Whats the purpose of using 3 transistors in an AND gate?

[Beamin]

What is the purpose of Q3? Wouldn't just two transistors work? Or is it to hold the circuit at ground when not on? To figure this out would you need the values of the R's?

[oPossum]

It would be a NAND gate without Q3.

[ledtester]

The transistors will either be off or driven into saturation, so the precise values of the R's are not important. For instance, using 1K for for all of the R's would probably work, or even 10K. The resistor values used will determine the drive capability (fan-out) of the gate.

[Zero999]

The collector load resistors should have a much lower value, than the base resistors.

If the NAND part isn't being used to drive other circuits, then R5 can be omitted.

[Beamin]

It would be a NAND gate without Q3.

Im reading about these and the same page where I got the above schematic from says this is how you would make a nand gate which is clearly different.

 

EDIT: here is another picture of an OR gate that might help me figure out whats going on, what is the point of R4 in the ORgate/2nd attachment pic circuit

[ledtester]

There a lot of ways of implementing logic gates. The NAND-gate with diodes is from of a logic family called "DTL" - diode transistor logic. But as you can tell from this Wikipedia page numerous other logic families have been invented:

https://en.wikipedia.org/wiki/Logic_family

Each logic family has its own way of implementing the various logic gates.

Perhaps what would help you understand what is going on is to realize that the last transistor and resistor in these examples (i.e. Q3/R4 in your first post, Q1/R2 in the NAND-gate, Q3/R4 in the OR-gate) function as a NOT gate.

If the voltage at the base of the transistor is HIGH, then the transistor is turned on and output voltage is LO. If the voltage at the base is LO, the transistor is off and the voltage at the output is HIGH due to the resistor.

[rstofer]

If the inputs at X and Y are 0V then neither transistor Q1 or Q2 will be on.  As a result, R3 pulls the base of Q3 high and that pulls the collector low and that's a 0V output.

If just one of X or Y goes high, the associated transistor will  be on, R3 will still pull the base of Q3 high and the output will be 0V.

If BOTH X and Y are high, Q1 an Q2 both conduct pulling R3 to ground and turning off Q3 which allow the collector resistor (R4) of Q3 to provide a high output signal.

It's an AND gate because the output is only high if both inputs are high.

Similar reasoning will work for all the other examples.  Figure out what the input transistors are doing and then figure out what is happening to the output signal.

[Beamin]

If the inputs at X and Y are 0V then neither transistor Q1 or Q2 will be on.  As a result, R3 pulls the base of Q3 high and that pulls the collector low and that's a 0V output.

If just one of X or Y goes high, the associated transistor will still be off, R3 will still pull the base of Q3 high and the output will be 0V.

If BOTH X and Y are high, Q1 an Q2 both conduct pulling R3 to ground and turning off Q3 which allow the collector resistor (R4) of Q3 to provide a high output signal.

It's an AND gate because the output is only high if both inputs are high.

Similar reasoning will work for all the other examples.  Figure out what the input transistors are doing and then figure out what is happening to the output signal.

[Beamin]

If the inputs at X and Y are 0V then neither transistor Q1 or Q2 will be on.  As a result, R3 pulls the base of Q3 high and that pulls the collector low and that's a 0V output.

If just one of X or Y goes high, the associated transistor will still be off, R3 will still pull the base of Q3 high and the output will be 0V.

If BOTH X and Y are high, Q1 an Q2 both conduct pulling R3 to ground and turning off Q3 which allow the collector resistor (R4) of Q3 to provide a high output signal.

It's an AND gate because the output is only high if both inputs are high.

Similar reasoning will work for all the other examples.  Figure out what the input transistors are doing and then figure out what is happening to the output signal.

Does dave have a video explaining this? I never put much thought into this just knowing that's how transistors work by putting current/voltage to the base, or wondering what was "under the hood" of logic/gate chips. What would it be called? Pull up/down resistor theory? 

[rstofer]

'resistor transistor logic'

Google will pop up a bunch of videos.

RTL is a very old logic family that came along before DTL and ECL and TTL and lsTTL and CMOS and <everything we use today>.  I started playing with in '69 and it had been on the market for a while.  Motorola had a nice family of chips at that time.

https://en.wikipedia.org/wiki/Logic_family

[amyk]

More famously, the Apollo Guidance Computer was entirely constructed of 3-input RTL NOR gates:

http://klabs.org/history/ech/agc_schematics/logic/5011-1.jpg
http://klabs.org/history/ech/agc_schematics/

[rhodges]

Thanks, that schematic is so easy to read. When I got my TI TTL databook almost four decades ago, I read the logic just fine, but the transistor diagrams made my eyes glaze over. I wonder if I still have that book somewhere...

It would be fun to read what resistor values were used in these RTL circuits, the expected hfe values, and the fan-outs. Could be a good story there.

[David Hess]

Fairchild included complete schematics with resistor values in their RTL datasheets.  They show base series resistors of 260 ohms and collector load resistors of 640 ohms.

Modern TTL and DTL has much better noise immunity through the use of additional diode junctions than RTL and early DTL.  The early stuff was actually pretty horrible.

http://semiconductormuseum.com/MuseumStore/MuseumStore_Fairchild_923_Index.htm

[rstofer]

We got to the Moon with RTL.  Pretty impressive.
Diode logic was prett cool.  1N914s everywhere!

[David Hess]

We got to the Moon with RTL.  Pretty impressive.
Diode logic was prett cool.  1N914s everywhere!

For the moon, I think it just came down to hybrid construction where screen printed thick film resistors are essentially free but every transistor or diode was an extra part to be wire bonded.

TTL eventually went back to using diode input structures but devices on complementary bipolar processes used PNP emitter followers.

[schmitt trigger]

And let’s not forget HTL High Threshold Logic.
Used in noisy industrial environments, it had zener diodes to increase the noise immunity.

If I recall correctly, it ran from +15v.

Motorola Semi had a decent HTL portfolio.