# Bipolar Current Source

Discussion in 'General Electronics Chat' started by Rolexme, Feb 18, 2008.

1. ### Rolexme Thread Starter New Member

Jan 8, 2008
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I learned a current source made by amplifier and transistor , I checked it in the book <<the art of electronics>> , but I can't understand how it work totally . How the transistor work in this circuit , does it fit the need of working condition itself ?

Thanks a million!

I don't know how to past the picture directly , I find the picture through books.google.com . The website address is as follows :

2. ### Audioguru New Member

Dec 20, 2007
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896
The opamp measures the voltage drop across the resistor R which is created by the current in this resistor and adjusts the conduction of the transistor so the current is constant.

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3. ### Rolexme Thread Starter New Member

Jan 8, 2008
7
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Thank you for your reply . I don't know how the transistor get it's DC operating point through the opamp. Is this a common-base circuit ? And we must limit the amplitude of load to make the transistor work well in this circuit ?

4. ### Audioguru New Member

Dec 20, 2007
9,411
896
The opamp adjusts the amount of drive to the transistor so that its input voltages are the same. So the reference voltage made by the two resistors in a voltage divider also appears at the emitter of the transistor since that is the other input of the opamp. Then if the load current increases the voltage across R increases and the opamp detects it and reduces the drive to the transistor to keep the current constant.
If the load current tries to reduce then the opamp detects it and drives the transistor harder to keep the current constant.

A common-base transistor has its emitter as its input. This transistor has its base as its input from the output of the opamp.
The load is at its collector. Only one type of transistor circuit is like that.

5. ### mark0908 New Member

Feb 18, 2008
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common-emitter ?

6. ### Rolexme Thread Starter New Member

Jan 8, 2008
7
0
Thanks !

I know this circuit is a negative-feedback , so the circuit can adjust itself to constant current . But I don't konw how the transistor work , is that a common-emitter ?common-base ? That confused me .

It 's great you are online , do you have a internet chat tools , like Skype or MSN or something ?