transistor NOT gate

I am trying to build a NOT gate similar to this one:

formatting link

and was wondering how the resistor values of 1k and 10k were determined.

Reply to
Michael
Loading thread data ...

Makes the arithmetic easy? Beta(min)=10?

Reply to
krw

They found the circuit somewhere else.

They found a circuit but used resistor values that they had.

From experience they had a general feel for things, so they guessed.

They used trimpots and adjusted the values until things worked.

They did some or all of the above and then made sure it worked, maybe making adjustments to the values to ensure it worked.

They got a calculator and formulas and planned it all out.

Michael

Reply to
Michael Black

Consider the 1k load or "pullup" resistor. You want high enough resistance that you don't waste a lot of power, but low enough resistance that you have enough current to drive whatever stage comes after the inverter (aka not gate). 1k is a pretty good value. At 5 volts it only dissipates a fortieth of a watt, so you know you won't have to use a big power resistor. Even at 12 volts, 1k only dissipates about a seventh of a watt. The base resistor just needs to deliver enough current to turn the transistor on. It's a function of the gain. In this circuit, if the input signal has the same voltage as Vs, then you would have about a 10 to 1 ratio of collector to base current. Just about any bipolar transistor you can find, of the sort used in a circuit like this, will have much more gain than that. So with a 10k base resistor you know the circuit will work. They could have used a bigger base resistor. I guess they picked 10k because it's a power of ten.

Reply to
Michael Robinson

low

mes

volts

e a

th of

input

you

in

Ok, thanks. I will have to ponder this information.

Michael

Reply to
Michael

When the transistor goes into saturation, the beta goes down -- down below 1, if you push it hard enough. Something like a 2N2222 or a

2N3904 could be expected to need that much base current to really work reliably.

Note that this thing will be slooooooow at turning off unless there's something positive to clear charges from the base. A resistor from transistor base to ground would do wonders for faster turn-off, at the cost of needing a lower series resistor for turn-on.

--
Tim Wescott
Wescott Design Services
 Click to see the full signature
Reply to
Tim Wescott

Pretty much by the seat of one's pants. ;-)

Cheers! Rich

Reply to
Rich Grise

Yes.

Yes, I suppose. Technically, it is possible to drive that much current into the base. No one will argue that. But for practical purposes, in that arrangement, I never see beta "down below 1" as a design input.

... would exhibit a typical beta of 100 at Vce=100mV and a beta of 30 at Vce=50mV. That's a "more than practical" Vce, and still way over 1 beta.

I'm not an expert. You are. No question. Could you explain this in more detail for a hobbyist type?

What do you mean by suggesting that one would need to plan Ib=Ic in that topology for it to "work reliably." I am mystified. I'd like to understand what you see here that makes you comment in that way.

Jon

Reply to
Jon Kirwan

I'm not saying that you'd need Ib = Ic -- the beta < 1 example was intentionally extreme, to show that you can't count on anything.

The answer, in all the detail you need, is -- look at the data sheet, and understand what it means. In fact, had _I_ done that, I'd have had my memory prompted about the 2N2222 being better than I remembered at low Vce (or newer ones are better).

While you're looking at the data sheet you also need to consider if the numbers are typical or guaranteed, and if they're for the whole temperature range of the device or just at 25C. Then ask if you're going to be happy with one prototype that works but can't be duplicated, and if you're going to be happy with a device that only works at room temperature or if you're ever going to take it outside.

--
Tim Wescott
Wescott Design Services
 Click to see the full signature
Reply to
Tim Wescott

They found the circuit somewhere else.

They found a circuit but used resistor values that they had.

From experience they had a general feel for things, so they guessed.

They used trimpots and adjusted the values until things worked.

They did some or all of the above and then made sure it worked, maybe making adjustments to the values to ensure it worked.

They got a calculator and formulas and planned it all out.

Michael

Or, they posted their problem on the net and asked for help.

Reply to
Tom Biasi

I suppose they could have plugged it into a SPICE program too... but I was trying to determine the proper way to find these values, since I know very little about transistors other than that they can be used as electronic switches, sometimes.

Michael

Reply to
Michael

formatting link

Have Fun! Rich

Reply to
Rich Grise

They may be assuming use with the old TTL logic chips that require about 2 milliamps for a low input, while the output is around 0.5 milliamp in the high state. The TTL output voltage can also be a lot less than 5 volts, maybe only

2.5.

The transistor e/b junction will drop around 0.7 volts, so that leaves

2.5 - 0.7 =3D 1.8 volts across the 10K resistor, which is 180 microamps of base current. The 1K resistor will require 5 milliamps to drop the 5 volt supply to near zero. So, the minimum gain for the transistor is .005 / .00018 =3D 27 with no load.

Now, if you connect the transistor output to another TTL input, you need about 2 milliamps to pull it low, so the transistor now has to provide 5 milliamps to the 1K resistor and another 2 milliamps to the load, so the minimum gain is now is .007 / .00018 =3D 39. Probably still in the ballpark for a single TTL load.

-Bill

Reply to
Bill Bowden

Yes, the standard TTL input(low) current was 1.6mA (16mA output low drive, for a fanout of 10).

Certainly. TTL is far from symmetric as we've gotten used to with CMOS. The output-high is current limited to contain smoke (output low, not so much) if outputs are shorted to ground or other outputs. The output LPUL is 2.4V, with at the input. The threshold is between 1.6V and 2.0V, IIRC for a 400mV noise margin. Wow, those are old memories. ;-)

But my bet is still that it works, and that 1K and 10K are nice looking numbers.

Reply to
krw

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.