# Transistor not gate function

1. Jun 24, 2011

### tmiddlet

Transistor "not gate" function

I've been trying to understand the following circuit:

I'm new to circuitry and electrical engineering, so I've had some trouble understanding the function of transistor Q3 and diode D2, I believe the circuit should function without them. If anybody could describe to me their function, that would be great, I under stand the rest.

Thanks!

2. Jun 24, 2011

### dlgoff

Re: Transistor "not gate" function

I don't understand the 0 V arrow pointing to Vcc as when the switch is grounded, the input would be 0 V but not Vcc which is 5 volts.

Anyway, when the switch/input is at 5volts, Q2 will saturate and its collector voltage will be low causing Q3 to not saturate also the emitter voltage of Q2 will be high causing Q4 to saturate, so the output would be low (saturation collector to emitter= ~0.2V). Hence the inverter; input high=5V, output low=0.2V.

Now when the switch/input is at 0volts, Q2 will not saturate and its collector voltage will be high causing Q3 to saturate also the emitter voltage of Q2 will be low causing Q4 to not saturate, so the output will be high (Vcc -saturation collector to emitter=~4.8V). Hence the inverter; input low=0V, output high=4.8V.

And I believe the diode, D2 is there to prevent whatever is connected to the output not to back feed current into your circuit. But I may be wrong on this point.

Hope this helps

3. Jun 24, 2011

### tmiddlet

Re: Transistor "not gate" function

The 0 volt arrows just indicate a zero volt difference, so the diode doesn't activate.

Because Q3 and D2 don't seem to significantly change the output voltage, I think you're probably right, they're probably protecting the circuit from back feed current.

They were confusing because they don't seem to affect the behavior of the circuit significantly.

4. Jun 24, 2011

### dlgoff

Re: Transistor "not gate" function

Well here's an explanation of what's happening and why the transistors of this circuit

http://en.wikipedia.org/wiki/Transistor–transistor_logic#TTL_with_a_.22totem-pole.22_output_stage"

Last edited by a moderator: Apr 26, 2017
5. Jun 24, 2011

### tmiddlet

Re: Transistor "not gate" function

Thanks for the link, that was a good description, the only part I don't understand is:
I don't see why R3 doesn't increase the output resistance

6. Jun 24, 2011

### dlgoff

Re: Transistor "not gate" function

The key words here are "voltage follower".

Bold emphasis by me.

http://en.wikipedia.org/wiki/Voltage_follower#Single-transistor_circuits"

Last edited by a moderator: Apr 26, 2017
7. Jun 25, 2011

### sophiecentaur

Re: Transistor "not gate" function

It's been a while since I was doing this stuff but I can't see why is there no 'pull down' resistor somewhere in the base circuit of Q2?
Any ideas?

8. Jun 25, 2011

### dlgoff

Re: Transistor "not gate" function

In the circuit note that "Q1" is a transistor like in this circuit,

It's from this site where the OP, I'm guessing, got the circuit.