EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: Infraviolet on February 19, 2023, 09:51:28 pm
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Ok, I'm wondering about the possibility for a circuit element involved in an optical communication emitter which can serve to produce the following "truth table":
InputA | InputB | Output
-------------------------------
High | High | Vcc
High | Low | Vcc/2
Low | High | Vcc/2
Low | Low | 0
So it is effectively adding the two signals.
I also want it to be able to handle fairly large output currents, up to 1A, so typical op amps are out of consideration i think.
Input currents would ideally be minimal, but could get as high s 10mA each if really needed.
And operate for fairly fast signals, input square waves in the 5Mhz to 50MHz range, where the output isn't hugel distorted by massively extended rise times. Again this probably prohibits normal op amps from being used.
What would be my options for such a circuit?
Thank you
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Use an analog switch like 74HC4051 or 74HC4052 to select your output voltages from a 1 of 4 or 8 inputs.
Take the output to drive the input of a cheap >1amp output linear opamp in voltage follower mode.
You may need to supply the opamp's VCC with a greater voltage than VCC to get the full +VCC output at 1 amp, even maybe so if it is a rail-to-rail output opamp. Digikey has a cheap 1.5amp op-amp, but, it only offers 12Mhz bandwidth.
Since I do not know where you derive the 1/2 vcc, or how you load the output, there may be better solutions for your design, however, my above solution allows for setting any 4 or 8 input voltages you like.
Done right, even 2x NPN transistors in emitter follower mode to buffer the 74HC4051's output with an added boost to your VCC reference could deliver a few good amps.
If you need push-pull, then think about a class A-B transistor output. Again, the class AB voltage follower will need an at least VCC+1 to +2v, and a -1 to -2 volt supply for clean sharp edges, but you will find this way you can drive fast square waves to 100MHz with a good complementary pair transistors.
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Thanks, that makes sense as a principle.