Why is the output impedance of the common emitter amplifier circuit Rc itself?

4pk

Why is the output impedance of the common emitter amplifier circuit Rc itself? [Copy link]

4pk

This is a capacitor in transistor circuit design... I don't quite understand it! As he said, the current source impedance is infinite, and it is connected in parallel with Rc, so isn't the load also connected in parallel with Rc? The signal is generated at the C pole. And Ie is fixed. Connecting a load will increase the Ic current, so the Rc voltage drop will increase. Then Uc will be reduced? Is this what you mean?

maychang

4pk published on 2018-11-18 21:02 This is a capacitor in transistor circuit design. . . I don't really understand it! As he said, the current source impedance is infinite, and when it is connected in parallel with Rc, then the load is also...

When two resistors are connected in parallel, the resistance value after parallel connection is mainly determined by the smaller resistor. The greater the difference between the two resistors, the more accurate this rule is. If the difference between the two resistors is dozens of times or more, the large resistor can be ignored. In the bipolar transistor common emitter amplifier circuit, the output is at the collector, and the collector output resistance is much larger than the load resistance, so the total output resistance is mainly determined by the load resistance.

maychang

4pk posted on 2018-11-18 21:02 This is a class of capacitors in transistor circuit design. . . I don’t quite understand it! As he said, the impedance of the current source is infinite, and when it is connected in parallel with Rc, then the load will also...

“Is that what you mean?” No. “And Ie is fixed, and connecting a load will increase the current of Ic” That’s not the case.

maychang

In the first post, there is a resistor (2k) between the emitter of the transistor and the ground. This resistor will produce negative feedback, making the collector output resistance larger than when there is no negative feedback, and much larger. In this way, the collector output resistance can be ignored.