# Calculating Component Values in this Low-Frequency CE Amplifier Circuit

• MiguelHut
In summary, the input impedance of a common emitter (CE) amplifier can be calculated by disconnecting the output load and considering the parallel connection of the two resistors connected to C1. The equivalent impedance seen through the resistor connected to the emitter depends on the transistor's AC current gain (β) and the emitter resistor. By assuming a value for β (e.g. 100) and calculating the impedance seen through the resistor, the size of C1 can be determined. However, it is important to note that the DC working point should not be used to determine the AC current gain. A tutorial on how to calculate the input impedance of a CE amplifier is provided for further reference.
MiguelHut
Hi guys , i need help. How do i calculate C1? fu=25Hz

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

berkeman
Is this ok?

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• bh.GIF
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MiguelHut said:
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 said:
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.

Is this right?

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• tgg.pdf
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MiguelHut said:
Is this right?
No.
1. 3mA/12μA = 3000μA/12μA = 250
2. 3mA and 12μA determines the DC working point, not the current gain

Svein said:
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

Averagesupernova, Tom.G and dlgoff

## 1. How do I calculate the value of the resistors in this low-frequency CE amplifier circuit?

To calculate the value of the resistors in this circuit, you will need to use Ohm's Law (V=IR) and the circuit equations for voltage and current. You will also need to consider the desired gain and input/output impedance of the amplifier. There are several online calculators and software programs that can assist with these calculations.

## 2. What is the purpose of the capacitors in this circuit?

The capacitors in this circuit serve several purposes. They are used to block DC voltage, allowing only AC signals to pass through. They also help to stabilize the gain and frequency response of the amplifier. Additionally, they can be used for coupling and decoupling signals between different stages of the amplifier.

## 3. How do I choose the appropriate transistor for this circuit?

Choosing the appropriate transistor for this circuit will depend on the desired gain, input/output impedance, and frequency response. You will also need to consider the power rating and voltage requirements of the circuit. It is important to consult datasheets and do research on different transistor types to determine the best fit for your specific circuit.

## 4. Can I use different values for the components in this circuit?

Yes, you can use different values for the components in this circuit. However, changing the values of the resistors and capacitors will affect the gain, frequency response, and stability of the amplifier. It is important to carefully calculate and test any changes to ensure the desired performance of the circuit.

## 5. How do I test the performance of this amplifier circuit?

To test the performance of this amplifier circuit, you can use a signal generator and an oscilloscope. Connect the input signal to the amplifier and measure the output signal on the oscilloscope. You can also use a multimeter to measure the gain and input/output impedance of the amplifier. It is important to compare these measurements to the calculated values to ensure the circuit is functioning as expected.

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