Common emitter amplifier circuit

3.2.1 Composition principles of amplifier circuits

When forming an amplifier circuit, the principles that must be followed are:

1. Set up a DC power supply to provide energy for the circuit.

2. The polarity and size of the power supply should ensure that the BJT emitter junction is in forward bias and the collector junction is in reverse bias, so that the BJT works in the amplification area. (For field effect tube amplifier circuits, it should work in the constant current area).

3. The value of the resistor in the circuit should be matched with the power supply voltage to ensure that the BJT has a suitable static operating point and avoid nonlinear distortion.

4. The input signal must be effectively transferred and act on the input circuit of the amplifier.

5. When connecting a load, it must be ensured that the load receives a current signal or voltage signal that is much larger than the input signal.

3.2.2 Composition of common emitter circuit

According to the composition principle of the amplifier circuit, a simple common emitter amplifier circuit is formed as shown in the figure. The functions of BJT, resistor, capacitor and other components in the circuit are as follows:

In the figure, T is an NPN transistor, which is the core of the entire circuit and plays an amplifying role.

The DC power supply VCC provides a reverse bias voltage for the transistor collector junction to ensure that the collector junction is reverse biased.

Rc is the collector load resistor, which converts the change of the transistor collector current into a change of voltage and sends it to the output. If there is no Rc, the voltage at the output end is always equal to the power supply voltage VCC and will not change with the input signal.

The DC power supply VBB provides a forward bias voltage for the transistor emitter junction through the base resistor Rb, and provides the required current IB (often called bias current) for the base.

Capacitors Cb1 and Cb2 are called DC blocking capacitors (or coupling capacitors). Their function is to "isolate DC and transmit AC". That is, for DC, the capacitive reactance of the capacitor is infinite, which is equivalent to an open circuit. But for AC signals, the capacitive reactance of the capacitor is very small and can be approximately considered as a short circuit. vi is the weak electrical signal to be amplified.

3.2.3 Working Principle of Common Emitter Amplifier Circuit

The voltage amplification effect of the common emitter basic amplifier circuit utilizes the current control function of the BJT and relies on Rc to convert the amplified current change into voltage change.

3.2.4 Two working states of the amplifier circuit

1. Static

When there is no input signal (vi=0), the working state of the amplifier circuit is called static state. Since the voltage and current at all points in the circuit are DC quantities in static state, static state is also called DC working state.

The basis for analyzing and calculating the DC quantity of the amplifier circuit when it is in static state is the DC path.

2. Dynamic

After the input signal is added to the input end of the circuit, the working state of the circuit is called dynamic. In dynamic state, the current of each electrode of the BJT and the voltage between each electrode are superimposed with the AC quantity that changes with the input signal on the basis of the static value. In dynamic state, the instantaneous total amount of current and voltage includes DC quantity and AC quantity.

The basis for analyzing and calculating the AC quantity of the amplifier circuit when it is in a dynamic state is the AC path.

3.2.5 DC path and AC path

DC path

1. The concept of DC path:

When there is no input signal, the path through which the DC current flows under the action of the DC power supply is called the DC path. The DC path is used to determine the static operating point (IB IC VCE) when the circuit is in the DC working state.

2. How to draw a DC path:

①The capacitor is considered as an open circuit;

②The inductor coil is considered as short circuit (ignore the coil resistance);

③The signal source is considered to be short-circuited, but its internal resistance should be retained.

According to the method of drawing the DC path, the DC path of the common-emitter amplifier circuit shown in Figure a can be drawn as shown in Figure b.

Communication Channel

1. The concept of communication channel:

The AC path is the path through which the AC signal flows under the action of the input signal. The AC path is used to analyze and calculate the dynamic performance indicators of the circuit (such as Av, Ri, Ro)

2. How to draw a communication path:

① Capacitors with large capacity (such as coupling capacitors and emitter bypass capacitors) are considered short circuits.

② The DC power supply (such as VCC) is considered short-circuited. Since the other terminal of the power supply is usually connected to the "^", the DC power supply should be short-circuited to the "^" at this time.

According to the method of drawing the AC path, the AC path of the amplifier circuit shown in Figure a can be drawn as shown in Figure c.

Figure C

3.2.6 The significance of setting a suitable static working point

Since BJT is a nonlinear device, it is very important to reasonably set the static operating point (Q point) so that the BJT always operates in the linear region when a small signal is input to the amplifier circuit.

We take the common-emitter circuit without static bias current as shown in Figure a as an example to illustrate.

Static state: Short-circuit the input terminal. According to circuit analysis, IB=0, IC=0, VCE=VCC, and BJT is in the cut-off state.

Dynamic: If the peak value of vi is less than the conduction voltage between be, the BJT will always work in the cut-off state during the entire cycle of the signal, and there will be no output signal at this time; if the amplitude of vi is large enough, the BJT can only be turned on during the time interval when the positive half cycle of the signal is greater than the conduction voltage between be, as shown in Figure b. From the above analysis, it can be seen that the output signal is seriously distorted.

Only when the BJT is always working in the amplification state during the entire cycle of the signal, the output signal will not be distorted. Therefore, a suitable static operating point must be set.