# How to measure Hie of bootstrap circuit (common emitter)

Discussion in 'Homework Help' started by Watershadow, Jul 10, 2011.

1. ### Watershadow Thread Starter New Member

Jul 6, 2011
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The text book I am using just assumes that the Hie is 2k. But says that its too complex to go into the detail for working out the Hie. Is there a specific way to work it out so that you would get roughly 2k specifically in a bootstrap setup for a common emitter.

2. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Show as the diagram

3. ### Watershadow Thread Starter New Member

Jul 6, 2011
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Not really sure how, thats why I am asking.

4. ### Jony130 AAC Fanatic!

Feb 17, 2009
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What book you are using.
And can you show as which type of the boostrap circuit you are talking.
This type of the circuit

In general case the Hie is BJT input resistance.
and Hie is always equal to
Hie = (β +1)*re = (β +1) * 26mV/Ic

So what is your definition fro Hie ??

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Last edited: Jul 10, 2011
5. ### Adjuster Late Member

Dec 26, 2010
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Normally you would understand hie as a parameter used in a model for a transistor by itself :-

hie= hfe*re, where re = 0.026/IC (approximately, at room temperature)

You might therefore get a value of around 2kΩ with a typical transistor operating with collector currents of a few mA.

If your text is applying h-parameters to describe the complete bootstrapped stage, that would be another matter. You would really have to supply more information to get a definite answer.

6. ### Watershadow Thread Starter New Member

Jul 6, 2011
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The text book is called Analogue Electronics : Design principle by Nico J. Oosthuysen.

Personally I think its a badly written book and I am studying in South Africa and the book is South African. It doesnt even show any information about Hie, I had to look online, but there is nothing about bootstrap and Hie. There are 2 resistors by the Emitter side with a capacitor connected across the one closest to the transistor. Will this make a difference to the formula considering there are now 2 "Re" resistors?

7. ### Jony130 AAC Fanatic!

Feb 17, 2009
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The diagram looks like this?

Or maybe this way

But this circuit has nothing to do withe the bootstrap

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Last edited: Jul 11, 2011