transistor parasitic

[fady232]

Hello, i had a question, as we know in a npn and pnp transistor there is an internal LC parasitic elements, which basically formes a LC resonant circuit ( in that way the circuit has a better frequency response to certain bandwith, i read that to change this frequency response, and u can obtain a wider frequency response by lowering the quality factor, and u can do that by introducing a resistor in parallel , which is basically adding a resistor to the transistor base.

Lower the resistor, lower is the quality factor, so better is the response to more frequencies.

But now, my question is: if u lower the resistor base u get a higher base current, and u may break the transistor?, then how u balance both things? having a low quality factor ( lets suppose in ur application u want to do that ) and having a lower current going into base?

Thanks all.

 

[betwixt]

The internal parasitics are actually very small and in most cases are insignificant compared to those of components around the transistor. However, you are correct, the resistor has the effect of discharging capacitance and absorbing energy released by an inductor. One of the design factors of high frequency transistors is that the parasitics are minimized and the legs are short so they don't add too much inductance.

In the case of your schematic, the BC547 is not intended to be used at high frequencies and although the parasitic LC is still there, in practical applications it can usually be ignored. Any LC resonance frequency in the transistor will be magnitudes higher than the transistor can operate at.

Brian.

 

[danadakk]

Generally speaking one is concerned about emitter follower stability
with C load in the emitter.

This might help : https://www.diyaudio.com/community/attachments/debella_base_stopper_emitter_follower-pdf.1396322/

To "de-Q" the response of an emitter follower at high freq, to increase BW, would be difficult
because of T and V sensitivities of parasitics (particularly in C).

L parasitic largely lead L internal and external. There are online calculators for
wire L

 

[FvM]

post #1 circuit neither shows a bias circuit nor a base resistor. I don't understand how it's related to discussed optimization.

 

[dick_freebird]

Your bond wire inductance is about 1nH/mm and external leads about the same. Until you get to gHz it's a don't-care.

However series base resistance can emulate inductance, as far as local instability goes - your cascodes want low Zbase in-looking.

 

[fady232]

post #1 circuit neither shows a bias circuit nor a base resistor. I don't understand how it's related to discussed optimization.

i meant to make an example, i guess some understood, but i could be more clear

 

[fady232]

The internal parasitics are actually very small and in most cases are insignificant compared to those of components around the transistor. However, you are correct, the resistor has the effect of discharging capacitance and absorbing energy released by an inductor. One of the design factors of high frequency transistors is that the parasitics are minimized and the legs are short so they don't add too much inductance.

In the case of your schematic, the BC547 is not intended to be used at high frequencies and although the parasitic LC is still there, in practical applications it can usually be ignored. Any LC resonance frequency in the transistor will be magnitudes higher than the transistor can operate at.

Brian.

i understood, thanks for ur explaination, and this kind of thing when i need to be aware of? like in what kind of projects i make?

 

[FvM]

i meant to make an example, i guess some understood, but i could be more clear

Your question is rather vague. You are talking about a parallel resistor, dick_freebird in constrast discussed a base series resistor, which is the only reasonable way to reduce Q when driving the amplifier with a low impedance source. I'm not sure if you thought about details at all, or have a different setup in mind.

--- Updated Apr 18, 2025 ---

My experience with discrete small signal GHz transistors is that package inductance often causes resonances in a 500 MHz range. Base series resistor may be required to prevent self oscillation.

 

[fady232]

Your question is rather vague. You are talking about a parallel resistor, dick_freebird in constrast discussed a base series resistor, which is the only reasonable way to reduce Q when driving the amplifier with a low impedance source. I'm not sure if you thought about details at all, or have a different setup in mind.

--- Updated Apr 18, 2025 ---

My experience with discrete small signal GHz transistors is that package inductance often causes resonances in a 500 MHz range. Base series resistor may be required to prevent self oscillation.

yes, series resistor to base, but for the capacitor, this resistor is seen as ''parallel'. but yeah i meant resistor in series to base, (thats what u can add to the circuit)

--- Updated Apr 18, 2025 ---

Your question is rather vague. You are talking about a parallel resistor, dick_freebird in constrast discussed a base series resistor, which is the only reasonable way to reduce Q when driving the amplifier with a low impedance source. I'm not sure if you thought about details at all, or have a different setup in mind.

--- Updated Apr 18, 2025 ---

My experience with discrete small signal GHz transistors is that package inductance often causes resonances in a 500 MHz range. Base series resistor may be required to prevent self oscillation.

sorry im bit newbie