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level convertor from 1.8v to 3.3v using npn transistor

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chethankp

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Hi,

I'm using npn transitor to do the level convertion from 1.8v to 3.3v .
And the maxium data rate will be 38400bs.
I have attached the ckt whcih I have drawn . pls see this and review .
And I have selected 1k at vcc to collector and 2k at the vcc to base . Suggest the resistor b/w emitter to base .
regards
chethan
 

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You need to ensure that when the emitter is low the transistor is turned ON and when it is high (1.8V) the transistor is OFF. If you target a turned ON base-emitter of 1V (which will obviously drop due to the base-emitter conduction) then you would need 870 ohms for the base-emitter resistor. When turned OFF, the base-emitter would be 0.45V. So, it would work. However, it is not a circuit I would be very happy with. Base-emitter voltage has a wide tolerance and significant temperature dependence.

Keith.
 

Hi,
Thanks for the reply....

I'll test this and 'll update the status
 

Hi,
I have used the NPN transistor , the datasheet of the trans is given in following link http://www.datasheetcatalog.org/datasheet/auk/S9013.pdf

I have tried with transistor , It did not work.. the collector output is always high for irrespective voltage at the emitter.
what may be the problem ..? is there any other simple circuit to do the above..?

regards
chethan
 

The base-emitter voltage on that datasheet is particularly low. I would suspect the wiring or circuit values, although the circuit does require a fairly high drive current from the 1.8V source. You could try increasing all resistor values by a factor of 3 or 4.

However, the sort of circuit I would normally use would be something like this:

41_1289235065.gif


It would invert the logic of the signal though. It would be fine with any general purpose NPN transistor at the frequencies you require.

Keith.
 

Hi,
Its working now.. I have connected 1k res b/w emitter and ground(parallel to the Txd).

regards
chethan
 

You should check your 1.8V logic levels - a resistor from emitter to ground should make no difference. It sounds like your 1.8V signal cannot sink current.

Keith
 

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