What is the role of Rb in the common emission basic amplifier circuit

“Rb is the base bias resistance of the transistor. Its function is to provide a small current for the base, that is, to provide a suitable operating point. If there is no such resistance, the static operating point will not be established and the transistor will not work. .

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Rb is the base bias resistance of the transistor. Its function is to provide a small current for the base, that is, to provide a suitable operating point. If there is no such resistance, the static operating point will not be established and the transistor will not work. .

The figure below is the schematic diagram of the common emitter amplifier circuit. The DC power supply VBB provides a forward bias voltage to the base of the transistor through Rb, and generates a base current Ib (often called a bias current, and the circuit that provides a bias current is called a bias circuit). The DC power supply Vcc passes through Rc and cooperates with VBB and Rb to provide a reverse bias voltage to the collector junction, so that the triode is in an amplified state. Another function of the resistor Rc is to convert the change in collector current into a change in voltage, and then send it to the output of the amplifier.

Because of the three amplifying circuits of transistors, the common emitter circuit is the most widely used. Its characteristics are: the phase of output and input voltage → reverse phase, input impedance → smaller (about a few hundred ohms), output impedance → larger (about tens of kiloohms), current amplification factor → large (tens to two Hundred times), voltage magnification→large (several hundreds to thousand times), power magnification→large (several thousand times), frequency characteristic→slightly poor, stability→poor, distortion→larger, power requirements→adopt The bias circuit only needs one power supply, and its application range→amplifier circuit, switch circuit and other occasions.

In order to understand the role of Rb, see the first circuit diagram at the bottom for details.

As shown in the figure, in the case of a common emitter, a reverse voltage must be applied to the collector junction and a forward voltage must be applied to the emitter junction. The figure below shows the basic amplifier circuit of the common emitter connection when no signal is applied. In the figure, Ec is the power supply of the collector to supply the reverse voltage of the collector junction; Eb is the power supply of the input loop, which supplies the forward voltage of the emitter junction to ensure that the emitter of the transistor can emit current. Rc is the load resistance, and Rb is the bias current resistance of the base b of the triode; changing Eb or Rb can change the value of the forward voltage on the emitter junction of the triode.

In order to explain the characteristics of the common emitter amplifier circuit in detail, please see its “DC Load Diagram”.

In the figure, the output loop is divided into two parts with a dotted line A, and the left side of the dotted line is the output terminal of the transistor. The output terminal voltage Uec of the transistor and the circuit Ic change according to the law described by the output characteristic curve. Draw the output characteristic curve of the transistor, which shows the electrical characteristics on the left side of the dotted line.

The right side of the dashed line A is the series circuit of Rc and Ec. The current flowing in the load resistance Rc is the collector current Ico of the transistor. According to Ohm’s law, the voltage at both ends of the branch on the right side of the dashed line; Uec=Ec-IcRc in this formula The relationship between Uec and current Ic is reflected in the output characteristics of the “DC load line diagram” as a straight line. We call it a DC load line. Because it is a straight line, it can be determined by setting two points;

① Short-circuit current point A,

Set; Uec=0, then Ic=Ec/Rc.

②Open circuit voltage point B,

Set; Ic=0, then Uec=Ec.

In fact, the dashed line is artificially assumed, which means that the voltage Uec at the terminals e and c and the current Ic in the loop must be on the characteristic curve of the transistor and on the DC load lines A and B. It is not difficult to see that the only point that must meet these two conditions is at their intersection.

Similarly, on the input circuit shown in the first figure, the circuit can also be separated by the dashed line B. The right side of the dashed line B is the input terminal of the transistor, and its voltage Ueb and current Ib will change according to the law of the input characteristics. The left side of the dashed line B is the series circuit of the power supply Eb and the bias resistance Rb. The current flowing in Rb is actually the base current Ib, so the voltage and current of the left branch of the line can also be calculated by Ohm’s law; Ueb=Eb-IbRb This formula is exactly the same as the DC load line of the output circuit, which is also a straight line.