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Logic Buffers using discrete components

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samsunil

New Member
Hello,

If the digital output (LVCMOS 3V3) is heavily loaded , we may need to use buffer to meet Vih & Ioh (output high current) level of the receiver circuit.
I prefer not use the stand-alone Buffer IC's . Can anyone guide me to design my own buffer circuit based on Transistors ??

Thanks
Antony
 

Daniel Wood

Member
I believe the component/configuration you're looking is an NPN low side switch. There are lots of examples if you do a google search.
 

crutschow

Well-Known Member
Most Helpful Member
If can tolerate about a 0.65V offset in the output levels, you could use a push-pull emitter follower circuit such as this:

upload_2017-7-31_10-45-1.png
 

crutschow

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dknguyen

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If voltage loss is an issue, couldn't you just replace the BJTs in crustchow's circuit with the corresponding MOSFETs?

You could also take a FET version of crustchow's circuit, except reverse so the P-device on the high-side and the N-device is on the low side (basically making an inverter) and then cascade two of them. Four transistors seems a little decadent, but that might just be the price you pay for going with discretes.

According to this:
http://m.eet.com/media/1103154/Fig1.gif
Losing 0.6V on the logic low could be problematic.
 
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crutschow

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"Shouldn't be more than about 0.7V."

Not so.
The base-emitter drop is typically 0.6-0.7V.
Why do you say it is otherwise? o_O

Here's a simple LTspice simulation that shows no more than 0.6V:

upload_2017-7-31_13-8-19.png
 

Daniel Wood

Member
If voltage loss is an issue, couldn't you just replace the BJTs in crustchow's circuit with the corresponding MOSFETs?
This may be a good solution with a 5V rail. But using 3.3V, the fet may not have an adequate Vgs threshold voltage to throw the mosfet into saturation. I did a little simulation of this with a 2n7002, single side only, and the Vds drop was worse than the equivalent Vce drop for a transistor.
 

crutschow

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What is the maximum frequency of the signal?
 

dknguyen

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This may be a good solution with a 5V rail. But using 3.3V, the fet may not have an adequate Vgs threshold voltage to throw the mosfet into saturation. I did a little simulation of this with a 2n7002, single side only, and the Vds drop was worse than the equivalent Vce drop for a transistor.
Well of course it wouldn't work well with a 2N7002 since that's designed to work with a gate voltage of 5V. You would use a 3.3V gate MOSFET.
 

ci139

Active Member
to design my own
it can be done but it must meet following
  • keep it's I/O specs. at your app. supply range
  • -- at the same time have it's threshold fn.of(Vdd,Vss) near SRC. tech.
  • have transition delay similar to your tech. e.g. LVCMOS
  • -- while having it's input impedance near SRC. tech. have it's output impedance much lesser (prefferredly matching the exact occasion)
  • perhaps more . . . (noise immunity) . . .
it may turn out that using off the shelf component weighs out the effort meeting all of the above
 
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