# Capacitors in Hybrid-pi Model and Frequency Response

• MyNameGoesHere
The input signal is split between the output and the input of the transistor. The output voltage of the low-pass filter will be a function of the input voltage and the transistor's frequency response.In summary, the conversation discussed the placement of C2 in a circuit using the hybrid pi model. It was determined that C2 is a power supply bypass capacitor that helps to ensure a more ideal voltage source for the circuit. It was also mentioned that for simulations, C2 can be ignored. Additionally, for the low-pass situation, the frequency response of the transistor should be taken into account.
MyNameGoesHere

This is what I have and I was wondering where C2 might go if I make apply the hybrid pi model on this circuit. I know that all the DC Voltages should be shorted if I were to make use of an equivalent circuit, so that in the hybrid pi model R1 should be in the input and R3 in the output. But what about C2? It was already grounded from the beginning. Does it go in the ground horizontally? If so, on the input or output?
I would normally ignore the capacitive reactances if the frequency was high enough that they would all go to zero. But I was told to do a frequency response and find -3 dB loss points, and they exist in both ends, in the low region and the high region. That means this circuit acts as a band pass filter which is a composite circuit of a low pass filter of RC such that I have R in series and C in parallel to the output and high pass filter of CR, C in series and R in parallel. Well, I can see that C1 and C3 are in series with the input and output. If C2 goes to the ground, I have no way of explaining decrease in the output in the high frequency range except arguing that the circuit is not perfect that it has capacitors and inductors everywhere.
This is as far as I was taught in my undergrad electronics physics class. I'm not an engineering student, so don't know a whole lot about circuits. It would be grateful if someone could give a hint. Thanks.

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C2 is a power supply bypass capacitor. In the real world it is often required because, as you state,
MyNameGoesHere said:
the circuit is not perfect that it has capacitors and inductors everywhere.
That is, there may be long leads back to the power supply and/or the power supply may use low quality electrolytic capacitors that do not work well at high frequencies. C2 works to ensure a more ideal (low impedance) voltage source for the circuit. For the usual, non rigorous, simulations just ignore C2 and leave it out. That leaves you assuming a perfect voltage source for the +15V

For the low-pass situation, take account of the frequency response/delay time of the transistor.

## 1. What is a capacitor in a hybrid-pi model?

A capacitor in a hybrid-pi model is a component that represents the capacitance between the collector and base of a transistor. It is used to model the high-frequency behavior of a transistor by considering the parasitic capacitances associated with the device.

## 2. How is a capacitor represented in a hybrid-pi model?

In a hybrid-pi model, a capacitor is represented by a parallel combination of two capacitors - Cbc (capacitance between collector and base) and Cbe (capacitance between base and emitter). This representation takes into account the non-ideal behavior of a transistor at high frequencies.

## 3. What is the role of capacitors in a hybrid-pi model?

The capacitors in a hybrid-pi model help to model the high-frequency response of a transistor. They account for the parasitic capacitances that exist between different terminals of a transistor, and allow for a more accurate analysis of the device's behavior at high frequencies.

## 4. How does the frequency response of a hybrid-pi model depend on the capacitors?

The frequency response of a hybrid-pi model is affected by the value of the capacitors used in the model. A larger value of capacitance will result in a lower cutoff frequency and a more dominant low-pass filter behavior, while a smaller value of capacitance will result in a higher cutoff frequency and a more dominant high-pass filter behavior.

## 5. Can capacitors in a hybrid-pi model be replaced with other components?

Yes, capacitors in a hybrid-pi model can be replaced with other equivalent components such as inductors or resistors to model the high-frequency behavior of a transistor. However, the choice of component will depend on the specific application and the desired frequency response of the circuit.

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