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input impedence or input resistance?

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Electronman

New Member
HI,
I am involving to a question these days,
I just want to know how to calculate the input impedance for an common emitter BJT configuration? Do we have a such phrase (ie input impedance for a bjt) or input impedance is equal to the input resistance which is calculated by small signal models like pi model?

suppose I want to calculate the input impedance of my BJT to see if my Mic is suitable to be used by my common emitter configuration, then what do I need to do? Do I need to calculate the input impedance (resistance?) of the BJT configuration or just knowing the high or low impedance of my configuration and the MIC is enough thing to be known?

Thank.
 
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Nigel Goodwin

Super Moderator
Most Helpful Member
You don't match impedances, it's mostly a very bad thing :D

You feed a low impedance into a higher one, a minimum of five times higher is the usual rule of thumb. So for a 600 ohm mike, you would use a minimum of 3000 ohms input.
 

audioguru

Well-Known Member
Most Helpful Member
Resistance is DC. Impedance is AC and the impedance is reduced when a coupling capacitor couples in more resistance in parallel.

The datasheet of the 2N3904 has a graph of its input impedance at 1kHz at various currents. It is its typical hfe x its internal emitter resistance.

An electret mic powered by a 10k resistor has a source impedance of about 3k ohms. To avoid loading it too much which reduces its output level it should have a load of 15k to 30k ohms.
 

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Electronman

New Member
So what's this:
rin= vin/iin ( the formula for calculating input resistance)?
r used to show AC (we know that R is for DC) so it is impedance right?!
second thing is that you seem to say that the exact VALUE of the input impedance is not important right? then why we calculate rin?
Yes I agree that there is no need for input impedance matching but what to do to see if a Mike is good for our configuration or not? any calculation needed?
 

Electronman

New Member
An electret mic powered by a 10k resistor has a source impedance of about 3k ohms. To avoid loading it too much which reduces its output level it should have a load of 15k to 30k ohms.



What do you mean by source impedance here? And what this means ""it should have a load of 15k to 30k ohms.""? Load??!
 

audioguru

Well-Known Member
Most Helpful Member
What do you mean by source impedance here?
An electret mic has a Jfet transistor that is a current source with an output impedance of about 4.3k ohms. In parallel with the 10k resistor that powers it its total impedance is 3k ohms. It is a signal source with a source impedance of 3k ohms.

And what this means ""it should have a load of 15k to 30k ohms.""? Load??!
The load on the mic is the input impedance of its preamp.
If the input impedance of its preamp transistor is 15k to 30k then the mic's output level will not be reduced much.
 

Electronman

New Member
Ok thanks, I want to know why we have to calculate rin and rout in small signal model?
Ok I change my question for the output of a bjt. Why we need to calculate ro in small signal models for a bjt? suppose I want to connect a small 8 ohms speaker to the output of a common emitter bjt, then I need impedance matching here, so can I tell that I have the impedance matching state when ro is equal to 8 ohms too?id not, then:
1: what's the usage of ro.
2: How to calculate the output resistance (impedance?) of the bjt so that I could relise if it is good for my speaker or not???
 

audioguru

Well-Known Member
Most Helpful Member
One transistor drives the very low impedance of an 8 ohm speaker poorly.

Audio amplifiers use a push-pull class-AB output which is a totem-pole NPN transistor and PNP transistor as emitter-followers.
Audio amplifiers have a very high internal gain so that negative feedback can reduce the gain to a useable amount and reduce the output impedance of the amplifier to less than 0.04 ohms so that it damps the resonances of the speaker.

Only old vacuum tube amplifiers matched their outout impedance to the speaker impedance and the speakers at that time had internal damping of their resonances.
 
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