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hellovinay
Joined: 06 Jul 2008
Posts: 6
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 06 Jul 2008 10:14 please help me out .......... |
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hi
i am struggling with this doubt which i had on differential ampilfier .here goes my doubt
in case of cb amplifiers the ac resistance is small beacause of presence of capacitor which allows ac current as the gain of amplifier is (Rc/re') where Rc is the collector resistance and re' is ac emmiter resistance where Rc>>re'
In a common emitter stage there is an ac path between emitter and ground which normally is of pretty low resistance - resulting in a rather high gain. And you are missing such a path in a differential amplifier.
Last edited by hellovinay on 06 Jul 2008 11:32; edited 3 times in total |
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LvW
Joined: 07 May 2008
Posts: 692
Helped: 134
Location: Germany
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| 06 Jul 2008 10:26 Re: differential amplifier .......... |
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| hellovinay wrote: |
hi
in case of normal amplifiers the ac resistance is small beacause of presence of capacitor which allows ac current as the gain of amplifier is (Rc/re') where Rc is the collector resistance and re' is ac emmiter resistance where Rc>>re'
but in differential ampliferu dont have any capacitor to provide low resistance path then in what way does the circuit is getting low resistance path
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Which "ac resistance" are you speaking of ? What is a "normal amp" ? (common emitter ?). Where and why do you want a "low resistance path" ?
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hellovinay
Joined: 06 Jul 2008
Posts: 6
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| 06 Jul 2008 10:48 Re: please help me out .......... |
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[quote="hellovinay"]hi
i am struggling with this doubt which i had on differential ampilfier .here goes my doubt
in case of cb amplifiers the ac resistance is small beacause of presence of capacitor which allows ac current as the gain of amplifier is (Rc/re') where Rc is the collector resistance and re' is ac emmiter resistance where Rc>>re'
but in differential ampliferu dont have any capacitor to provide low resistance path then in what way does the circuit is getting low resistance path
PLEASE GIVE UR REPLY OR SUGGEST ME ANY WEBSITE FOR THIS IF POSSIBLE
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FvM
Joined: 22 Jan 2008
Posts: 2488
Helped: 409
Location: Bochum, Germany
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| 06 Jul 2008 10:49 Re: please help me out .......... |
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As LvW said, please clarify which circuit topology you're considering here. Generally, if you operate two amplifiers differentially, (e. g. of common-base type), you have to distinguish common mode and differential parameters. The differential input impedance of a cb amplifier pair would be always low.
P.S.: Please show a circuit!
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jony130
Joined: 06 Sep 2006
Posts: 51
Helped: 5
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| 06 Jul 2008 11:04 Re: please help me out .......... |
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Long-tailed pair is a emitter-follower + common-base amplifier. And as we now the input resistance of CB amplifier is small and output resistance of emitter-follower is also small. So here you have your "low resistance path".
http://paginas.fe.up.pt/~fff/eBook/MDA/MDA.html
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LvW
Joined: 07 May 2008
Posts: 692
Helped: 134
Location: Germany
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| 06 Jul 2008 11:13 Re: please help me out .......... |
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| hellovinay wrote: |
in case of cb amplifiers the ac resistance is small beacause of presence of capacitor which allows ac current as the gain of amplifier is (Rc/re') where Rc is the collector resistance and re' is ac emmiter resistance where Rc>>re'
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Hi, hellovinay,
I like to make some general remarks to your question:
If you expect a usable answer from somebody, you should try to formulate your problem as clear as possible. Example: When you speak about an "ac resistance" in a circuit you have to specify between which two points ! And when you refer to an "ac path" you should give information which path you mean. OK ? In your very own interest !
Nevertheless, I believe to know what you mean:
In a common emitter stage there is an ac path between emitter and ground which normally is of pretty low resistance - resulting in a rather high gain. And you are missing such a path in a differential amplifier. Right ?
Answer: You cannot compare common emitter principle with the diff. stage. Instead you should think of a series of a common collector and a common base arrangement. The first transistor works in common collector topologie because the output at its emitter is fed to the emitter of the 2nd stage, which therefore is operating in a common base configuration.
The gain of the first stage (related to its emitter) loaded by the low input impedance of the 2nd stage is only 1/2. The gain of the 2nd stage is (+gRc) - resulting in an overall gain (base 1 to collector 2) of 0.5*g*Rc.
Any further question ?
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