impedance of amplifier

Discussion in 'Homework Help' started by bhuvanesh, Mar 17, 2015.

Aug 10, 2013
268
2
i am studying bjt amplifiers,i am often getting through impedance of input and outputs . Its messing up with me
example:common collector configuration have high input impedance and low output impedance
does that input impedance is impedance between the terminal( base and collector)?
it is also said as impedance matching circuit.why common base and common emitter are not being said impedance matching circuits.Thank you in advance

2. Papabravo Expert

Feb 24, 2006
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In the context of amplifiers the word "common' refers to the terminal that is common to the input of the amplifier and the output of the amplifier.
Common Emitter --> the emitter is common to the input circuit and the output circuit. The base is the other input terminal and the collector is the other output terminal.
Common Collector --> the collector is common to the input circuit and the output circuit. The base is the other input terminal and the emitter is the other output terminal.
Common Base --> the base is common to the input circuit and the output circuit. The emitter is the other input terminal and the collector is the other output terminal.
Is that clear?

Aug 10, 2013
268
2
now my doubt is about the impedance sir, does common collector have high input impedance and low output impedance mean that collector base terminal have high input impedance and collector emitter terminal have low input impedance respectively ?
Thank you

4. Papabravo Expert

Feb 24, 2006
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Yes, that is what it means.

Aug 10, 2013
268
2
okay then ,why this configuration(common collector ) is called as impedance matching configuration?

6. Papabravo Expert

Feb 24, 2006
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It is called that because it is used for that purpose. You would get poor performance if you connected a high impedance output stage to a speaker for example.

bhuvanesh likes this.

Aug 10, 2013
268
2
okay mic is high impedance and speaker is low impedance ,so to match both we using common collector configuration .If suppose we need to connect low impedance source to high impedance load ,will we go common base.Is this also called as impedance matching configuration.

8. Papabravo Expert

Feb 24, 2006
10,340
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I'm not that familiar with the common base configuration, since it is not common in low frequency discrete circuits.

So I'll defer to someone else.

bhuvanesh likes this.
9. DickCappels Moderator

Aug 21, 2008
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"Impedance match" is probably not the best term since transistors don't transform the signal to a new impedance the way a transformers and LC networks can me used to match impedances by transforming the same energy at a different impedance.

"Impedance converter" might be a more appropriate term for the common collector configuration, since it has a high impedance input and a low impedance output. Unlike transformers the common collector circuit does not transfer the energy, but instead adds collector current to the base current to bring the emitter signal to within on Vbe drop of the base, taking only enough base current to do so.

If you want to drive a high impedance load with a low impedance source you can merely connect the two together forgetting about converting or matching networks. If you want to transfer as much of the power available at the input as possible to the output, or if you are concerned about reflections on a transmission line, a transformer or matching network should be used.

https://en.wikipedia.org/wiki/Impedance_matching

Last edited: Mar 18, 2015
bhuvanesh likes this.

Aug 10, 2013
268
2
if we connect high impedance source with low impedance load directly ,we will get poor performance said by papbravo
we do get poor performance when we connect high impedance load to low impedance source directly ,dont we ?

11. DickCappels Moderator

Aug 21, 2008
2,758
667
It depends on what you mean by "poor performance".

I suspect Papabravo was referring to poor performance as the loss of voltage amplitude when driving a lower impedance source from a high impedance source.

When driving a high impedance load with a low impedance source the voltage amplitude of the source is not affected much.

Maximum power transfer does not occur when the impedances are not matched, but most transistor amplifiers, maximum power transfer is not the objective. Where it is an objective, the impedances are matched using a matching network or transformer.

12. w2aew Member

Jan 3, 2012
219
64
Maybe my video on the three configurations (CE, CB, CC) will help: