1. The composition of the basic amplifier circuit in the common emitter configuration

The common emitter configuration basic amplifier circuit is shown in the figure.

Common emitter configuration AC basic amplifier circuit

(1) Basic composition

Transistor T–Amplifies.

Load resistor RC, RL–converts the changing collector current into a voltage output.

Bias circuit UCC (Vcc), RB – make the triode work in the linear region.

Coupling capacitors C1 and C2—act as a DC block, and the input capacitor C1 ensures that the signal is added to the emitter junction without affecting the bias of the emitter junction. Output capacitor C2 ensures that the signal is delivered to the load without affecting the collector junction bias.

(2) Static and dynamic

Static—when ui=0, the working state of the amplifying circuit is also called the DC working state.

Dynamic—when ui≠0, the working state of the amplifying circuit is also called the AC working state.

The correct static state of the amplifying circuit is the premise to ensure the dynamic work. To analyze the amplifying circuit, it is necessary to correctly distinguish between static and dynamic, and to correctly distinguish between DC and AC paths.

(3) DC path and AC path

The DC path and AC path of the amplifying circuit are shown in (a) and (b) in the figure below.

A direct current path is a path through which direct current can pass. Looking outward from C, B, E, there are DC load resistors, Rc, RB.

AC path, that is, a circuit path through which AC can pass. If viewed from C, B, E, there are equivalent AC load resistances, Rc//RL, RB.

DC power supply and coupling capacitor are equivalent to short circuit to AC. Because according to the principle of superposition, when the AC current flows through the DC power supply, there is no voltage drop. Assuming that C1 and C2 are large enough, the AC voltage drop on them is approximately zero for the signal, and the coupling capacitor can be short-circuited in the AC path.

(a) DC path (b) AC path

DC path and AC path of basic amplifier circuit

2. Static analysis

1. Calculation and analysis method of static working state

According to the DC path Figure 5-2(a), the static state of the amplifier circuit can be calculated

The working state represented by these quantities of IB, IC and UCE is called the static working point, expressed by Q.

2. Use the graphic method to find the static working point

The graphic analysis of the static working state of the amplifier circuit is shown in the figure below.

(1) Determine two special points—UCC and UCC/Rc on the X-axis and Y-axis of the output characteristic curve, and then draw the DC load line.

(2) From the formula UBE =UCC-IBRb, draw the input load line on the input characteristic curve, and the intersection point of the two lines is Q.

(3) Obtain the parameters IB, IC and UCE of point Q.

Diagram Analysis of Static Working State of Amplifying Circuit

3. Dynamic analysis

Slightly variable equivalent circuit method and graphical method are the basic methods of dynamic analysis.

(1) Establishment of micro-variation equivalent circuit

① The triode is equivalent to a linear element.

② For the low frequency model, the influence of junction capacitance can be ignored.

The input and output characteristic curves of transistors are shown in the figure (a) and figure 5-4(b) below.

(a)    (b)

The equivalent circuit of its input circuit is shown in the figure below.


(2) Dynamic performance index calculation

The common emitter AC basic amplifier circuit is shown in Figure (a) below.

(a) Common emitter basic amplifier circuit (b) Micro-variation equivalent circuit

Common Emitter Amplifier Circuit and Its Microvariant Equivalent Circuit

Voltage magnification Av

Off = = -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

Reviewing editor: Tang Zihong

Leave a Reply

Your email address will not be published.