I. Composition of the basic amplifier circuit of the common emitter configuration
The basic amplification circuit of the common emitter configuration is shown in Fig.
Basic amplification circuit for common-shot configuration AC
(1) Basic components
Triode T – acts as an amplifier.
Load resistor RC, RL – converts the varying collector current into a voltage output.
Bias circuit UCC (Vcc), RB – enables the triode to operate in the linear region.
The coupling capacitors C1, C2 – act as a direct separation. The input capacitor C1 ensures that the signal is added to the emitter junction and does not affect the emitter junction bias. The output capacitor C2 ensures that the signal is delivered to the load and does not affect the collector junction bias.
(2) Static and dynamic
Static – ui = 0, the operating state of the amplifier circuit, also known as the DC operating state.
Dynamic – when ui ≠ 0, the operating state of the amplifier circuit, also known as the AC operating state.
The amplifier circuit establishes the correct static, which is a prerequisite for dynamic operation. The analysis of amplification circuits must correctly distinguish between static and dynamic, and between the direct and AC paths.
(3) Direct and AC paths
The direct and AC paths of an amplifier circuit are shown in the diagrams below (a) and (b).
The direct current path, i.e. the path through which DC can be passed. Looking out from C, B and E, there are DC load resistors, Rc and RB.
AC path, i.e. the circuit path that can pass through AC. As seen from C, B, E outwards, there is the equivalent AC load resistance, Rc//RL, RB.
The DC supply and coupling capacitor are equivalent to a short circuit to AC. This is because, according to the iterative principle, there is no voltage drop when the AC current flows through the DC supply. Let C1, C2 be large enough for the signal, the AC voltage drop across them is approximately zero, and the coupling capacitor can be shorted out in the AC path.
(a) Direct current path (b) AC path
The direct and AC paths of a basic amplifier circuit
II. Static analysis
- Calculation and analysis method of static working state
According to the direct current circuit Figure 5-2 (a) can be calculated for the static amplification circuit
IB, IC and UCE these quantities represent the operating state called the static operating point, expressed in Q.
2, the graphical method to find the static operating point
The graphical analysis of the static operating state of the amplifier circuit is shown below.
(1) In the output characteristic curve X axis and Y axis to determine two special points – UCC and UCC / Rc, you can draw the DC load line.
(2) From the equation UBE =UCC-IBRb on the input characteristic curve, make the input load line, the intersection of the two lines is Q.
(3) Obtain the parameters IB, IC and UCE for the Q point.
Diagrammatic analysis of the static operating state of the amplifier circuit
- Dynamic analysis
The microvariable equivalent circuit method and the diagrammatic method are the basic methods of dynamic analysis.
(1) Establishment of a micro-variable equivalent circuit
① The triode is equated to a linear element.
② For low frequency models the effect of junction capacitance can be disregarded.
The input and output characteristic curves of the transistor are shown in Fig. (a) and Fig. 5-4(b) below.
The equivalent circuit for its input circuit is shown in the diagram below.
(2) Calculation of dynamic performance indicators
The common emitter AC basic amplifier circuit is shown in figure (a) below.
(a) Common-emitter basic amplifier circuit (b) Microvariable equivalent circuit
Common-radio amplifier circuit and its micro-variable equivalent circuit
Voltage amplification Av
Av = = -βRL’ / rbe
Input resistance ri
ri = = rbe // Rb1// Rb2≈ rbe = rbb’ + (1+β)26 / IE = 300Ω + (1+β)26/ IE
Output resistance Ro
Ro = rce∥Rc≈Rc