Parasitic Capacitance in Transistor Amp

In summary, parasitic capacitance is a problem with transistors at high frequencies that causes them to cut off. It is the result of unintentional capacitance in the circuit and can reduce the gain of the transistor if it is wired as an inverter. This capacitance can come from various sources, such as the natural capacitance between the legs of the transistor. A circuit diagram can be provided for further clarification.
  • #1
Hairbaby
2
0
Hi all :)
I need some help with the concept of parasitic capacitance. I'm currently doing some analysis on a basic tuned transistor circuit. I know the LCR portion of the circuit is there to remove parasitic capacitance. The only problem I'm having is I don't exactly know what parasitic capacitance is. As far as I can tell, it's a problem with the transistor at high frequencies which causes it to cut off. Can anyone please elaborate or correct me?

I can post a circuit diagram if needed.

Thanks.
 
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  • #2
Parasitic capacitance is exactly what would happen if you soldered a capacitor on top of your transistor. For a MOS transistor that amplifies from the gate and the drain (or source, whichever is used as output), or a bipolar transistor that amplifies from the base to the collector (or emitter if that is used as output), the effect of the parasitic capacitor is to copy whatever is sent to the input over to the output. If the transistor is wired up as an inverter, then this will reduce the output amplitude and therefore reduce the gain.

Parasitic capacitance comes about from all kinds of ways including, for instance, the natural capacitance between the legs of the transistor.

But this is probably what you already knew.

Carl
 

1. What is parasitic capacitance in a transistor amplifier?

Parasitic capacitance in a transistor amplifier refers to the unintentional or unwanted capacitance that exists between different parts of the transistor, such as between the collector and base or between the base and emitter. This capacitance can affect the performance of the amplifier and must be taken into account during design.

2. How does parasitic capacitance affect the performance of a transistor amplifier?

Parasitic capacitance can cause several issues in a transistor amplifier, including reduced gain, distortion of the output signal, and instability. It can also limit the high frequency response of the amplifier and increase power consumption.

3. What causes parasitic capacitance in a transistor amplifier?

Parasitic capacitance is mainly caused by the physical structure of the transistor and the materials used in its construction. The close proximity of different layers and components within the transistor can result in unintentional capacitance being formed.

4. How can parasitic capacitance be reduced in a transistor amplifier?

There are several techniques that can be used to reduce parasitic capacitance in a transistor amplifier, including careful design of the transistor structure, using materials with lower dielectric constants, and adding additional shielding or isolation layers.

5. Can parasitic capacitance be completely eliminated in a transistor amplifier?

While it is not possible to completely eliminate parasitic capacitance in a transistor amplifier, it can be minimized through careful design and construction. However, some level of parasitic capacitance will always exist and must be accounted for in the amplifier's performance specifications.

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