# impedance

Discussion in 'General Electronics Chat' started by vijaytej, Jan 25, 2015.

1. ### vijaytej Thread Starter New Member

Jan 11, 2015
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0
hello guys......what is the advantage of having high input impedance as well as output impedance in case of designing amplifiers(using bjt s)????

2. ### crutschow Expert

Mar 14, 2008
16,537
4,455
Normally you want a relatively high input impedance to minimize any loading of the signal source.

Generally you want the output impedance to be low so the signal is not attenuated by any load.
One exception is if you are driving a high frequency signal into a transmission line and need to match its characteristic impedance, such as 50 or 75 ohms, then you want the output impedance to be the same.

3. ### vijaytej Thread Starter New Member

Jan 11, 2015
11
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thank you .... can you be more specific by citing an example.....

4. ### wayneh Expert

Sep 9, 2010
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4,416
Capturing the signal from a guitar string or microphone element requires a high impedance input. A low impedance would draw the voltage of the signal to near zero. There would be no signal left to amplify.

At the speaker end, you need a low impedance so that connecting a, e.g. 8Ω speaker does not short out the output. Very little power would be transferred to the speaker if the voltage is pulled almost to ground.

An ideal amplifier has an infinite input impedance and zero output impedance.

5. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
519
vijaytej
Do you know what a potential divider is?

Jan 11, 2015
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Yes

7. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
519
OK,

Well the output impedence of the previous stage (Zo or Ro) or input device and the input impedence of the following stage or device (Zi or Ri) form a potential divider.

The following device (amplifier) sees the voltage across Zi as its input. That is it sees Zi/(Zi+Zo) times the source voltage.

If Zi > Zo then more of the source voltage appears across Zi (the input signal is bigger) If Zi>>Zo then most of the source voltage appears across Zi

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8. ### vijaytej Thread Starter New Member

Jan 11, 2015
11
0
Thank you ,what are you referring to in the 1st para.is that a transistor or a cascade combination??