Author Topic: Level Shifter circuits  (Read 1623 times)

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Offline foxabiloTopic starter

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Level Shifter circuits
« on: February 02, 2019, 01:43:00 pm »
I've been into electronics for many years and even made a living out of it, today I put in my boggo standard low speed level translator for 1.8 to 3.3 volt systems and it hit me I have NO bloody idea how this actually works, when the INPUT (yes I know it's the wrong right to left not left to right) is low I get that the NPN base is at 1.8v (ok current is flowing through the 4k7 always on state) so both the input and output drag the 3.3V side to approximately ground through the 10K, but when the INPUT is high the base is still receiving current, is it that the Gate to Emitter differential is now 0 that lets the output side rise to 3.3V?
 

Offline T3sl4co1l

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Re: Level Shifter circuits
« Reply #1 on: February 02, 2019, 04:53:16 pm »
It's a common-base amplifier, so the voltage gain is positive and can be greater than one (which of course is necessary to get more output swing from less input swing).

Current gain is less than one, so the 1.8V logic has to be able to sink the 3.3V logic pullup.

This is a saturating circuit, so the base supply has to be current-limited, and a speed-up cap is optional (although 1nF seems way too much -- 100pF would be enough for a 2N3904).

For greater output voltage swings, this circuit has the advantage that Miller effect is nearly eliminated.  So you can go from a very low logic level (1.2V or more), up to a full voltage swing, which might include applications from gate drivers (10-30V swing) to nixie drivers or CRT blanking (50-200V swing).

Here's an example where I used a 1.2V logic level (set by D2 Vf + Q3 Vbe) into a common-base level shifter (Q4, D3, D4), with a current source pull-up for better speed (Q5, Q6 and such), then emitter follower for gate drive:
https://www.seventransistorlabs.com/Class_D_Amp.pdf

It works just fine with MOSFETs too, with the difference that the gate voltage bias and input swing need to be adequate for the part.  A 2N7002 needs 2 or 3V (gate to VDD, source to low-level logic, drain to pull-up).  A depletion type would have gate tied to GND instead, which means an open-collector/drain pin can drive it and it supplies its own pull-up through drain-source current (though a pull-up from source to VDD will speed things up a bit).

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
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Offline foxabiloTopic starter

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Re: Level Shifter circuits
« Reply #2 on: February 06, 2019, 05:57:17 pm »
thank you ..that explanation was perfect. I get it now
 


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