent operating point: the polarity of the DC power supply must match the type of transistor, and the resistor settings must match the power supply to ensure that the device operates in the amplification region. The input signal must be effectively applied to the input terminal of the amplifying device, causing the current or voltage at the transistor input terminal to change proportionally with the input signal. The output signal after amplification by the transistor (e.g., ic = β*ib) should be effectively converted into an output voltage signal across the load.
Voltage transfer characteristics and quiescent operating point
I. Voltage Transfer Characteristics of Single-Transistor Amplifier Circuits
Graphical analysis method:
Output loop equation:
Output characteristic curve:
AB segment: Cutoff region, corresponding to the part of the output characteristic curve where iB < 0.
BCDEFG segment: Enlarged area
GHI segment: saturation region
When used for amplification applications: Point Q should be placed at point E (the center of the amplification area). Setting point Q at point C can easily cause load distortion. Setting point Q at point F can easily cause saturation distortion.
For switching control applications: operates in the cutoff and saturation regions.
II. Quiescent Operating Point of a Single-Transistor Amplifier Circuit (Calculated Using Formulas)
Single-supply fixed bias circuit: Select appropriate Rb and Rc to make the circuit work in the amplification state.
A bias circuit with a stable operating point: This method is an approximate estimation method.
Voltage divider bias circuit:
Another interpretation of the stable operating point: Temperature T↑→IC↑→IE↑→VE↑(=IERe)↓(VB is fixed), then IC↓ IB↓ VBE↓ (=VB-VE).
Under static conditions, an increase in temperature causes an increase in IC. Since the base potential VB is essentially constant, this current increment generates negative feedback through Re, forcing IC to decrease automatically and keeping the Q point stable. The larger Re is, the stronger the negative feedback and the better the stability.
However, if Re is too large, the output dynamic range (ΔVCE) decreases, which can easily cause distortion. The smaller Rb1 and Rb2 are, the more stable VB becomes. However, if they are too small, the amplification capability will decrease. In engineering design, the influence of resistor values should be considered comprehensively.
Empirical formula: I1=(5～10)IBQ, VEQ=IEQRe=0.2VCC (or VEQ=1～3V).
