Calculating Component Values in this Low-Frequency CE Amplifier Circuit

[MiguelHut]

Hi guys , i need help. How do i calculate C1? fu=25Hz

 

[NascentOxygen]

Determine the input impedance of that stage as seen at the junction of the 3 resistors.

 

[MiguelHut]

Is this ok?

 

[Svein]

Is this ok?

Well, no. Let us start to the left. If you disconnect R6, C1 sees the parallel connection of R7 and R8 (13k). Now reconnect R6. The equivalent impedance seen through R6 depends on the transistor (the β of the transistor at the working point) and the emitter resistor. This is a somewhat cumbersome calculation, but to get a ballpark value, assume β=100. Then the impedance seen through R6 is approximately R6 + 100*R3. This impedance appears in parallel with the 13k calculated above.

So - estimating the AC impedance on the right side of C1 to be about 10k, you can calculate the size of C1.

 

[Tom.G]

assume β=100.

There is no need to guess the transistor β. Both the Base current and Collector current are given.

https://www.physicsforums.com/attachments/rt-gif.249461/

 

[Svein]

There is no need to guess the transistor β. Both the Base current and Collector current are given.

Yes. But - we are not talking about the DC working point, but the AC current gain.

 

[MiguelHut]

Is this right?

 

[Svein]

Is this right?

No.

1. 3mA/12μA = 3000μA/12μA = 250
2. 3mA and 12μA determines the DC working point, not the current gain

 

[berkeman]

Yes. But - we are not talking about the DC working point, but the AC current gain.

Here is a pretty good tutorial on how to calculate the input impedance of a CE amplifier:

https://www.electronics-tutorials.ws/amplifier/input-impedance-of-an-amplifier.html