output buffers

[Ldanielrosa]

I'm interested in a parts minimal tri-state output buffer. No enable pin, just high-OC-low output from the input signal. I'm inspired by the capability of uCs to multiplex outputs, but find the use of support chips wanting. For applications like clocks and gauges the uC will spend a lot of time doing nothing, so the space weight and power used doesn't seem justified.

I have a couple ideas I'll submit once they've been drawn. I'd like see what others have been using.

[Ldanielrosa]

I'm sorry about the delay, I had a couple other things to attend to.

This one is inverting. The output has resistors in the emitter legs, which are optional. I thought that for multiplexed LEDs, some current control in the buffer would be good. If this is not desirable, the resistors (Re) and the diodes (unlabeled) should be reoved. The zeners will probably be LEDs (common, cheap) but leave the device with a narrow operating voltage range. Ideally the 'zeners' should have a voltage drop more than half and less than all the supply voltage.

I haven't yet built one. Once I do I'll let you know what I find out.

[Ldanielrosa]

I don't know if I can attach two images to one post, so here's the other idea I have.

This one does not invert, and is a bit more forgiving of supply voltage. The first stage (Q1 and Q2) steers the current away from the second stage bases. The quiescent current, IRc, will flow when the output is high-Z (this buffer's chief liability in my opinion). This one also requires that the input come within a half volt of the supply rails to guarantee reliable operation.

Let the opinions roll in!