Simple transistor NOT gate

Allen,

Yes, it is possible, and your circuit should work. When you ground the input, the transistor should be in cut-off and the collector voltage should be at Vcc. When you apply Vcc to the input, the transistor should saturate and the collector voltage should be at zero.

Check all connections and check you transistor. If your ciruit still does not work, in what way is it not working? What type of transistor are you using? What value of Vcc?

Other options? Google: transistor not gate

Richard

What do you get at the output when input is at ground? At Vcc?

What kind of transistor? How many volts is Vcc?

You may need to select different resistor values for a particular transistor type, but it should work.

The overall circuit is a classic inverting amplifier, used just about everywhere.

You've described a perfect transistor inverter, that should work for logic level inputs. A couple of things to check:

• Make sure you're using an NPN transistor instead of a PNP, a MOSFET, or something else.
• Recheck your wiring, looking particularly at the pinout of the transistor. It's easy to assume EBC when it's ECB, or vice versa.
• Use a voltmeter to actually measure the voltage at the input. If you're driving the input from an open collector transistor or gate output, no current will be injected into the base of your transistor when your logic level is "high".

If you run through these three, the transistor has to be defective. Replace it.

You've got the right answer right in front of you. Make it work.

Cheers Chris

Classic RTL gate: should work.

But if you used this to drive another such gate, the second gate would serious load down the output voltage of the first one. You could juggle the resistor values to reduce this effect. Like maybe...

John

TPUT

TPUT

Thanks very much Richard, Randy, Chris & John,

The gate was used to replace a 74LS04 TTL inverter used between the output of 74LS138 and the LE input of a 74LS373 latch. The clock was about 1.8Mhz. During a high the transistor conducts fine but during a low input of short duration, I would not be able to get a high with my logic probe nor the LE is triggered. The Vcc is 5V and the transistor is a Japanese 2SC945 general purpose transistor.

I checked with google for a transistor NOT gate and it used 1K at collector and 10K for the base and input. There was no resistor between Base and Emitter. And I tried that circuit, it doesn't work too. Where can I get the schematics for a RTL or DTL not gate?

TIA and regards,

Allen

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I have got the datasheet of 2sc945 at

http://www.ortodoxism.ro/datasheets2/8/0u9h25xojqxp8qwzrfi8kxtjsswy.pdf

and the RTL inverter gate at

Should I used HFE1 or HFE2 for the current gain of the 2sc945 or should I get a 2N4124 to make the circuit work?

Allen

Plain 'ol transistor circuits can be slooow if you aren't careful. You should either use a smaller (220 ohm?) resistor on the base, or you should use a 220 in series with a 1k in parallel with a cap -- something between 100pF and 1nF should do, but I refuse to go through the math in my head.

You can get Tiny Logic inverters -- I assume you are purposely avoiding surface mount parts?

OUTPUT

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OUTPUT

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en

here

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Is this the circuit you have in mind?

VCC | .-. | | | |1K '-' | 10k +------o ___ |/ o-|___|-+ -| | |>

.-. | | | | 220| | =3D=3D=3D '-' GND | +---+ | | .-. | | | --- 1k| | ---1n '-' | | | +---+ | =3D=3D=3D GND

(created by AACircuit v1.28.6 beta 04/19/05

Yes, this is just a hobby & fun project and I dont have the gears to solder a SMD. I am constructing the circuit on a breadboard and if it works, I would solder it onto a stripboard.

I saw the inverter SMD on the back of the databook. It was made by Toshiba and part number is TC4S69F. It was a 1989 databook.

Allen

Alan: Your transtor is saturating when on, and it is probably very slow to come out of saturation. Try putting a diode from base to collector; that will keep it from saturating. Here is the circuit:

VCC --------------------------. | | .-. | | 10k | | '-' | | .--->|----o----- out | | | | | | 1k | | ___ | |/ in -------|___|----o-------| |>

| | GND --------------------------' (created by AACircuit v1.28.6 beta 04/19/05

This trick is called a 'baker clamp' for some reason. The diode prevents the transistor from saturating, thereby increasing the speed at which it can turn off. This circuit probably won't do more than a few MHz, but you never know.

Regards, Bob Monsen

No no! You want the capacitor in the path from the drive to the gate, to speed up the current into and out of the base:

If you have enough drive this will even forcibly de-saturate a transistor

-- look around on the Zetex web site & you'll find some app notes on this.

o come

he

Hi Bob,

Your circuit works like magic. I am going to read more about this 'baker clamp' diode. Was it mentioned in the AoE?

Thanks again for your help!

Allen

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HI Tim,

I tried out the circuit but it doesnt work in my application. What do you meant by "if you have enough drive"? I am driving the circuit with the output of a 74LS138. Does the 74LS138 have enough drive to unsaturate the transistor?

I am going to google the Zetex web site to get more info as suggested.

Thanks.

Allen

It sounds like Bob Monsen's solution worked for you. I've done this with

-- Tim Wescott Control systems and communications consulting

Need to learn how to apply control theory in your embedded system? "Applied Control Theory for Embedded Systems" by Tim Wescott Elsevier/Newnes,

Man, Monson put a lot of empty space in his art ...and Google added a lot of crap to it when it got a hold of it.

Allen B>Your circuit works like magic. I am going to read more

Yup.

ide quoted text -

Hi, you're right! My copy of AoE does have a description on Baker Clamp on Page 584 and also a full chapter on the 8085 micro too. What a find! Looks like the book was sitting in my book shelf for toooooo long and I'm going to reread it from the begining again.

Cheers,

Allen

If you want even faster switching, try this: [Bob Monsen drew, but I've modified:]

Cheers! Rich

Thanks Rich,

I do have an 8051 and a PIC16F877 SBC working at 12Mhz and 20Mhz resp. I will try out the circuit there.

Rgds,

Allen

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Hi Tim,

I've found the application notes on the zetex web site. It was on P2 & P4 of the pdf file which explains the speeding up of switching, both the capacitor speedup and baker clamp methods are described.

Allen