In the TTL inverter circuit, there is no power supply on the collector of VT4. How does VT4 saturate?

zlb0183

In the TTL inverter circuit, there is no power supply on the collector of VT4. How does VT4 saturate? [Copy link]

In the TTL inverter circuit, the book mentions that when the input is high level, VT4 is saturated due to the large base current. However, I think that at this time, because VT3 is cut off, there is no power supplied to the collector of VT4, and the emitter junction cannot be fully forward biased. The collector junction is forward biased, so how can VT4 be saturated?

maychang

For the transistor to enter saturation, it is not necessary to have a power supply on the collector. It only requires that the collector current is less than the base current multiplied by the current gain factor.

In the specific current, the collector of VT4 may be powered by an external circuit (load, input terminal of the next stage circuit, etc.). For example, if the next stage is the same inverter, the input terminal of the next stage (equivalent to the u1 terminal of this stage) will provide power for VT4 of this stage. Note that the emitter current of the input transistor of the next stage (equivalent to VT1 of this stage) flows outside the chip.

maychang

If the subsequent stage is not an inverter, but other digital circuits such as a NAND gate or a NOT gate, the input end of the subsequent stage is the same as the inverter, and can also provide power for VT4 in the first post.

zlb0183

maychang published on 2019-6-13 09:38 When a transistor enters saturation, it is not necessary to add a power supply to the collector. It only requires that the collector current is less than the base current multiplied by the current amplification factor. The specific...

"For a transistor to enter saturation, it is not necessary to have a power supply on the collector, but only the collector current must be less than the base current multiplied by the current magnification factor." Can this sentence be understood like this: if there is no power supply on the collector, then no collector current can be provided, so the collector current is 0, which must be less than the base current multiplied by the current magnification factor, so the transistor enters saturation

maychang

zlb0183 Published on 2019-6-13 13:04 ''For a transistor to enter saturation, it is not necessary to have a power supply on the collector. It only requires that the collector current is less than the base current multiplied by the current amplifier...

"Can this sentence be understood like this? If there is no power supply on the collector, then the collector current cannot be provided. In this way, the collector current is 0, which must be less than the base current multiplied by the current amplification factor, so the transistor enters saturation."

That is exactly the understanding.

zlb0183

maychang posted on 2019-6-13 14:53 "Can this sentence be understood in this way? If there is no power supply on the collector, then the collector current cannot be provided. In this way, the collector current is 0, which must be small...

Thank you for your answer. It changed my understanding of the saturation state of transistors. I originally thought that for VT4 to reach saturation, there must be a lower-level external circuit to provide power to meet the forward bias of the emitter junction and the collector junction. Now I know that for a transistor, as long as there is a suitable current at the base, even if the collector is open, the transistor will definitely be saturated (not necessarily with collector current).

人去搂空

Understand Understand