Hello !
Looking at these two circuits of a transistor constant current source circuit taken from talkingelectronics.com :
I have a few questions regarding this arrangement , but first let me try to explain what I understand is going on there:
****
In the version with the two diodes the base is biased at 2*0.7 (1.4) volts.
VBE is 0.7 volts so that means that the emitter has to be at 0.7 in respect to ground which means that the transistor will push a magnitude of current through the emitter resistor so that there is 0.7V across it , therefore satisfying the condition that the emitter is at 0.7v IRT ground therefore giving us a constant current irregardless of the value of the collector resistor(if we have one and assuing VCC is large enough) , right ?
The circuit with the two transistors works in a similar fashion where if the emitter resistor drops more than 0.7 then it turns on the left transistor which turns off the right transistor therefore keeping 0.7 across the emitter resistor which gives us a constant current.
***
Now for some questions :
- Is this a good/accurate constant current source ?
-What is the diffrence between the two versions of the circuit in terms of performance?
- Are there any parameters that will cause my constant current to drift without me adjusting anything? perhaps VBE or something else?
- Is there any way that this circuit could be improved to provide constant current with a high frequency square wave input ? Say.. a few hundred khz or more assuming that the transistors are up for the job.
Thanks in advance .
Tim
Looking at these two circuits of a transistor constant current source circuit taken from talkingelectronics.com :
I have a few questions regarding this arrangement , but first let me try to explain what I understand is going on there:
****
In the version with the two diodes the base is biased at 2*0.7 (1.4) volts.
VBE is 0.7 volts so that means that the emitter has to be at 0.7 in respect to ground which means that the transistor will push a magnitude of current through the emitter resistor so that there is 0.7V across it , therefore satisfying the condition that the emitter is at 0.7v IRT ground therefore giving us a constant current irregardless of the value of the collector resistor(if we have one and assuing VCC is large enough) , right ?
The circuit with the two transistors works in a similar fashion where if the emitter resistor drops more than 0.7 then it turns on the left transistor which turns off the right transistor therefore keeping 0.7 across the emitter resistor which gives us a constant current.
***
Now for some questions :
- Is this a good/accurate constant current source ?
-What is the diffrence between the two versions of the circuit in terms of performance?
- Are there any parameters that will cause my constant current to drift without me adjusting anything? perhaps VBE or something else?
- Is there any way that this circuit could be improved to provide constant current with a high frequency square wave input ? Say.. a few hundred khz or more assuming that the transistors are up for the job.
Thanks in advance .
Tim