# Input Resistance and Open Circuit Gain of CB, CC, and CE

Discussion in 'General Electronics Chat' started by staticd, Feb 12, 2011.

## Which transistor has a better high frequency response?

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1. ### staticd Thread Starter New Member

Sep 7, 2010
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I just couldn't handle it any more. I had to work it out myself and figured that I would share.

Here is a link that shows how to work out the input resistance (or, impedance) for a common base, common collector, and common emitter amplifiers.

http://goo.gl/kkPoh

The cool part is that you can also use these results for MOSFETs!

Just switch Rb to Rg, Re to Rs, and Rc to Rd! Booya.

Enjoy.

2. ### Audioguru New Member

Dec 20, 2007
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I don't calculate anything. I simply look at the graph of the input resistance of a 2N3904 transistor on its datasheet. It is accurate for most small transistors. But it is common-emitter only and of course the input resistance is affected by the beta of the transistor that varies.

3. ### staticd Thread Starter New Member

Sep 7, 2010
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These equations are relative to the circuit topology, which is independent of any values on the data sheet. The data sheet allows you to "plug" those numbers into these equations.

4. ### Audioguru New Member

Dec 20, 2007
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896
No.
The input resistance of a common-emitter and a common collector transistor depends on its beta multiplying its internal emitter resistance (0.025/emitter current) plus multiplying its external emitter resistance.

5. ### staticd Thread Starter New Member

Sep 7, 2010
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My bad. I thought you had actually looked at my calculations.

6. ### Audioguru New Member

Dec 20, 2007
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The simple graph on the datasheet does not look like your page that is completely full of calculations.

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7. ### staticd Thread Starter New Member

Sep 7, 2010
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Right, that graph is showing you the value of Rpi.