Embarrassing electronics problem (logic gates)

SredniVashtar

Have you even tried to replace the diodes with switches?
Because it looks like you are experiencing problems with basic circuit theory, not diodes.

In logic gates like the one you've shown here, diodes and transistors are nothing more than controlled switches. Transistors can act as closed or open switches by providing a suitable base or gate voltage. Diodes can act as closed or open switches by providing a suitable voltage at the 'gate input' terminal (if Vanode>Vcathode then it's a closed switch, otherwise it's open).

Once you understood how the gate works with ideal devices, you can refine your analysis by considering the effects of real devices (like diode drop voltage, internal resistances, parasitic capacitances...).

rbj

wannab, think of diodes as one-way valves. if the applied voltage is positive (or exceeds about 0.6 v), it conducts and if the applied voltage is negative, it does not conduct.

sophiecentaur

I think your problem is that you think you can jump into a topic like Electronics half way through and pick it up from there. This is just not true. The basics are absolutely essential for making progress further on. To have the remotest clue about what a transistor is doing and how it can work in a circuit, you need to understand, first, how simple circuits with just resistors operate. If you think that you already have that sussed then you should be able to answer most of the questions in this link.. If not, then there is little point in trying to understand Logic Circuits at the component level (you can always design combinational logic circuits by treating the units as black boxes because all the different logic technologies follow the same 'rules' of logic).

If you have started or completed any Physics or Electronics course then you should have or will have the knowledge. Otherwise, you will need to do a bit of self-education. There is no easy way to 'get' Electronics. Some people approach it by building and testing circuits on their own. That can work but you can't beat being taught the basics first.

There is no point in saying that you just don't like diodes. You can't be selective in this way. The expression "blocking the current" means very little, in the wrong context.

wannab

Ignoring one silly misreading I got 100%

sophiecentaur

OK then. What's the problem with diodes, which go low resistance when forward biased and high resistance when reverse biased? That's all you need to remember when you look at the diode logic circuit. You can't let diodes spoil your day. Have another go.
Are you doing a course at the moment?

wannab

It makes some sense to me if the diodes let some current through the wrong direction, but not as much as the transistor AND gate does.

Nope.

sophiecentaur

As you say, the leakage current for a diode is fractions of a microAmp and for transistor circuits the leakage could be higher. But is this relevant, when all a logic gate needs to detect is whether an input is near one or the other discrete levels? What do you think that tiny reverse current can do to effect the operation of the circuit? It has 'no significant' effect at all.

In the Diode version of an OR gate, if one input OR the other OR both are held at a 'low' voltage, then current will flow through the common resistor (at the top) and the output voltage will be low (a logic 0). This output level (0.6V) will be low enough to act as a 'zero' input for a subsequent gate. This process can only be taken so far, because each gate will have an output voltage value which is 0.6V higher than the previous one. The 0.6Vs will add up, eventually the resulting ('0') voltage output will be almost at the input supply level . This is where a transistor (active) gate works better because the logic levels are maintained/ regenerated from gate to gate.