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I know that this is a very basic question but I really do not know the answer.

I was making simple logic gates using transistors and I used PNP transistor as NOT gate.

I made the following circuit: (A) AND (B) AND (NOT C)

This circuit does not work without the resistors R1,R2 and R3. Why are these resistors important ?

schematic

simulate this circuit – Schematic created using CircuitLab

The Following NOT gate does not need a base resistor. I connect Vcc or ground directly to base and it works.

How can it work without resistors although it is very similar to the previous circuit ?

schematic

simulate this circuit

Michael George
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  • Most people don't like it when their magic smoke escapes... – PlasmaHH Sep 24 '15 at 20:16
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    If you drive "D" with something that can sink significant current, and use a battery with low internal resistance, then the second circuit is not likely to work for very long. – The Photon Sep 24 '15 at 20:35
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    Resistors in the bases might just save the LED from failing on over-current. Have you heard of BJT current gain? – Andy aka Sep 24 '15 at 20:38
  • @Andyaka yes, I've heard of BJT current gain. "Resistors in the bases might just save the LED from failing on over-current". The LED did not light at all, It is not burnt. So, I wonder why the led is not "on" at the first circuit when I removed base resistors, it should receive big current ? – Michael George Sep 24 '15 at 20:46
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    Can you show us what is driving the bases of the transistors? As the others have hinted at, you need to limit the current through the LED. If it's a red, green or yellow LED it will pass a very large current if you power it from 6V as shown. A resistor of about 470 ohms would limit the current to a safe value - about 20 mA. – Transistor Sep 24 '15 at 21:59
  • The resistor, in series, at the base of a BJT transistor is, among other things, to ensure that the base-emitter voltage does not exceed 0.7v or 0.3v (silicon-germanium) since the current curve of the transistor it is very sensitive to temperature variations and an increase in the base-emitter voltage always increases the temperature of the junction. – APO69 Feb 13 '22 at 11:03

2 Answers2

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Without resistors, your circuit looks like

schematic

simulate this circuit – Schematic created using CircuitLab

The problem is that there is no voltage across the emitter-base of Q1, so it cannot turn on.

WhatRoughBeast
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The second circuit works because when you connect the base to ground you have shorted out the base-collector junction and, effectively, turned your transistor into a diode (between the emitter and the base. Since that "diode" is forward biased current will flow.

As per my comment above, we need a little more info on what was driving the transistor bases to understand what was happening with the first circuit.

Transistor
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  • Wires were driving the transistor bases. I connect them manually to vcc or ground. The circuit does not work without resistors. It should draw large current without resistors, shouldn't it ? – Michael George Sep 24 '15 at 23:05
  • The second circuit is a voltage follower with VIN = -6 V (with respect to the positive rail) that tries to set 5.3 V across the LED. The transistor functions normally in active mode. The only problem is the unlimited current. The reason for not burning the LED may be that the source is weak (it has some internal resistance)... or the LED has high threshold voltage... or both. – Circuit fantasist Nov 23 '19 at 21:31