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# What is the input impedance of this HF preamplifier?

#### neazoi

What is the input impedance of this HF preamplifier fed by 9v?

1. At the transistor base.
2. At the input of the potentiometer.
3. At the transistor base, if the input potentiometer was missing.

Thanks a lot!

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1) Impedance is a function of frequency; your post is NOT a function of frequency
2) Impedance is also a function of the setting of your pot.

1) Impedance is a function of frequency; your post is NOT a function of frequency
2) Impedance is also a function of the setting of your pot.
It is to be used in HF as said, so 2-30MHz.
In different pot settings.
A rough estimation is enough. Will be a low impedance, high impedance, how high 1k, 10k or what?

Rough estimation is it will be like most common emitter configurations, middle impedance, probably several hundred Ohms to a few K. That is assuming the pot is omitted or set to maximum. Note that most potentiometers will work badly at 30MHz too.

Brian.

### neazoi

Points: 2
Rough estimation is it will be like most common emitter configurations, middle impedance, probably several hundred Ohms to a few K. That is assuming the pot is omitted or set to maximum. Note that most potentiometers will work badly at 30MHz too.

Brian.
Brian, To present a constant input impedance and at the same time allow the pot to adjust the input signal, I was thinking of a stereo pot, say 2x 500 ohms. One gang will be connected as a usual potential divider. Its wiper, will be connected through a 500R resistor to the wiper of the second gand. i.e. the second gang will be connected in reverse.

This will present 500 ohms at the input of whatever stage is to be connected to the amplifier (eg a filter) and 500 ohms to the amplifier input as well. Even at it's high position there will be a 500R series resistor, so the impedance would be somewhat presented.

How does it sound to you?

This is the input impedance at various frequencies between 1MHz and 30MHz (reflection coefficient in Magnitude-Angle format) at the transistor base without potentiometer. I think the other measurements are meaningless.

To convert a reflection coefficient in Magnitude-Angle format into Impedance Zs(Rs +/- jXs) format, use the calculator below:

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Points: 2