Vol IV - Chap 3 - The NOT Gate - Transistor Q1

Thread Starter

Florian Mainguy

Joined Apr 30, 2015
2
Hello everyone,

I'm a bit confused about the behaviour of the transistor Q1 in the NOT circuit showed in one of the textbooks. It says that it behaves as a back-to-back pair of diodes. I always try to understand at a low level so I would like to know what's going on beetwen the PN junctions.

So it behaves as a back-to-back pair of diodes. Why there isn't a current flow beetwen Emitter and Collector ? Is it because:

- The collector is linked to the Base of a NPN transistor Q2. If Q2 was a PNP transistor, there would be a flow.

- Q1 is a different transistor. The P doped area is bigger, so that there can't be any flow beetwen Emitter and Collector.

I hope I have been clear.
Thanks.
 

MrChips

Joined Oct 2, 2009
19,768
As pointed out in the text, Q1 is not used as a transistor but as a back-to-back pair of diodes.
Pay attention to the current through R1. The current can only go through diode A or diode B.
When the input is at logic-HIGH, the current through R1 goes to the base-emitter junction of Q1.
When the input is at logic-LOW, the current is steered away from the base of Q1.
 

Thread Starter

Florian Mainguy

Joined Apr 30, 2015
2
Actually I understand the circuit once I consider the transistor Q1 as a back-to-back pair of diodes, but what I don't understand is why we consider the transistor as a back-to-back pair of diodes.

When the input is at logic-low, there is a current going through the base-emitter of Q1. And I thought that as soon there is a base-emitter current, there is a much bigger emitter-collector current. But if I'm right, this is not possible here because the collector of Q1 is connected to the P Base of Q2.
So no emitter-collector current in Q1.
So the transistor = back-to-back pair of diodes.

If I'm right, of course.
 
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